From 34cd7e2b1a1b5be175d79c3233a97393b01b8bad Mon Sep 17 00:00:00 2001 From: Vijay Dev Date: Sat, 20 Dec 2014 15:45:17 +0000 Subject: Renaming the autoloading guide [ci skip] The term 'constant autoloading' can be mistakenly construed as 'constantly autoloading'. --- .../source/autoloading_and_reloading_constants.md | 1297 ++++++++++++++++++++ .../source/constant_autoloading_and_reloading.md | 1297 -------------------- 2 files changed, 1297 insertions(+), 1297 deletions(-) create mode 100644 guides/source/autoloading_and_reloading_constants.md delete mode 100644 guides/source/constant_autoloading_and_reloading.md (limited to 'guides') diff --git a/guides/source/autoloading_and_reloading_constants.md b/guides/source/autoloading_and_reloading_constants.md new file mode 100644 index 0000000000..c39210383b --- /dev/null +++ b/guides/source/autoloading_and_reloading_constants.md @@ -0,0 +1,1297 @@ +Autoloading and Reloading Constants +=================================== + +This guide documents how constant autoloading and reloading works. + +After reading this guide, you will know: + +* Key aspects of Ruby constants +* What is `autoload_paths` +* How constant autoloading works +* What is `require_dependency` +* How constant reloading works +* Solutions to common autoloading gotchas + +-------------------------------------------------------------------------------- + + +Introduction +------------ + +Ruby on Rails allows applications to be written as if their code was preloaded. + +In a normal Ruby program classes need to load their dependencies: + +```ruby +require 'application_controller' +require 'post' + +class PostsController < ApplicationController + def index + @posts = Post.all + end +end +``` + +Our Rubyist instinct quickly sees some redundancy in there: If classes were +defined in files matching their name, couldn't their loading be automated +somehow? We could save scanning the file for dependencies, which is brittle. + +Moreover, `Kernel#require` loads files once, but development is much more smooth +if code gets refreshed when it changes without restarting the server. It would +be nice to be able to use `Kernel#load` in development, and `Kernel#require` in +production. + +Indeed, those features are provided by Ruby on Rails, where we just write + +```ruby +class PostsController < ApplicationController + def index + @posts = Post.all + end +end +``` + +This guide documents how that works. + + +Constants Refresher +------------------- + +While constants are trivial in most programming languages, they are a rich +topic in Ruby. + +It is beyond the scope of this guide to document Ruby constants, but we are +nevertheless going to highlight a few key topics. Truly grasping the following +sections is instrumental to understanding constant autoloading and reloading. + +### Nesting + +Class and module definitions can be nested to create namespaces: + +```ruby +module XML + class SAXParser + # (1) + end +end +``` + +The *nesting* at any given place is the collection of enclosing nested class and +module objects outwards. For example, in the previous example, the nesting at +(1) is + +```ruby +[XML::SAXParser, XML] +``` + +It is important to understand that the nesting is composed of class and module +*objects*, it has nothing to do with the constants used to access them, and is +also unrelated to their names. + +For instance, while this definition is similar to the previous one: + +```ruby +class XML::SAXParser + # (2) +end +``` + +the nesting in (2) is different: + +```ruby +[XML::SAXParser] +``` + +`XML` does not belong to it. + +We can see in this example that the name of a class or module that belongs to a +certain nesting does not necessarily correlate with the namespaces at the spot. + +Even more, they are totally independent, take for instance + +```ruby +module X::Y + module A::B + # (3) + end +end +``` + +The nesting in (3) consists of two module objects: + +```ruby +[A::B, X::Y] +``` + +So, it not only doesn't end in `A`, which does not even belong to the nesting, +but it also contains `X::Y`, which is independent from `A::B`. + +The nesting is an internal stack maintained by the interpreter, and it gets +modified according to these rules: + +* The class object following a `class` keyword gets pushed when its body is +executed, and popped after it. + +* The module object following a `module` keyword gets pushed when its body is +executed, and popped after it. + +* A singleton class opened with `class << object` gets pushed, and popped later. + +* When any of the `*_eval` family of methods is called using a string argument, +the singleton class of the receiver is pushed to the nesting of the eval'ed +code. + +* The nesting at the top-level of code interpreted by `Kernel#load` is empty +unless the `load` call receives a true value as second argument, in which case +a newly created anonymous module is pushed by Ruby. + +It is interesting to observe that blocks do not modify the stack. In particular +the blocks that may be passed to `Class.new` and `Module.new` do not get the +class or module being defined pushed to their nesting. That's one of the +differences between defining classes and modules in one way or another. + +The nesting at any given place can be inspected with `Module.nesting`. + +### Class and Module Definitions are Constant Assignments + +Let's suppose the following snippet creates a class (rather than reopening it): + +```ruby +class C +end +``` + +Ruby creates a constant `C` in `Object` and stores in that constant a class +object. The name of the class instance is "C", a string, named after the +constant. + +That is, + +```ruby +class Project < ActiveRecord::Base +end +``` + +performs a constant assignment equivalent to + +```ruby +Project = Class.new(ActiveRecord::Base) +``` + +including setting the name of the class as a side-effect: + +```ruby +Project.name # => "Project" +``` + +Constant assignment has a special rule to make that happen: if the object +being assigned is an anonymous class or module, Ruby sets the object's name to +the name of the constant. + +INFO. From then on, what happens to the constant and the instance does not +matter. For example, the constant could be deleted, the class object could be +assigned to a different constant, be stored in no constant anymore, etc. Once +the name is set, it doesn't change. + +Similarly, module creation using the `module` keyword as in + +```ruby +module Admin +end +``` + +performs a constant assignment equivalent to + +```ruby +Admin = Module.new +``` + +including setting the name as a side-effect: + +```ruby +Admin.name # => "Admin" +``` + +WARNING. The execution context of a block passed to `Class.new` or `Module.new` +is not entirely equivalent to the one of the body of the definitions using the +`class` and `module` keywords. But both idioms result in the same constant +assignment. + +Thus, when one informally says "the `String` class", that really means: the +class object stored in the constant called "String" in the class object stored +in the `Object` constant. `String` is otherwise an ordinary Ruby constant and +everything related to constants such as resolution algorithms applies to it. + +Likewise, in the controller + +```ruby +class PostsController < ApplicationController + def index + @posts = Post.all + end +end +``` + +`Post` is not syntax for a class. Rather, `Post` is a regular Ruby constant. If +all is good, the constant evaluates to an object that responds to `all`. + +That is why we talk about *constant* autoloading, Rails has the ability to +load constants on the fly. + +### Constants are Stored in Modules + +Constants belong to modules in a very literal sense. Classes and modules have +a constant table; think of it as a hash table. + +Let's analyze an example to really understand what that means. While common +abuses of language like "the `String` class" are convenient, the exposition is +going to be precise here for didactic purposes. + +Let's consider the following module definition: + +```ruby +module Colors + RED = '0xff0000' +end +``` + +First, when the `module` keyword is processed the interpreter creates a new +entry in the constant table of the class object stored in the `Object` constant. +Said entry associates the name "Colors" to a newly created module object. +Furthermore, the interpreter sets the name of the new module object to be the +string "Colors". + +Later, when the body of the module definition is interpreted, a new entry is +created in the constant table of the module object stored in the `Colors` +constant. That entry maps the name "RED" to the string "0xff0000". + +In particular, `Colors::RED` is totally unrelated to any other `RED` constant +that may live in any other class or module object. If there were any, they +would have separate entries in their respective constant tables. + +Pay special attention in the previous paragraphs to the distinction between +class and module objects, constant names, and value objects associated to them +in constant tables. + +### Resolution Algorithms + +#### Resolution Algorithm for Relative Constants + +At any given place in the code, let's define *cref* to be the first element of +the nesting if it is not empty, or `Object` otherwise. + +Without getting too much into the details, the resolution algorithm for relative +constant references goes like this: + +1. If the nesting is not empty the constant is looked up in its elements and in +order. The ancestors of those elements are ignored. + +2. If not found, then the algorithm walks up the ancestor chain of the cref. + +3. If not found, `const_missing` is invoked on the cref. The default +implementation of `const_missing` raises `NameError`, but it can be overridden. + +Rails autoloading **does not emulate this algorithm**, but its starting point is +the name of the constant to be autoloaded, and the cref. See more in [Relative +References](#autoloading-algorithms-relative-references). + +#### Resolution