diff options
Diffstat (limited to 'guides/source/association_basics.md')
| -rw-r--r-- | guides/source/association_basics.md | 640 |
1 files changed, 327 insertions, 313 deletions
diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md index 74cd9bdc7b..09ab64837a 100644 --- a/guides/source/association_basics.md +++ b/guides/source/association_basics.md @@ -16,54 +16,54 @@ After reading this guide, you will know: Why Associations? ----------------- -Why do we need associations between models? Because they make common operations simpler and easier in your code. For example, consider a simple Rails application that includes a model for customers and a model for orders. Each customer can have many orders. Without associations, the model declarations would look like this: +In Rails, an _association_ is a connection between two Active Record models. Why do we need associations between models? Because they make common operations simpler and easier in your code. For example, consider a simple Rails application that includes a model for authors and a model for books. Each author can have many books. Without associations, the model declarations would look like this: ```ruby -class Customer < ActiveRecord::Base +class Author < ApplicationRecord end -class Order < ActiveRecord::Base +class Book < ApplicationRecord end ``` -Now, suppose we wanted to add a new order for an existing customer. We'd need to do something like this: +Now, suppose we wanted to add a new book for an existing author. We'd need to do something like this: ```ruby -@order = Order.create(order_date: Time.now, customer_id: @customer.id) +@book = Book.create(published_at: Time.now, author_id: @author.id) ``` -Or consider deleting a customer, and ensuring that all of its orders get deleted as well: +Or consider deleting an author, and ensuring that all of its books get deleted as well: ```ruby -@orders = Order.where(customer_id: @customer.id) -@orders.each do |order| - order.destroy +@books = Book.where(author_id: @author.id) +@books.each do |book| + book.destroy end -@customer.destroy +@author.destroy ``` -With Active Record associations, we can streamline these - and other - operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up customers and orders: +With Active Record associations, we can streamline these - and other - operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up authors and books: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, dependent: :destroy +class Author < ApplicationRecord + has_many :books, dependent: :destroy end -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author end ``` -With this change, creating a new order for a particular customer is easier: +With this change, creating a new book for a particular author is easier: ```ruby -@order = @customer.orders.create(order_date: Time.now) +@book = @author.books.create(published_at: Time.now) ``` -Deleting a customer and all of its orders is *much* easier: +Deleting an author and all of its books is *much* easier: ```ruby -@customer.destroy +@author.destroy ``` To learn more about the different types of associations, read the next section of this guide. That's followed by some tips and tricks for working with associations, and then by a complete reference to the methods and options for associations in Rails. @@ -71,7 +71,7 @@ To learn more about the different types of associations, read the next section o The Types of Associations ------------------------- -In Rails, an _association_ is a connection between two Active Record models. Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model `belongs_to` another, you instruct Rails to maintain Primary Key-Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model. Rails supports six types of associations: +Rails supports six types of associations: * `belongs_to` * `has_one` @@ -80,35 +80,37 @@ In Rails, an _association_ is a connection between two Active Record models. Ass * `has_one :through` * `has_and_belongs_to_many` +Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model `belongs_to` another, you instruct Rails to maintain [Primary Key](https://en.wikipedia.org/wiki/Unique_key)-[Foreign Key](https://en.wikipedia.org/wiki/Foreign_key) information between instances of the two models, and you also get a number of utility methods added to your model. + In the remainder of this guide, you'll learn how to declare and use the various forms of associations. But first, a quick introduction to the situations where each association type is appropriate. ### The `belongs_to` Association -A `belongs_to` association sets up a one-to-one connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model. For example, if your application includes customers and orders, and each order can be assigned to exactly one customer, you'd declare the order model this way: +A `belongs_to` association sets up a one-to-one connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model. For example, if your application includes authors and books, and each book can be assigned to exactly one author, you'd declare the book model this way: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author end ```  -NOTE: `belongs_to` associations _must_ use the singular term. If you used the pluralized form in the above example for the `customer` association in the `Order` model, you would be told that there was an "uninitialized constant Order::Customers". