Renaming the autoloading guide [ci skip]

The term 'constant autoloading' can be mistakenly
construed as 'constantly autoloading'.

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+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
+<full path to application_controller.rb> 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
+```