Algorithm for Qualified Constants + +Qualified constants look like this: + +```ruby +Billing::Invoice +``` + +`Billing::Invoice` is composed of two constants: `Billing` is relative and is +resolved using the algorithm of the previous section. + +INFO. Leading colons would make the first segment absolute rather than +relative: `::Billing::Invoice`. That would force `Billing` to be looked up +only as a top-level constant. + +`Invoice` on the other hand is qualified by `Billing` and we are going to see +its resolution next. Let's call *parent* to that qualifying class or module +object, that is, `Billing` in the example above. The algorithm for qualified +constants goes like this: + +1. The constant is looked up in the parent and its ancestors. + +2. If the lookup fails, `const_missing` is invoked in the parent. The default +implementation of `const_missing` raises `NameError`, but it can be overridden. + +As you see, this algorithm is simpler than the one for relative constants. In +particular, the nesting plays no role here, and modules are not special-cased, +if neither they nor their ancestors have the constants, `Object` is **not** +checked. + +Rails autoloading **does not emulate this algorithm**, but its starting point is +the name of the constant to be autoloaded, and the parent. See more in +[Qualified References](#qualified-references). + + +Vocabulary +---------- + +### Parent Namespaces + +Given a string with a constant path we define its *parent namespace* to be the +string that results from removing its rightmost segment. + +For example, the parent namespace of the string "A::B::C" is the string "A::B", +the parent namespace of "A::B" is "A", and the parent namespace of "A" is "". + +The interpretation of a parent namespace when thinking about classes and modules +is tricky though. Let's consider a module M named "A::B": + +* The parent namespace, "A", may not reflect nesting at a given spot. + +* The constant `A` may no longer exist, some code could have removed it from +`Object`. + +* If `A` exists, the class or module that was originally in `A` may not be there +anymore. For example, if after a constant removal there was another constant +assignment there would generally be a different object in there. + +* In such case, it could even happen that the reassigned `A` held a new class or +module called also "A"! + +* In the previous scenarios M would no longer be reachable through `A::B` but +the module object itself could still be alive somewhere and its name would +still be "A::B". + +The idea of a parent namespace is at the core of the autoloading algorithms +and helps explain and understand their motivation intuitively, but as you see +that metaphor leaks easily. Given an edge case to reason about, take always into +account that by "parent namespace" the guide means exactly that specific string +derivation. + +### Loading Mechanism + +Rails autoloads files with `Kernel#load` when `config.cache_classes` is false, +the default in development mode, and with `Kernel#require` otherwise, the +default in production mode. + +`Kernel#load` allows Rails to execute files more than once if [constant +reloading](#constant-reloading) is enabled. + +This guide uses the word "load" freely to mean a given file is interpreted, but +the actual mechanism can be `Kernel#load` or `Kernel#require` depending on that +flag. + + +Autoloading Availability +------------------------ + +Rails is always able to autoload provided its environment is in place. For +example the `runner` command autoloads: + +``` +$ bin/rails runner 'p User.column_names' +["id", "email", "created_at", "updated_at"] +``` + +The console autoloads, the test suite autoloads, and of course the application +autoloads. + +By default, Rails eager loads the application files when it boots in production +mode, so most of the autoloading going on in development does not happen. But +autoloading may still be triggered during eager loading. + +For example, given + +```ruby +class BeachHouse < House +end +``` + +if `House` is still unknown when `app/models/beach_house.rb` is being eager +loaded, Rails autoloads it. + + +autoload_paths +-------------- + +As you probably know, when `require` gets a relative file name: + +```ruby +require 'erb' +``` + +Ruby looks for the file in the directories listed in `$LOAD_PATH`. That is, Ruby +iterates over all its directories and for each one of them checks whether they +have a file called "erb.rb", or "erb.so", or "erb.o", or "erb.dll". If it finds +any of them, the interpreter loads it and ends the search. Otherwise, it tries +again in the next directory of the list. If the list gets exhausted, `LoadError` +is raised. + +We are going to cover how constant autoloading works in more detail later, but +the idea is that when a constant like `Post` is hit and missing, if there's a +`post.rb` file for example in `app/models` Rails is going to find it, evaluate +it, and have `Post` defined as a side-effect. + +Alright, Rails has a collection of directories similar to `$LOAD_PATH` in which +to look up `post.rb`. That collection is called `autoload_paths` and by +default it contains: + +* All subdirectories of `app` in the application and engines. For example, + `app/controllers`. They do not need to be the default ones, any custom + directories like `app/workers` belong automatically to `autoload_paths`. + +* Any existing second level directories called `app/*/concerns` in the + application and engines. + +* The directory `test/mailers/previews`. + +Also, this collection is configurable via `config.autoload_paths`. For example, +`lib` was in the list years ago, but no longer is. An application can opt-in +by adding this to `config/application.rb`: + +```ruby +config.autoload_paths += "#{Rails.root}/lib" +``` + +The value of `autoload_paths` can be inspected. In a just generated application +it is (edited): + +``` +$ bin/rails r 'puts ActiveSupport::Dependencies.autoload_paths' +.../app/assets +.../app/controllers +.../app/helpers +.../app/mailers +.../app/models +.../app/controllers/concerns +.../app/models/concerns +.../test/mailers/previews +``` + +INFO. `autoload_paths` is computed and cached during the initialization process. +The application needs to be restarted to reflect any changes in the directory +structure. + + +Autoloading Algorithms +---------------------- + +### Relative References + +A relative constant reference may appear in several places, for example, in + +```ruby +class PostsController < ApplicationController + def index + @posts = Post.all + end +end +``` + +all three constant references are relative. + +#### Constants after the `class` and `module` Keywords + +Ruby performs a lookup for the constant that follows a `class` or `module` +keyword because it needs to know if the class or module is going to be created +or reopened. + +If the constant is not defined at that point it is not considered to be a +missing constant, autoloading is **not** triggered. + +So, in the previous example, if `PostsController` is not defined when the file +is interpreted Rails autoloading is not going to be triggered, Ruby will just +define the controller. + +#### Top-Level Constants + +On the contrary, if `ApplicationController` is unknown, the constant is +considered missing and an autoload is going to be attempted by Rails. + +In order to load `ApplicationController`, Rails iterates over `autoload_paths`. +First checks if `app/assets/application_controller.rb` exists. If it does not, +which is normally the case, it continues and finds +`app/controllers/application_controller.rb`. + +If the file defines the constant `ApplicationController` all is fine, otherwise +`LoadError` is raised: + +``` +unable to autoload constant ApplicationController, expected + to define it (LoadError) +``` + +INFO. Rails does not require the value of autoloaded constants to be a class or +module object. For example, if the file `app/models/max_clients.rb` defines +`MAX_CLIENTS = 100` autoloading `MAX_CLIENTS` works just fine. + +#### Namespaces + +Autoloading `ApplicationController` looks directly under the directories of +`autoload_paths` because the nesting in that spot is empty. The situation of +`Post` is different, the nesting in that line is `[PostsController]` and support +for namespaces comes into play. + +The basic idea is that given + +```ruby +module Admin + class BaseController < ApplicationController + @@all_roles = Role.all + end +end +``` + +to autoload `Role` we are going to check if it is defined in the current or +parent namespaces, one at a time. So, conceptually we want to try to autoload +any of + +``` +Admin::BaseController::Role +Admin::Role +Role +``` + +in that order. That's the idea. To do so, Rails looks in `autoload_paths` +respectively for file names like these: + +``` +admin/base_controller/role.rb +admin/role.rb +role.rb +``` + +modulus some additional directory lookups we are going to cover soon. + +INFO. `'Constant::Name'.underscore` gives the relative path without extension of +the file name where `Constant::Name` is expected to be defined. + +Let's see how Rails autoloads the `Post` constant in the `PostsController` +above assuming the application has a `Post` model defined in +`app/models/post.rb`. + +First it checks for `posts_controller/post.rb` in `autoload_paths`: + +``` +app/assets/posts_controller/post.rb +app/controllers/posts_controller/post.rb +app/helpers/posts_controller/post.rb +... +test/mailers/previews/posts_controller/post.rb +``` + +Since the lookup is exhausted without success, a similar search for a directory +is performed, we are going to see why in the [next section](#automatic-modules): + +``` +app/assets/posts_controller/post +app/controllers/posts_controller/post +app/helpers/posts_controller/post +... +test/mailers/previews/posts_controller/post +``` + +If all those attempts fail, then Rails starts the lookup again in the parent +namespace. In this case