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too. +NOTE: `belongs_to` associations _must_ use the singular term. If you used the pluralized form in the above example for the `author` association in the `Book` model, you would be told that there was an "uninitialized constant Book::Authors". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too. The corresponding migration might look like this: ```ruby -class CreateOrders < ActiveRecord::Migration +class CreateBooks < ActiveRecord::Migration[5.0] def change - create_table :customers do |t| + create_table :authors do |t| t.string :name t.timestamps null: false end - create_table :orders do |t| - t.belongs_to :customer, index: true - t.datetime :order_date + create_table :books do |t| + t.belongs_to :author, index: true + t.datetime :published_at t.timestamps null: false end end @@ -120,7 +122,7 @@ end A `has_one` association also sets up a one-to-one connection with another model, but with somewhat different semantics (and consequences). This association indicates that each instance of a model contains or possesses one instance of another model. For example, if each supplier in your application has only one account, you'd declare the supplier model like this: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account end ``` @@ -130,7 +132,7 @@ end The corresponding migration might look like this: ```ruby -class CreateSuppliers < ActiveRecord::Migration +class CreateSuppliers < ActiveRecord::Migration[5.0] def change create_table :suppliers do |t| t.string :name @@ -159,11 +161,11 @@ end ### The `has_many` Association -A `has_many` association indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a `belongs_to` association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing customers and orders, the customer model could be declared like this: +A `has_many` association indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a `belongs_to` association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing authors and books, the author model could be declared like this: ```ruby -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` @@ -174,16 +176,16 @@ NOTE: The name of the other model is pluralized when declaring a `has_many` asso The corresponding migration might look like this: ```ruby -class CreateCustomers < ActiveRecord::Migration +class CreateAuthors < ActiveRecord::Migration[5.0] def change - create_table :customers do |t| + create_table :authors do |t| t.string :name t.timestamps null: false end - create_table :orders do |t| - t.belongs_to :customer, index: true - t.datetime :order_date + create_table :books do |t| + t.belongs_to :author, index: true + t.datetime :published_at t.timestamps null: false end end @@ -195,17 +197,17 @@ end A `has_many :through` association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding _through_ a third model. For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this: ```ruby -class Physician < ActiveRecord::Base +class Physician < ApplicationRecord has_many :appointments has_many :patients, through: :appointments end -class Appointment < ActiveRecord::Base +class Appointment < ApplicationRecord belongs_to :physician belongs_to :patient end -class Patient < ActiveRecord::Base +class Patient < ApplicationRecord has_many :appointments has_many :physicians, through: :appointments end @@ -216,7 +218,7 @@ end The corresponding migration might look like this: ```ruby -class CreateAppointments < ActiveRecord::Migration +class CreateAppointments < ActiveRecord::Migration[5.0] def change create_table :physicians do |t| t.string :name @@ -238,30 +240,32 @@ class CreateAppointments < ActiveRecord::Migration end ``` -The collection of join models can be managed via the API. For example, if you assign +The collection of join models can be managed via the [`has_many` association methods](#has-many-association-reference). +For example, if you assign: ```ruby physician.patients = patients ``` -new join models are created for newly associated objects, and if some are gone their rows are deleted. +Then new join models are automatically created for the newly associated objects. +If some that existed previously are now missing, then their join rows are automatically deleted. WARNING: Automatic deletion of join models is direct, no destroy callbacks are triggered. The `has_many :through` association is also useful for setting up "shortcuts" through nested `has_many` associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way: ```ruby -class Document < ActiveRecord::Base +class Document < ApplicationRecord