only the top-level remains: + +``` +app/assets/post.rb +app/controllers/post.rb +app/helpers/post.rb +app/mailers/post.rb +app/models/post.rb +``` + +A matching file is found in `app/models/post.rb`. The lookup stops there and the +file is loaded. If the file actually defines `Post` all is fine, otherwise +`LoadError` is raised. + +### Qualified References + +When a qualified constant is missing Rails does not look for it in the parent +namespaces. But there is a caveat: When a constant is missing, Rails is +unable to tell if the trigger was a relative reference or a qualified one. + +For example, consider + +```ruby +module Admin + User +end +``` + +and + +```ruby +Admin::User +``` + +If `User` is missing, in either case all Rails knows is that a constant called +"User" was missing in a module called "Admin". + +If there is a top-level `User` Ruby would resolve it in the former example, but +wouldn't in the latter. In general, Rails does not emulate the Ruby constant +resolution algorithms, but in this case it tries using the following heuristic: + +> If none of the parent namespaces of the class or module has the missing +> constant then Rails assumes the reference is relative. Otherwise qualified. + +For example, if this code triggers autoloading + +```ruby +Admin::User +``` + +and the `User` constant is already present in `Object`, it is not possible that +the situation is + +```ruby +module Admin + User +end +``` + +because otherwise Ruby would have resolved `User` and no autoloading would have +been triggered in the first place. Thus, Rails assumes a qualified reference and +considers the file `admin/user.rb` and directory `admin/user` to be the only +valid options. + +In practice, this works quite well as long as the nesting matches all parent +namespaces respectively and the constants that make the rule apply are known at +that time. + +However, autoloading happens on demand. If by chance the top-level `User` was +not yet loaded, then Rails assumes a relative reference by contract. + +Naming conflicts of this kind are rare in practice, but if one occurs, +`require_dependency` provides a solution by ensuring that the constant needed +to trigger the heuristic is defined in the conflicting place. + +### Automatic Modules + +When a module acts as a namespace, Rails does not require the application to +defines a file for it, a directory matching the namespace is enough. + +Suppose an application has a back office whose controllers are stored in +`app/controllers/admin`. If the `Admin` module is not yet loaded when +`Admin::UsersController` is hit, Rails needs first to autoload the constant +`Admin`. + +If `autoload_paths` has a file called `admin.rb` Rails is going to load that +one, but if there's no such file and a directory called `admin` is found, Rails +creates an empty module and assigns it to the `Admin` constant on the fly. + +### Generic Procedure + +Relative references are reported to be missing in the cref where they were hit, +and qualified references are reported to be missing in their parent. (See +[Resolution Algorithm for Relative +Constants](#resolution-algorithm-for-relative-constants) at the beginning of +this guide for the definition of *cref*, and [Resolution Algorithm for Qualified +Constants](#resolution-algorithm-for-qualified-constants) for the definition of +*parent*.) + +The procedure to autoload constant `C` in an arbitrary situation is as follows: + +``` +if the class or module in which C is missing is Object + let ns = '' +else + let M = the class or module in which C is missing + + if M is anonymous + let ns = '' + else + let ns = M.name + end +end + +loop do + # Look for a regular file. + for dir in autoload_paths + if the file "#{dir}/#{ns.underscore}/c.rb" exists + load/require "#{dir}/#{ns.underscore}/c.rb" + + if C is now defined + return + else + raise LoadError + end + end + end + + # Look for an automatic module. + for dir in autoload_paths + if the directory "#{dir}/#{ns.underscore}/c" exists + if ns is an empty string + let C = Module.new in Object and return + else + let C = Module.new in ns.constantize and return + end + end + end + + if ns is empty + # We reached the top-level without finding the constant. + raise NameError + else + if C exists in any of the parent namespaces + # Qualified constants heuristic. + raise NameError + else + # Try again in the parent namespace. + let ns = the parent namespace of ns and retry + end + end +end +``` + + +require_dependency +------------------ + +Constant autoloading is triggered on demand and therefore code that uses a +certain constant may have it already defined or may trigger an autoload. That +depends on the execution path and it may vary between runs. + +There are times, however, in which you want to make sure a certain constant is +known when the execution reaches some code. `require_dependency` provides a way +to load a file using the current [loading mechanism](#loading-mechanism), and +keeping track of constants defined in that file as if they were autoloaded to +have them reloaded as needed. + +`require_dependency` is rarely needed, but see a couple of use-cases in +[Autoloading and STI](#autoloading-and-sti) and [When Constants aren't +Triggered](#when-constants-aren-t-missed). + +WARNING. Unlike autoloading, `require_dependency` does not expect the file to +define any particular constant. Exploiting this behavior would be a bad practice +though, file and constant paths should match. + + +Constant Reloading +------------------ + +When `config.cache_classes` is false Rails is able to reload autoloaded +constants. + +For example, in you're in a console session and edit some file behind the +scenes, the code can be reloaded with the `reload!` command: + +``` +> reload! +``` + +When the application runs, code is reloaded when something relevant to this +logic changes. In order to do that, Rails monitors a number of things: + +* `config/routes.rb`. + +* Locales. + +* Ruby files under `autoload_paths`. + +* `db/schema.rb` and `db/structure.sql`. + +If anything in there changes, there is a middleware that detects it and reloads +the code. + +Autoloading keeps track of autoloaded constants. Reloading is implemented by +removing them all from their respective classes and modules using +`Module#remove_const`. That way, when the code goes on, those constants are +going to be unknown again, and files reloaded on demand. + +INFO. This is an all-or-nothing operation, Rails does not attempt to reload only +what changed since dependencies between classes makes that really tricky. +Instead, everything is wiped. + + +Module#autoload isn't Involved +------------------------------ + +`Module#autoload` provides a lazy way to load constants that is fully integrated +with the Ruby constant lookup algorithms, dynamic constant API, etc. It is quite +transparent. + +Rails internals make extensive use of it to defer as much work as possible from +the boot process. But constant autoloading in Rails is **not** implemented with +`Module#autoload`. + +One possible implementation based on `Module#autoload` would be to walk the +application tree and issue `autoload` calls that map existing file names to +their conventional constant name. + +There are a number of reasons that prevent Rails from using that implementation. + +For example, `Module#autoload` is only capable of loading files using `require`, +so reloading would not be possible. Not only that, it uses an internal `require` +which is not `Kernel#require`. + +Then, it provides no way to remove declarations in case a file is deleted. If a +constant gets removed with `Module#remove_const` its `autoload` is not triggered +again. Also, it doesn't support qualified names, so files with namespaces should +be interpreted during the walk tree to install their own `autoload` calls, but +those files could have constant references not yet configured. + +An implementation based on `Module#autoload` would be awesome but, as you see, +at least as of today it is not possible. Constant autoloading in Rails is +implemented with `Module#const_missing`, and that's why it has its own contract, +documented in this guide. + + +Common Gotchas +-------------- + +### Nesting and Qualified Constants + +Let's consider + +```ruby +module Admin + class UsersController < ApplicationController + def index + @users = User.all + end + end +end +``` + +and + +```ruby +class Admin::UsersController < ApplicationController + def index + @users = User.all + end +end +``` + +To resolve `User` Ruby checks `Admin` in the former case, but it does not in +the latter because it does not belong to the nesting. (See [Nesting](#nesting) +and [Resolution Algorithms](#resolution-algorithms).) + +Unfortunately Rails autoloading does not know the nesting in the spot where the +constant was missing and so it is not able to act as Ruby would. In particular, +`Admin::User` will get autoloaded in either case. + +Albeit qualified constants with `class` and `module` keywords may technically +work with autoloading in some cases, it is preferable to use relative constants +instead: + +```ruby +module Admin + class UsersController < ApplicationController + def index + @users = User.all + end + end +end +``` + +### Autoloading and STI + +Single Table Inheritance (STI) is a feature of Active Record that enables +storing a hierarchy of models in one single table. The API of such models is +aware of the hierarchy and encapsulates some common needs. For example, given +these classes: + +```ruby +# app/models/polygon.rb +class Polygon < ActiveRecord::Base +end + +# app/models/triangle.rb +class Triangle < Polygon +end + +# app/models/rectangle.rb +class Rectangle < Polygon +end +``` + +`Triangle.create` creates a row that represents a triangle, and +`Rectangle.create` creates a row that represents a rectangle. If `id` is the +ID of an existing record, `Polygon.find(id)` returns an object of the correct +type. + +Methods that operate on collections are also aware of the hierarchy. For +example, `Polygon.all` returns all the records of the table, because all +rectangles and triangles are polygons. Active Record takes care of returning +instances of their corresponding class in the result set. + +Types are autoloaded as needed. For example, if `Polygon.first` is a rectangle +and `Rectangle` has not yet been loaded, Active Record autoloads it and the +record is correctly instantiated. + +All good, but if instead of performing queries based on the root class we need +to work on some subclass, things get interesting. + +While working with `Polygon` you do not need to be aware of all its descendants, +because anything in the table is by definition a polygon, but when working with +subclasses Active Record needs to be able to enumerate the types it is looking +for. Let’s see an example. + +`Rectangle.all` only loads rectangles by adding a type constraint to the query: + +```sql +SELECT "polygons".* FROM "polygons" +WHERE "polygons"."type" IN ("Rectangle") +``` + +Let’s introduce now a subclass of `Rectangle`: + +```ruby +# app/models/square.rb +class Square < Rectangle +end +``` + +`Rectangle.all` should now return rectangles **and** squares: + +```sql +SELECT "polygons".* FROM "polygons" +WHERE "polygons"."type" IN ("Rectangle", "Square") +``` + +But there’s a caveat here: How does Active Record know that the class `Square` +exists at all? + +Even if the file `app/models/square.rb` exists and defines the `Square` class, +if no code yet used that class, `Rectangle.all` issues the query + +```sql +SELECT "polygons".* FROM "polygons" +WHERE "polygons"."type" IN ("Rectangle") +``` + +That is not a bug, the query includes all *known* descendants of `Rectangle`. + +A way to ensure this works correctly regardless of the order of execution is to +load the leaves of the tree by hand at the bottom of the file that defines the +root class: + +```ruby +# app/models/polygon.rb +class Polygon < ActiveRecord::Base +end +require_dependency ‘square’ +``` + +Only the leaves that are **at least grandchildren** need to be loaded this +way. Direct subclasses do not need to be preloaded. If the hierarchy is +deeper, intermediate classes will be autoloaded recursively from the bottom +because their constant will appear in the class definitions as superclass. + +### Autoloading and `require` + +Files defining constants to be autoloaded should never be `require`d: + +```ruby +require 'user' # DO NOT DO THIS + +class UsersController < ApplicationController + ... +end +``` + +There are two possible gotchas here in development mode: + +1. If `User` is autoloaded before reaching the `require`, `app/models/user.rb` +runs again because `load` does not update `$LOADED_FEATURES`. + +2. If the `require` runs first Rails does not mark `User` as an autoloaded +constant and changes to `app/models/user.rb` aren't reloaded. + +Just follow the flow and use constant autoloading always, never mix +autoloading and `require`. As a last resort, if some file absolutely needs to +load a certain file use `require_dependency` to play nice with constant +autoloading. This option is rarely needed in practice, though. + +Of course, using `require` in autoloaded files to load ordinary 3rd party +libraries is fine, and Rails is able to distinguish their constants, they are +not marked as autoloaded. + +### Autoloading and Initializers + +Consider this assignment in `config/initializers/set_auth_service.rb`: + +```ruby +AUTH_SERVICE = if Rails.env.production? + RealAuthService +else + MockedAuthService +end +``` + +The purpose of this setup would be that the application uses the class that +corresponds to the environment via `AUTH_SERVICE`. In development mode +`MockedAuthService` gets autoloaded when the initializer runs. Let’s suppose +we do some requests, change its implementation, and hit the application again. +To our surprise the changes are not reflected. Why? + +As [we saw earlier](#constant-reloading), Rails removes autoloaded constants, +but `AUTH_SERVICE` stores the original class object. Stale, non-reachable +using the original constant, but perfectly functional. + +The following code summarizes the situation: + +```ruby +class C + def quack + 'quack!' + end +end + +X = C +Object.instance_eval { remove_const(:C) } +X.new.quack # => quack! +X.name # => C +C # => uninitialized constant C (NameError) +``` + +Because of that, it is not a good idea to autoload constants on application +initialization. + +In the case above we could implement a dynamic access point: + +```ruby +# app/models/auth_service.rb +class AuthService + if Rails.env.production? + def self.instance + RealAuthService + end + else + def self.instance + MockedAuthService + end + end +end +``` + +and have the application use `AuthService.instance` instead. `AuthService` +would be loaded on demand and be autoload-friendly. + +### `require_dependency` and Initializers + +As we saw before, `require_dependency` loads files in an autoloading-friendly +way. Normally, though, such a call does not make sense in an initializer. + +One could think about doing some [`require_dependency`](#require-dependency) +calls in an initializer to make sure certain constants are loaded upfront, for +example as an attempt to address the [gotcha with STIs](#autoloading-and-sti). + +Problem is, in development mode [autoloaded constants are wiped](#constant-reloading) +if there is any relevant change in the file system. If that happens then +we are in the very same situation the initializer wanted to avoid! + +Calls to `require_dependency` have to be strategically written in autoloaded +spots. + +### When Constants aren't Missed + +#### Relative References + +Let's consider a flight simulator. The application has a default flight model + +```ruby +# app/models/flight_model.rb +class FlightModel +end +``` + +that can be overridden by each airplane, for instance + +```ruby +# app/models/bell_x1/flight_model.rb +module BellX1 + class FlightModel < FlightModel + end +end + +# app/models/bell_x1/aircraft.rb +module BellX1 + class Aircraft + def initialize + @flight_model = FlightModel.new + end + end +end +``` + +The initializer wants to create a `BellX1::FlightModel` and nesting has +`BellX1`, that looks good. But if the default flight model is loaded and the +one for the Bell-X1 is not, the interpreter is able to resolve the top-level +`FlightModel` and autoloading is thus not triggered for `BellX1::FlightModel`. + +That code depends on the execution path. + +These kind of ambiguities can often be resolved using qualified constants: + +```ruby +module BellX1 + class Plane + def flight_model + @flight_model ||= BellX1::FlightModel.new + end + end +end +``` + +Also, `require_dependency` is a solution: + +```ruby +require_dependency 'bell_x1/flight_model' + +module BellX1 + class Plane + def flight_model + @flight_model ||= FlightModel.new + end + end +end +``` + +#### Qualified References + +Given + +```ruby +# app/models/hotel.rb +class Hotel +end + +# app/models/image.rb +class Image +end + +# app/models/hotel/image.rb +class Hotel + class Image < Image + end +end +``` + +the expression `Hotel::Image` is ambiguous, depends on the execution path. + +As [we saw before](#resolution-algorithm-for-qualified-constants), Ruby looks +up the constant in `Hotel` and its ancestors. If `app/models/image.rb` has +been loaded but `app/models/hotel/image.rb` hasn't, Ruby does not find `Image` +in `Hotel`, but it does in `Object`: + +``` +$ bin/rails r 'Image; p Hotel::Image' 2>/dev/null +Image # NOT Hotel::Image! +``` + +The code evaluating `Hotel::Image` needs to make sure +`app/models/hotel/image.rb` has been loaded, possibly with +`require_dependency`. + +In these cases the interpreter issues a warning though: + +``` +warning: toplevel constant Image referenced by Hotel::Image +``` + +This surprising constant resolution can be observed with any qualifying class: + +``` +2.1.5 :001 > String::Array +(irb):1: warning: toplevel constant Array referenced by String::Array + => Array +``` + +WARNING. To find this gotcha the qualifying namespace has to be a class, +`Object` is not an ancestor of modules. + +### Autoloading within Singleton Classes + +Let's suppose we have these class definitions: + +```ruby +# app/models/hotel/services.rb +module Hotel + class Services + end +end + +# app/models/hotel/geo_location.rb +module Hotel + class GeoLocation + class << self + Services + end + end +end +``` + +If `Hotel::Services` is known by the time `app/models/hotel/geo_location.rb` +is being loaded, `Services` is resolved by Ruby because `Hotel` belongs to the +nesting when the singleton class of `Hotel::GeoLocation` is opened. + +But if `Hotel::Services` is not known, Rails is not able to autoload it, the +application raises `NameError`. + +The reason is that autoloading is triggered for the singleton class, which is +anonymous, and as [we saw before](#generic-procedure), Rails only checks the +top-level namespace in that edge case. + +An easy solution to this caveat is to qualify the constant: + +```ruby +module Hotel + class GeoLocation + class << self + Hotel::Services + end + end +end +``` + +### Autoloading in `BasicObject` + +Direct descendants of `BasicObject` do not have `Object` among their ancestors +and cannot resolve top-level constants: + +```ruby +class C < BasicObject + String # NameError: uninitialized constant C::String +end +``` + +When autoloading is involved that plot has a twist. Let's consider: + +```ruby +class C < BasicObject + def user + User # WRONG + end +end +``` + +Since Rails checks the top-level namespace `User` gets autoloaded just fine the +first time the `user` method is invoked. You only get the exception if the +`User` constant is known at that point, in particular in a *second* call to +`user`: + +```ruby +c = C.new +c.user # surprisingly fine, User +c.user # NameError: uninitialized constant C::User +``` + +because it detects