has_many :sections has_many :paragraphs, through: :sections end -class Section < ActiveRecord::Base +class Section < ApplicationRecord belongs_to :document has_many :paragraphs end -class Paragraph < ActiveRecord::Base +class Paragraph < ApplicationRecord belongs_to :section end ``` @@ -280,17 +284,17 @@ For example, if each supplier has one account, and each account is associated wi supplier model could look like this: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account has_one :account_history, through: :account end -class Account < ActiveRecord::Base +class Account < ApplicationRecord belongs_to :supplier has_one :account_history end -class AccountHistory < ActiveRecord::Base +class AccountHistory < ApplicationRecord belongs_to :account end ``` @@ -300,7 +304,7 @@ end The corresponding migration might look like this: ```ruby -class CreateAccountHistories < ActiveRecord::Migration +class CreateAccountHistories < ActiveRecord::Migration[5.0] def change create_table :suppliers do |t| t.string :name @@ -327,11 +331,11 @@ end A `has_and_belongs_to_many` association creates a direct many-to-many connection with another model, with no intervening model. For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way: ```ruby -class Assembly < ActiveRecord::Base +class Assembly < ApplicationRecord has_and_belongs_to_many :parts end -class Part < ActiveRecord::Base +class Part < ApplicationRecord has_and_belongs_to_many :assemblies end ``` @@ -341,7 +345,7 @@ end The corresponding migration might look like this: ```ruby -class CreateAssembliesAndParts < ActiveRecord::Migration +class CreateAssembliesAndParts < ActiveRecord::Migration[5.0] def change create_table :assemblies do |t| t.string :name @@ -368,11 +372,11 @@ If you want to set up a one-to-one relationship between two models, you'll need The distinction is in where you place the foreign key (it goes on the table for the class declaring the `belongs_to` association), but you should give some thought to the actual meaning of the data as well. The `has_one` relationship says that one of something is yours - that is, that something points back to you. For example, it makes more sense to say that a supplier owns an account than that an account owns a supplier. This suggests that the correct relationships are like this: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account end -class Account < ActiveRecord::Base +class Account < ApplicationRecord belongs_to :supplier end ``` @@ -380,7 +384,7 @@ end The corresponding migration might look like this: ```ruby -class CreateSuppliers < ActiveRecord::Migration +class CreateSuppliers < ActiveRecord::Migration[5.0] def change create_table :suppliers do |t| t.string :name @@ -405,11 +409,11 @@ NOTE: Using `t.integer :supplier_id` makes the foreign key naming obvious and ex Rails offers two different ways to declare a many-to-many relationship between models. The simpler way is to use `has_and_belongs_to_many`, which allows you to make the association directly: ```ruby -class Assembly < ActiveRecord::Base +class Assembly < ApplicationRecord has_and_belongs_to_many :parts end -class Part < ActiveRecord::Base +class Part < ApplicationRecord has_and_belongs_to_many :assemblies end ``` @@ -417,17 +421,17 @@ end The second way to declare a many-to-many relationship is to use `has_many :through`. This makes the association indirectly, through a join model: ```ruby -class Assembly < ActiveRecord::Base +class Assembly < ApplicationRecord has_many :manifests has_many :parts, through: :manifests end -class Manifest < ActiveRecord::Base +class Manifest < ApplicationRecord belongs_to :assembly belongs_to :part end -class Part < ActiveRecord::Base +class Part < ApplicationRecord has_many :manifests has_many :assemblies, through: :manifests end @@ -442,15 +446,15 @@ You should use `has_many :through` if you need validations, callbacks or extra a A slightly more advanced twist on associations is the _polymorphic association_. With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model. Here's how this could be declared: ```ruby -class Picture < ActiveRecord::Base +class Picture < ApplicationRecord belongs_to :imageable, polymorphic: true end -class Employee < ActiveRecord::Base +class Employee < ApplicationRecord has_many :pictures, as: :imageable end -class Product < ActiveRecord::Base +class Product < ApplicationRecord has_many :pictures, as: :imageable end ``` @@ -462,7 +466,7 @@ Similarly, you can retrieve `@product.pictures`. If you have an instance of the `Picture` model, you can get to its parent via `@picture.imageable`. To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface: ```ruby -class CreatePictures < ActiveRecord::Migration +class CreatePictures < ActiveRecord::Migration[5.0] def change create_table :pictures do |t| t.string :name @@ -479,7 +483,7 @@ end This migration can be simplified by using the `t.references` form: ```ruby -class CreatePictures < ActiveRecord::Migration +class CreatePictures < ActiveRecord::Migration[5.0] def change create_table :pictures do |t| t.string :name @@ -497,7 +501,7 @@ end In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations: ```ruby -class Employee < ActiveRecord::Base +class Employee < ApplicationRecord has_many :subordinates, class_name: "Employee", foreign_key: "manager_id" @@ -510,7 +514,7 @@ With this setup, you can retrieve `@employee.subordinates` and `@employee.manage In your migrations/schema, you will add a references column to the model itself. ```ruby -class CreateEmployees < ActiveRecord::Migration +class CreateEmployees < ActiveRecord::Migration[5.0] def change create_table :employees do |t| t.references :manager, index: true @@ -536,17 +540,17 @@ Here are a few things you should know to make efficient use of Active Record ass All of the association methods are built around caching, which keeps the result of the most recent query available for further operations. The cache is even shared across methods. For example: ```ruby -customer.orders # retrieves orders from the database -customer.orders.size # uses the cached copy of orders -customer.orders.empty? # uses the cached copy of orders +author.books # retrieves books from the database +author.books.size # uses the cached copy of books +author.books.empty? # uses the cached copy of books ``` But what if you want to reload the cache, because data might have been changed by some other part of the application? Just pass `true` to the association call: ```ruby -customer.orders # retrieves orders from the database -customer.orders.size # uses the cached copy of orders -customer.orders(true).empty? # discards the cached copy of orders +author.books # retrieves books from the database +author.books.size # uses the cached copy of books +author.books(true).empty? # discards the cached copy of books # and goes back to the database ``` @@ -563,23 +567,23 @@ Associations are extremely useful, but they are not magic. You are responsible f When you declare a `belongs_to` association, you need to create foreign keys as appropriate. For example, consider this model: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author end ``` -This declaration needs to be backed up by the proper foreign key declaration on the orders table: +This declaration needs to be backed up by the proper foreign key declaration on the books table: ```ruby -class CreateOrders < ActiveRecord::Migration +class CreateBooks < ActiveRecord::Migration[5.0] def change - create_table :orders do |t| - t.datetime :order_date - t.string :order_number - t.integer :customer_id + create_table :books do |t| + t.datetime :published_at + t.string :book_number + t.integer :author_id end - add_index :orders, :customer_id + add_index :books, :author_id end end ``` @@ -588,18 +592,18 @@ If you create an association some time after you build the underlying model, you #### Creating Join Tables for `has_and_belongs_to_many` Associations -If you create a `has_and_belongs_to_many` association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the `:join_table` option, Active Record creates the name by using the lexical order of the class names. So a join between customer and order models will give the default join table name of "customers_orders" because "c" outranks "o" in lexical ordering. +If you create a `has_and_belongs_to_many` association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the `:join_table` option, Active Record creates the name by using the lexical book of the class names. So a join between author and book models will give the default join table name of "authors_books" because "a" outranks "b" in lexical ordering. WARNING: The precedence between model names is calculated using the `<=>` operator for `String`. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '\_' is lexicographically _less_ than 's' in common encodings). Whatever the name, you must manually generate the join table with an appropriate migration. For example, consider these associations: ```ruby -class Assembly < ActiveRecord::Base +class Assembly < ApplicationRecord has_and_belongs_to_many :parts end -class Part < ActiveRecord::Base +class Part < ApplicationRecord has_and_belongs_to_many :assemblies end ``` @@ -607,7 +611,7 @@ end These need to be backed up by a migration to create the `assemblies_parts` table. This table should be created without a primary key: ```ruby -class CreateAssembliesPartsJoinTable < ActiveRecord::Migration +class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[5.0] def change create_table :assemblies_parts, id: false do |t| t.integer :assembly_id @@ -625,7 +629,7 @@ We pass `id: false` to `create_table` because that table does not represent a mo You can also use the method `create_join_table` ```ruby -class CreateAssembliesPartsJoinTable < ActiveRecord::Migration +class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[5.0] def change create_join_table :assemblies, :parts do |t| t.index :assembly_id @@ -642,11 +646,11 @@ By