a parent namespace already has the constant (see [Qualified +References](#qualified-references).) + +As with pure Ruby, within the body of a direct descendant of `BasicObject` use +always absolute constant paths: + +```ruby +class C < BasicObject + ::String # RIGHT + + def user + ::User # RIGHT + end +end +``` diff --git a/guides/source/constant_autoloading_and_reloading.md b/guides/source/constant_autoloading_and_reloading.md deleted file mode 100644 index c39210383b..0000000000 --- a/guides/source/constant_autoloading_and_reloading.md +++ /dev/null @@ -1,1297 +0,0 @@ -Autoloading and Reloading Constants -=================================== - -This guide documents how constant autoloading and reloading works. - -After reading this guide, you will know: - -* Key aspects of Ruby constants -* What is `autoload_paths` -* How constant autoloading works -* What is `require_dependency` -* How constant reloading works -* Solutions to common autoloading gotchas - --------------------------------------------------------------------------------- - - -Introduction ------------- - -Ruby on Rails allows applications to be written as if their code was preloaded. - -In a normal Ruby program classes need to load their dependencies: - -```ruby -require 'application_controller' -require 'post' - -class PostsController < ApplicationController - def index - @posts = Post.all - end -end -``` - -Our Rubyist instinct quickly sees some redundancy in there: If classes were -defined in files matching their name, couldn't their loading be automated -somehow? We could save scanning the file for dependencies, which is brittle. - -Moreover, `Kernel#require` loads files once, but development is much more smooth -if code gets refreshed when it changes without restarting the server. It would -be nice to be able to use `Kernel#load` in development, and `Kernel#require` in -production. - -Indeed, those features are provided by Ruby on Rails, where we just write - -```ruby -class PostsController < ApplicationController - def index - @posts = Post.all - end -end -``` - -This guide documents how that works. - - -Constants Refresher -------------------- - -While constants are trivial in most programming languages, they are a rich -topic in Ruby. - -It is beyond the scope of this guide to document Ruby constants, but we are -nevertheless going to highlight a few key topics. Truly grasping the following -sections is instrumental to understanding constant autoloading and reloading. - -### Nesting - -Class and module definitions can be nested to create namespaces: - -```ruby -module XML - class SAXParser - # (1) - end -end -``` - -The *nesting* at any given place is the collection of enclosing nested class and -module objects outwards. For example, in the previous example, the nesting at -(1) is - -```ruby -[XML::SAXParser, XML] -``` - -It is important to understand that the nesting is composed of class and module -*objects*, it has nothing to do with the constants used to access them, and is -also unrelated to their names. - -For instance, while this definition is similar to the previous one: - -```ruby -class XML::SAXParser - # (2) -end -``` - -the nesting in (2) is different: - -```ruby -[XML::SAXParser] -``` - -`XML` does not belong to it. - -We can see in this example that the name of a class or module that belongs to a -certain nesting does not necessarily correlate with the namespaces at the spot. - -Even more, they are totally independent, take for instance - -```ruby -module X::Y - module A::B - # (3) - end -end -``` - -The nesting in (3) consists of two module objects: - -```ruby -[A::B, X::Y] -``` - -So, it not only doesn't end in `A`, which does not even belong to the nesting, -but it also contains `X::Y`, which is independent from `A::B`. - -The nesting is an internal stack maintained by the interpreter, and it gets -modified according to these rules: - -* The class object following a `class` keyword gets pushed when its body is -executed, and popped after it. - -* The module object following a `module` keyword gets pushed when its body is -executed, and popped after it. - -* A singleton class opened with `class << object` gets pushed, and popped later. - -* When any of the `*_eval` family of methods is called using a string argument, -the singleton class of the receiver is pushed to the nesting of the eval'ed -code. - -* The nesting at the top-level of code interpreted by `Kernel#load` is empty -unless the `load` call receives a true value as second argument, in which case -a newly created anonymous module is pushed by Ruby. - -It is interesting to observe that blocks do not modify the stack. In particular -the blocks that may be passed to `Class.new` and `Module.new` do not get the -class or module being defined pushed to their nesting. That's one of the -differences between defining classes and modules in one way or another. - -The nesting at any given place can be inspected with `Module.nesting`. - -### Class and Module Definitions are Constant Assignments - -Let's suppose the following snippet creates a class (rather than reopening it): - -```ruby -class C -end -``` - -Ruby creates a constant `C` in `Object` and stores in that constant a class -object. The name of the class instance is "C", a string, named after the -constant. - -That is, - -```ruby -class Project < ActiveRecord::Base -end -``` - -performs a constant assignment equivalent to - -```ruby -Project = Class.new(ActiveRecord::Base) -``` - -including setting the name of the class as a side-effect: - -```ruby -Project.name # => "Project" -``` - -Constant assignment has a special rule to make that happen: if the object -being assigned is an anonymous class or module, Ruby sets the object's name to -the name of the constant. - -INFO. From then on, what happens to the constant and the instance does not -matter. For example, the constant could be deleted, the class object could be -assigned to a different constant, be stored in no constant anymore, etc. Once -the name is set, it doesn't change. - -Similarly, module creation using the `module` keyword as in - -```ruby -module Admin -end -``` - -performs a constant assignment equivalent to - -```ruby -Admin = Module.new -``` - -including setting the name as a side-effect: - -```ruby -Admin.name # => "Admin" -``` - -WARNING. The execution context of a block passed to `Class.new` or `Module.new` -is not entirely equivalent to the one of the body of the definitions using the -`class` and `module` keywords. But both idioms result in the same constant -assignment. - -Thus, when one informally says "the `String` class", that really means: the -class object stored in the constant called "String" in the class object stored -in the `Object` constant. `String` is otherwise an ordinary Ruby constant and -everything related to constants such as resolution algorithms applies to it. - -Likewise, in the controller - -```ruby -class PostsController < ApplicationController - def index - @posts = Post.all - end -end -``` - -`Post` is not syntax for a class. Rather, `Post` is a regular Ruby constant. If -all is good, the constant evaluates to an object that responds to `all`. - -That is why we talk about *constant* autoloading, Rails has the ability to -load constants on the fly. - -### Constants are Stored in Modules - -Constants belong to modules in a very literal sense. Classes and modules have -a constant table; think of it as a hash table. - -Let's analyze an example to really understand what that means. While common -abuses of language like "the `String` class" are convenient, the exposition is -going to be precise here for didactic purposes. - -Let's consider the following module definition: - -```ruby -module Colors - RED = '0xff0000' -end -``` - -First, when the `module` keyword is processed the interpreter creates a new -entry in the constant table of the class object stored in the `Object` constant. -Said entry associates the name "Colors" to a newly created module object. -Furthermore, the interpreter sets the name of the new module object to be the -string "Colors". - -Later, when the body of the module definition is interpreted, a new entry is -created in the constant table of the module object stored in the `Colors` -constant. That entry maps the name "RED" to the string "0xff0000". - -In particular, `Colors::RED` is totally unrelated to any other `RED` constant -that may live in any other class or module object. If there were any, they -would have separate entries in their respective constant tables. - -Pay special attention in the previous paragraphs to the distinction between -class and module objects, constant names, and value objects associated to them -in constant tables. - -### Resolution Algorithms - -#### Resolution Algorithm for Relative Constants - -At any given place in the code, let's define *cref* to be the first element of -the nesting if it is not empty, or `Object` otherwise. - -Without getting too much into the details, the resolution algorithm for relative -constant references goes like this: - -1. If the nesting is not empty the constant is looked up in its elements and in -order. The ancestors of those elements are ignored. - -2. If not found, then the algorithm walks up the ancestor chain of the cref. - -3. If not found, `const_missing` is invoked on the cref. The default -implementation of `const_missing` raises `NameError`, but it can be overridden. - -Rails autoloading **does not emulate this algorithm**, but its starting point is -the name of the constant to be autoloaded, and the cref. See more in [Relative -References](#autoloading-algorithms-relative-references). - -#### Resolution Algorithm for Qualified Constants - -Qualified constants look like this: - -```ruby -Billing::Invoice -``` - -`Billing::Invoice` is composed of two constants: `Billing` is relative and is -resolved using the algorithm of the previous section. - -INFO. Leading colons would make the first segment absolute rather than -relative: `::Billing::Invoice`. That would force `Billing` to be looked up -only as a top-level constant. - -`Invoice` on the other hand is qualified by `Billing` and we are going to see -its resolution next. Let's call *parent* to that qualifying class or module -object, that is, `Billing` in the example above. The algorithm for qualified -constants goes like this: - -1. The constant is looked up in the parent and its ancestors. - -2. If the lookup fails, `const_missing` is invoked in the parent. The default -implementation of `const_missing` raises `NameError`, but it can be overridden. - -As you see, this algorithm is simpler than the one for relative constants. In -particular, the nesting plays no role here, and modules are not special-cased, -if neither they nor their ancestors have the constants, `Object` is **not** -checked. - -Rails autoloading **does not emulate this algorithm**, but its starting point is -the name of the constant to be autoloaded, and the parent. See more in -[Qualified References](#qualified-references). - - -Vocabulary ----------- - -### Parent Namespaces - -Given a string with a constant path we define its *parent namespace* to be the -string that results from removing its rightmost segment. - -For example, the parent namespace of the string "A::B::C" is the string "A::B", -the parent namespace of "A::B" is "A", and the parent namespace of "A" is "". - -The interpretation of a parent namespace when thinking about classes and modules -is tricky though. Let's consider a module M named "A::B": - -* The parent namespace, "A", may not reflect nesting at a given spot. - -* The constant `A` may no longer exist, some code could have removed it from -`Object`. - -* If `A` exists, the class or module that was originally in `A` may not be there -anymore. For example, if after a constant removal there was another constant -assignment there would generally be a different object in there. - -* In such case, it could even happen that the reassigned `A` held a new class or -module called also "A"! - -* In the previous scenarios M would no longer be reachable through `A::B` but -the module object itself could still be alive somewhere and its name would -still be "A::B". - -The idea of a parent namespace is at the core of the autoloading algorithms -and helps explain and understand their motivation intuitively, but as you see -that metaphor leaks easily. Given an edge case to reason about, take always into -account that by "parent namespace" the guide means exactly that specific string -derivation. - -### Loading Mechanism - -Rails autoloads files with `Kernel#load` when `config.cache_classes` is false, -the default in development mode, and with `Kernel#require` otherwise, the -default in production mode. - -`Kernel#load` allows Rails to execute files more than once if [constant -reloading](#constant-reloading) is enabled. - -This guide uses the word "load" freely to mean a given file is interpreted, but -the actual mechanism can be `Kernel#load` or `Kernel#require` depending on that -flag. - - -Autoloading Availability ------------------------- - -Rails is always able to autoload provided its environment is in place. For -example the `runner` command autoloads: - -``` -$ bin/rails runner 'p User.column_names' -["id", "email", "created_at", "updated_at"] -``` - -The console autoloads, the test suite autoloads, and of course the application -autoloads. - -By default, Rails eager loads the application files when it boots in production -mode, so most of the autoloading going on in development does not happen. But -autoloading may still be triggered during eager loading. - -For example, given - -```ruby -class BeachHouse < House -end -``` - -if `House` is still unknown when `app/models/beach_house.rb` is being eager -loaded, Rails autoloads it. - - -autoload_paths --------------- - -As you probably know, when `require` gets a relative file name: - -```ruby -require 'erb' -``` - -Ruby looks for the file in the directories listed in `$LOAD_PATH`. That is, Ruby -iterates over all its directories and for each one of them checks whether they -have a file called "erb.rb", or "erb.so", or "erb.o", or "erb.dll". If it finds -any of them, the interpreter loads it and ends the search. Otherwise, it tries -again in the next directory of the list. If the list gets exhausted, `LoadError` -is raised. - -We are going to cover how constant autoloading works in more detail later, but -the idea is that when a constant like `Post` is hit and missing, if there's a -`post.rb` file for example in `app/models` Rails is going to find it, evaluate -it, and have `Post` defined as a side-effect. - -Alright, Rails has a collection of directories similar to `$LOAD_PATH` in which -to look up `post.rb`. That collection is called `autoload_paths` and by -default it contains: - -* All subdirectories of `app` in the application and engines. For example, - `app/controllers`. They do not need to be the default ones, any custom - directories like `app/workers` belong automatically to `autoload_paths`. - -* Any existing second level directories called `app/*/concerns` in the - application and engines. - -* The directory `test/mailers/previews`. - -Also, this collection is configurable via `config.autoload_paths`. For example, -`lib` was in the list years ago, but no longer is. An application can opt-in -by adding this to `config/application.rb`: - -```ruby -config.autoload_paths += "#{Rails.root}/lib" -``` - -The value of `autoload_paths` can be inspected. In a just generated application -it is (edited): - -``` -$ bin/rails r 'puts ActiveSupport::Dependencies.autoload_paths' -.../app/assets -.../app/controllers -.../app/helpers -.../app/mailers -.../app/models -.../app/controllers/concerns -.../app/models/concerns -.../test/mailers/previews -``` - -INFO. `autoload_paths` is computed and cached during the initialization process. -The application needs to be restarted to reflect any changes in the directory -structure. - - -Autoloading Algorithms ----------------------- - -### Relative References - -A relative constant reference may appear in several places, for example, in - -```ruby -class PostsController < ApplicationController - def index - @posts = Post.all - end -end -``` - -all three constant references are relative. - -#### Constants after the `class` and `module` Keywords - -Ruby performs a lookup for the constant that follows a `class` or `module` -keyword because it needs to know if the class or module is going to be created -or reopened. - -If the constant is not defined at that point it is not considered to be a -missing constant, autoloading is **not** triggered. - -So, in the previous example, if `PostsController` is not defined when the file -is interpreted Rails autoloading is not going to be triggered, Ruby will just -define the controller. - -#### Top-Level Constants - -On the contrary, if `ApplicationController` is unknown, the constant is -considered missing and an autoload is going to be attempted by Rails. - -In order to load `ApplicationController`, Rails iterates over `autoload_paths`. -First checks if `app/assets/application_controller.rb` exists. If it does not, -which is normally the case, it continues and finds -`app/controllers/application_controller.rb`. - -If the file defines the constant `ApplicationController` all is fine, otherwise -`LoadError` is raised: - -``` -unable to autoload constant ApplicationController, expected - to define it (LoadError) -``` - -INFO. Rails does not require the value of autoloaded constants to be a class or -module object. For example, if the file `app/models/max_clients.rb` defines -`MAX_CLIENTS = 100` autoloading `MAX_CLIENTS` works just fine. - -#### Namespaces - -Autoloading `ApplicationController` looks directly under the directories of -`autoload_paths` because the nesting in that spot is empty. The situation of -`Post` is different, the nesting in that line is `[PostsController]` and support -for namespaces comes into play. - -The basic idea is that given - -```ruby -module Admin - class BaseController < ApplicationController - @@all_roles = Role.all - end -end -``` - -to autoload `Role` we are going to check if it is defined in the current or -parent namespaces, one at a time. So, conceptually we want to try to autoload -any of - -``` -Admin::BaseController::Role -Admin::Role -Role -``` - -in that order. That's the idea. To do so, Rails looks in `autoload_paths` -respectively for file names like these: - -``` -admin/base_controller/role.rb -admin/role.rb -role.rb -``` - -modulus some additional directory lookups we are going to cover soon. - -INFO. `'Constant::Name'.underscore` gives the relative path without extension of -the file name where `Constant::Name` is expected to be defined. - -Let's see how Rails autoloads the `Post` constant in the `PostsController` -above assuming the application has a `Post` model defined in -`app/models/post.rb`. - -First it checks for `posts_controller/post.rb` in `autoload_paths`: - -``` -app/assets/posts_controller/post.rb -app/controllers/posts_controller/post.rb -app/helpers/posts_controller/post.rb -... -test/mailers/previews/posts_controller/post.rb -``` - -Since the lookup is exhausted without success, a similar search for a directory -is performed, we are going to see why in the [next section](#automatic-modules): - -``` -app/assets/posts_controller/post -app/controllers/posts_controller/post -app/helpers/posts_controller/post -... -test/mailers/previews/posts_controller/post -``` - -If all those attempts fail, then Rails starts the lookup again in the parent -namespace. In this case only the top-level remains: - -``` -app/assets/post.rb -app/controllers/post.rb -app/helpers/post.rb -app/mailers/post.rb -app/models/post.rb -``` - -A matching file is found in `app/models/post.rb`. The lookup stops there and the -file is loaded. If the file actually defines `Post` all is fine, otherwise -`LoadError` is raised. - -### Qualified References - -When a qualified constant is missing Rails does not look for it in the parent -namespaces. But there is a caveat: When a constant is missing, Rails is -unable to tell if the trigger was a relative reference or a qualified one. - -For example, consider - -```ruby -module Admin - User -end -``` - -and - -```ruby -Admin::User -``` - -If `User` is missing, in either case all Rails knows is that a constant called -"User" was missing in a module called "Admin". - -If there is a top-level `User` Ruby would resolve it in the former example, but -wouldn't in the latter. In general, Rails does not emulate the Ruby constant -resolution algorithms, but in this case it tries using the following heuristic: - -> If none of the parent namespaces of the class or module has the missing -> constant then Rails assumes the reference is relative. Otherwise qualified. - -For example, if this code triggers autoloading - -```ruby -Admin::User -``` - -and the `User` constant is already present in `Object`, it is not possible that -the situation is - -```ruby -module Admin - User -end -``` - -because otherwise Ruby would have resolved `User` and no autoloading would have -been triggered in the first place. Thus, Rails assumes a qualified reference and -considers the file `admin/user.rb` and directory `admin/user` to be the only -valid options. - -In practice, this works quite well as long as the nesting matches all parent -namespaces respectively and the constants that make the rule apply are known at -that time. - -However, autoloading happens on demand. If by chance the top-level `User` was -not yet loaded, then Rails assumes a relative reference by contract. - -Naming conflicts of this kind are rare in practice, but if one occurs, -`require_dependency` provides a solution by ensuring that the constant needed -to trigger the heuristic is defined in the conflicting place. - -### Automatic Modules - -When a module acts as a namespace, Rails does not require the application to -defines a file for it, a directory matching the namespace is enough. - -Suppose an application has a back office whose controllers are stored in -`app/controllers/admin`. If the `Admin` module is not yet loaded when -`Admin::UsersController` is hit, Rails needs first to autoload the constant -`Admin`. - -If `autoload_paths` has a file called `admin.rb` Rails is going to load that -one, but if there's no such file and a directory called `admin` is found, Rails -creates an empty module and assigns it to the `Admin` constant on the fly. - -### Generic Procedure - -Relative references are reported to be missing in the cref where they were hit, -and qualified references are reported to be missing in their parent. (See -[Resolution Algorithm for Relative -Constants](#resolution-algorithm-for-relative-constants) at the beginning of -this guide for the definition of *cref*, and [Resolution Algorithm for Qualified -Constants](#resolution-algorithm-for-qualified-constants) for the definition of -*parent*.) - -The procedure to autoload constant `C` in an arbitrary situation is as follows: - -``` -if the class or module in which C is missing is Object - let ns = '' -else - let M = the class or module in which C is missing - - if M is anonymous - let ns = '' - else - let ns = M.name - end -end - -loop do - # Look for a regular file. - for dir in autoload_paths - if the file "#{dir}/#{ns.underscore}/c.rb" exists - load/require "#{dir}/#{ns.underscore}/c.rb" - - if C is now defined - return - else - raise LoadError - end - end - end - - # Look for an automatic module. - for dir in autoload_paths - if the directory "#{dir}/#{ns.underscore}/c" exists - if ns is an empty string - let C = Module.new in Object and return - else - let C = Module.new in ns.constantize and return - end - end - end - - if ns is empty - # We reached the top-level without finding the constant. - raise NameError - else - if C exists in any of the parent namespaces - # Qualified constants heuristic. - raise NameError - else - # Try again in the parent namespace. - let ns = the parent namespace of ns and retry - end - end -end -``` - - -require_dependency ------------------- - -Constant autoloading is triggered on demand and therefore code that uses a -certain constant may have it already defined or may trigger an autoload. That -depends on the execution path and it may vary between runs. - -There are times, however, in which you want to make sure a certain constant is -known when the execution reaches some code. `require_dependency` provides a way -to load a file using the current [loading mechanism](#loading-mechanism), and -keeping track of constants defined in that file as if they were autoloaded to -have them reloaded as needed. - -`require_dependency` is rarely needed, but see a couple of use-cases in -[Autoloading and STI](#autoloading-and-sti) and [When Constants aren't -Triggered](#when-constants-aren-t-missed). - -WARNING. Unlike autoloading, `require_dependency` does not expect the file to -define any particular constant. Exploiting this behavior would be a bad practice -though, file and constant paths should match. - - -Constant Reloading ------------------- - -When `config.cache_classes` is false Rails is able to reload autoloaded -constants. - -For example, in you're in a console session and edit some file behind the -scenes, the code can be reloaded with the `reload!` command: - -``` -> reload! -``` - -When the application runs, code is reloaded when something relevant to this -logic changes. In order to do that, Rails monitors a number of things: - -* `config/routes.rb`. - -* Locales. - -* Ruby files under `autoload_paths`. - -* `db/schema.rb` and `db/structure.sql`. - -If anything in there changes, there is a middleware that detects it and reloads -the code. - -Autoloading keeps track of autoloaded constants. Reloading is implemented by -removing them all from their respective classes and modules using -`Module#remove_const`. That way, when the code goes on, those constants are -going to be unknown again, and files reloaded on demand. - -INFO. This is an all-or-nothing operation, Rails does not attempt to reload only -what changed since dependencies between classes makes that really tricky. -Instead, everything is wiped. - - -Module#autoload isn't Involved ------------------------------- - -`Module#autoload` provides a lazy way to load constants that is fully integrated -with the Ruby constant lookup algorithms, dynamic constant API, etc. It is quite -transparent. - -Rails internals make extensive use of it to defer as much work as possible from -the boot process. But constant autoloading in Rails is **not** implemented with -`Module#autoload`. - -One possible implementation based on `Module#autoload` would be to walk the -application tree and issue `autoload` calls that map existing file names to -their conventional constant name. - -There are a number of reasons that prevent Rails from using that implementation. - -For example, `Module#autoload` is only capable of loading files using `require`, -so reloading would not be possible. Not only that, it uses an internal `require` -which is not `Kernel#require`. - -Then, it provides no way to remove declarations in case a file is deleted. If a -constant gets removed with `Module#remove_const` its `autoload` is not triggered -again. Also, it doesn't support qualified names, so files with namespaces should -be interpreted during the walk tree to install their own `autoload` calls, but -those files could have constant references not yet configured. - -An implementation based on `Module#autoload` would be awesome but, as you see, -at least as of today it is not possible. Constant autoloading in Rails is -implemented with `Module#const_missing`, and that's why it has its own contract, -documented in this guide. - - -Common Gotchas --------------- - -### Nesting and Qualified Constants - -Let's consider - -```ruby -module Admin - class UsersController < ApplicationController - def index - @users = User.all - end - end -end -``` - -and - -```ruby -class Admin::UsersController < ApplicationController - def index - @users = User.all - end -end -``` - -To resolve `User` Ruby checks `Admin` in the former case, but it does not in -the latter because it does not belong to the nesting. (See [Nesting](#nesting) -and [Resolution Algorithms](#resolution-algorithms).) - -Unfortunately Rails autoloading does not know the nesting in the spot where the -constant was missing and so it is not able to act as Ruby would. In particular, -`Admin::User` will get autoloaded in either case. - -Albeit qualified constants with `class` and `module` keywords may technically -work with autoloading in some cases, it is preferable to use relative constants -instead: - -```ruby -module Admin - class UsersController < ApplicationController - def index - @users = User.all - end - end -end -``` - -### Autoloading and STI - -Single Table Inheritance (STI) is a feature of Active Record that enables -storing a hierarchy of models in one single table. The API of such models is -aware of the hierarchy and encapsulates some common needs. For example, given -these classes: - -```ruby -# app/models/polygon.rb -class Polygon < ActiveRecord::Base -end - -# app/models/triangle.rb -class Triangle < Polygon -end - -# app/models/rectangle.rb -class Rectangle < Polygon -end -``` - -`Triangle.create` creates a row that represents a triangle, and -`Rectangle.create` creates a row that represents a rectangle. If `id` is the -ID of an existing record, `Polygon.find(id)` returns an object of the correct -type. - -Methods that operate on collections are also aware of the hierarchy. For -example, `Polygon.all` returns all the records of the table, because all -rectangles and triangles are polygons. Active Record takes care of returning -instances of their corresponding class in the result set. - -Types are autoloaded as needed. For example, if `Polygon.first` is a rectangle -and `Rectangle` has not yet been loaded, Active Record autoloads it and the -record is correctly instantiated. - -All good, but if instead of performing queries based on the root class we need -to work on some subclass, things get interesting. - -While working with `Polygon` you do not need to be aware of all its descendants, -because anything in the table is by definition a polygon, but when working with -subclasses Active Record needs to be able to enumerate the types it is looking -for. Let’s see an example. - -`Rectangle.all` only loads rectangles by adding a type constraint to the query: - -```sql -SELECT "polygons".* FROM "polygons" -WHERE "polygons"."type" IN ("Rectangle") -``` - -Let’s introduce now a subclass of `Rectangle`: - -```ruby -# app/models/square.rb -class Square < Rectangle -end -``` - -`Rectangle.all` should now return rectangles **and** squares: - -```sql -SELECT "polygons".* FROM "polygons" -WHERE "polygons"."type" IN ("Rectangle", "Square") -``` - -But there’s a caveat here: How does Active Record know that the class `Square` -exists at all? - -Even if the file `app/models/square.rb` exists and defines the `Square` class, -if no code yet used that class, `Rectangle.all` issues the query - -```sql -SELECT "polygons".* FROM "polygons" -WHERE "polygons"."type" IN ("Rectangle") -``` - -That is not a bug, the query includes all *known* descendants of `Rectangle`. - -A way to ensure this works correctly regardless of the order of execution is to -load the leaves of the tree by hand at the bottom of the file that defines the -root class: - -```ruby -# app/models/polygon.rb -class Polygon < ActiveRecord::Base -end -require_dependency ‘square’ -``` - -Only the leaves that are **at least grandchildren** need to be loaded this -way. Direct subclasses do not need to be preloaded. If the hierarchy is -deeper, intermediate classes will be autoloaded recursively from the bottom -because their constant will appear in the class definitions as superclass. - -### Autoloading and `require` - -Files defining constants to be autoloaded should never be `require`d: - -```ruby -require 'user' # DO NOT DO THIS - -class UsersController < ApplicationController - ... -end -``` - -There are two possible gotchas here in development mode: - -1. If `User` is autoloaded before reaching the `require`, `app/models/user.rb` -runs again because `load` does not update `$LOADED_FEATURES`. - -2. If the `require` runs first Rails does not mark `User` as an autoloaded -constant and changes to `app/models/user.rb` aren't reloaded. - -Just follow the flow and use constant autoloading always, never mix -autoloading and `require`. As a last resort, if some file absolutely needs to -load a certain file use `require_dependency` to play nice with constant -autoloading. This option is rarely needed in practice, though. - -Of course, using `require` in autoloaded files to load ordinary 3rd party -libraries is fine, and Rails is able to distinguish their constants, they are -not marked as autoloaded. - -### Autoloading and Initializers - -Consider this assignment in `config/initializers/set_auth_service.rb`: - -```ruby -AUTH_SERVICE = if Rails.env.production? - RealAuthService -else - MockedAuthService -end -``` - -The purpose of this setup would be that the application uses the class that -corresponds to the environment via `AUTH_SERVICE`. In development mode -`MockedAuthService` gets autoloaded when the initializer runs. Let’s suppose -we do some requests, change its implementation, and hit the application again. -To our surprise the changes are not reflected. Why? - -As [we saw earlier](#constant-reloading), Rails removes autoloaded constants, -but `AUTH_SERVICE` stores the original class object. Stale, non-reachable -using the original constant, but perfectly functional. - -The following code summarizes the situation: - -```ruby -class C - def quack - 'quack!' - end -end - -X = C -Object.instance_eval { remove_const(:C) } -X.new.quack # => quack! -X.name # => C -C # => uninitialized constant C (NameError) -``` - -Because of that, it is not a good idea to autoload constants on application -initialization. - -In the case above we could implement a dynamic access point: - -```ruby -# app/models/auth_service.rb -class AuthService - if Rails.env.production? - def self.instance - RealAuthService - end - else - def self.instance - MockedAuthService - end - end -end -``` - -and have the application use `AuthService.instance` instead. `AuthService` -would be loaded on demand and be autoload-friendly. - -### `require_dependency` and Initializers - -As we saw before, `require_dependency` loads files in an autoloading-friendly -way. Normally, though, such a call does not make sense in an initializer. - -One could think about doing some [`require_dependency`](#require-dependency) -calls in an initializer to make sure certain constants are loaded upfront, for -example as an attempt to address the [gotcha with STIs](#autoloading-and-sti). - -Problem is, in development mode [autoloaded constants are wiped](#constant-reloading) -if there is any relevant change in the file system. If that happens then -we are in the very same situation the initializer wanted to avoid! - -Calls to `require_dependency` have to be strategically written in autoloaded -spots. - -### When Constants aren't Missed - -#### Relative References - -Let's consider a flight simulator. The application has a default flight model - -```ruby -# app/models/flight_model.rb -class FlightModel -end -``` - -that can be overridden by each airplane, for instance - -```ruby -# app/models/bell_x1/flight_model.rb -module BellX1 - class FlightModel < FlightModel - end -end - -# app/models/bell_x1/aircraft.rb -module BellX1 - class Aircraft - def initialize - @flight_model = FlightModel.new - end - end -end -``` - -The initializer wants to create a `BellX1::FlightModel` and nesting has -`BellX1`, that looks good. But if the default flight model is loaded and the -one for the Bell-X1 is not, the interpreter is able to resolve the top-level -`FlightModel` and autoloading is thus not triggered for `BellX1::FlightModel`. - -That code depends on the execution path. - -These kind of ambiguities can often be resolved using qualified constants: - -```ruby -module BellX1 - class Plane - def flight_model - @flight_model ||= BellX1::FlightModel.new - end - end -end -``` - -Also, `require_dependency` is a solution: - -```ruby -require_dependency 'bell_x1/flight_model' - -module BellX1 - class Plane - def flight_model - @flight_model ||= FlightModel.new - end - end -end -``` - -#### Qualified References - -Given - -```ruby -# app/models/hotel.rb -class Hotel -end - -# app/models/image.rb -class Image -end - -# app/models/hotel/image.rb -class Hotel - class Image < Image - end -end -``` - -the expression `Hotel::Image` is ambiguous, depends on the execution path. - -As [we saw before](#resolution-algorithm-for-qualified-constants), Ruby looks -up the constant in `Hotel` and its ancestors. If `app/models/image.rb` has -been loaded but `app/models/hotel/image.rb` hasn't, Ruby does not find `Image` -in `Hotel`, but it does in `Object`: - -``` -$ bin/rails r 'Image; p Hotel::Image' 2>/dev/null -Image # NOT Hotel::Image! -``` - -The code evaluating `Hotel::Image` needs to make sure -`app/models/hotel/image.rb` has been loaded, possibly with -`require_dependency`. - -In these cases the interpreter issues a warning though: - -``` -warning: toplevel constant Image referenced by Hotel::Image -``` - -This surprising constant resolution can be observed with any qualifying class: - -``` -2.1.5 :001 > String::Array -(irb):1: warning: toplevel constant Array referenced by String::Array - => Array -``` - -WARNING. To find this gotcha the qualifying namespace has to be a class, -`Object` is not an ancestor of modules. - -### Autoloading within Singleton Classes - -Let's suppose we have these class definitions: - -```ruby -# app/models/hotel/services.rb -module Hotel - class Services - end -end - -# app/models/hotel/geo_location.rb -module Hotel - class GeoLocation - class << self - Services - end - end -end -``` - -If `Hotel::Services` is known by the time `app/models/hotel/geo_location.rb` -is being loaded, `Services` is resolved by Ruby because `Hotel` belongs to the -nesting when the singleton class of `Hotel::GeoLocation` is opened. - -But if `Hotel::Services` is not known, Rails is not able to autoload it, the -application raises `NameError`. - -The reason is that autoloading is triggered for the singleton class, which is -anonymous, and as [we saw before](#generic-procedure), Rails only checks the -top-level namespace in that edge case. - -An easy solution to this caveat is to qualify the constant: - -```ruby -module Hotel - class GeoLocation - class << self - Hotel::Services - end - end -end -``` - -### Autoloading in `BasicObject` - -Direct descendants of `BasicObject` do not have `Object` among their ancestors -and cannot resolve top-level constants: - -```ruby -class C < BasicObject - String # NameError: uninitialized constant C::String -end -``` - -When autoloading is involved that plot has a twist. Let's consider: - -```ruby -class C < BasicObject - def user - User # WRONG - end -end -``` - -Since Rails checks the top-level namespace `User` gets autoloaded just fine the -first time the `user` method is invoked. You only get the exception if the -`User` constant is known at that point, in particular in a *second* call to -`user`: - -```ruby -c = C.new -c.user # surprisingly fine, User -c.user # NameError: uninitialized constant C::User -``` - -because it detects a parent namespace already has the constant (see [Qualified -References](#qualified-references).) - -As with pure Ruby, within the body of a direct descendant of `BasicObject` use -always absolute constant paths: - -```ruby -class C < BasicObject - ::String # RIGHT - - def user - ::User # RIGHT - end -end -``` -- cgit v1.2.3