default, associations look for objects only within the current module's scope ```ruby module MyApplication module Business - class Supplier < ActiveRecord::Base + class Supplier < ApplicationRecord has_one :account end - class Account < ActiveRecord::Base + class Account < ApplicationRecord belongs_to :supplier end end @@ -658,13 +662,13 @@ This will work fine, because both the `Supplier` and the `Account` class are def ```ruby module MyApplication module Business - class Supplier < ActiveRecord::Base + class Supplier < ApplicationRecord has_one :account end end module Billing - class Account < ActiveRecord::Base + class Account < ApplicationRecord belongs_to :supplier end end @@ -676,14 +680,14 @@ To associate a model with a model in a different namespace, you must specify the ```ruby module MyApplication module Business - class Supplier < ActiveRecord::Base + class Supplier < ApplicationRecord has_one :account, class_name: "MyApplication::Billing::Account" end end module Billing - class Account < ActiveRecord::Base + class Account < ApplicationRecord belongs_to :supplier, class_name: "MyApplication::Business::Supplier" end @@ -696,45 +700,45 @@ end It's normal for associations to work in two directions, requiring declaration on two different models: ```ruby -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author end ``` By default, Active Record doesn't know about the connection between these associations. This can lead to two copies of an object getting out of sync: ```ruby -c = Customer.first -o = c.orders.first -c.first_name == o.customer.first_name # => true -c.first_name = 'Manny' -c.first_name == o.customer.first_name # => false +a = Author.first +b = a.books.first +a.first_name == b.author.first_name # => true +a.first_name = 'Manny' +a.first_name == b.author.first_name # => false ``` -This happens because `c` and `o.customer` are two different in-memory representations of the same data, and neither one is automatically refreshed from changes to the other. Active Record provides the `:inverse_of` option so that you can inform it of these relations: +This happens because `a` and `b.author` are two different in-memory representations of the same data, and neither one is automatically refreshed from changes to the other. Active Record provides the `:inverse_of` option so that you can inform it of these relations: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, inverse_of: :customer +class Author < ApplicationRecord + has_many :books, inverse_of: :author end -class Order < ActiveRecord::Base - belongs_to :customer, inverse_of: :orders +class Book < ApplicationRecord + belongs_to :author, inverse_of: :books end ``` -With these changes, Active Record will only load one copy of the customer object, preventing inconsistencies and making your application more efficient: +With these changes, Active Record will only load one copy of the author object, preventing inconsistencies and making your application more efficient: ```ruby -c = Customer.first -o = c.orders.first -c.first_name == o.customer.first_name # => true -c.first_name = 'Manny' -c.first_name == o.customer.first_name # => true +a = author.first +b = a.books.first +a.first_name == b.author.first_name # => true +a.first_name = 'Manny' +a.first_name == b.author.first_name # => true ``` There are a few limitations to `inverse_of` support: @@ -777,19 +781,19 @@ When you declare a `belongs_to` association, the declaring class automatically g In all of these methods, `association` is replaced with the symbol passed as the first argument to `belongs_to`. For example, given the declaration: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author end ``` -Each instance of the `Order` model will have these methods: +Each instance of the `Book` model will have these methods: ```ruby -customer -customer= -build_customer -create_customer -create_customer! +author +author= +build_author +create_author +create_author! ``` NOTE: When initializing a new `has_one` or `belongs_to` association you must use the `build_` prefix to build the association, rather than the `association.build` method that would be used for `has_many` or `has_and_belongs_to_many` associations. To create one, use the `create_` prefix. @@ -799,13 +803,13 @@ NOTE: When initializing a new `has_one` or `belongs_to` association you must use The `association` method returns the associated object, if any. If no associated object is found, it returns `nil`. ```ruby -@customer = @order.customer +@author = @book.author ``` If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), call `#reload` on the parent object. ```ruby -@customer = @order.reload.customer +@author = @book.reload.author ``` ##### `association=(associate)` @@ -813,7 +817,7 @@ If the associated object has already been retrieved from the database for this o The `association=` method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from the associated object and setting this object's foreign key to the same value. ```ruby -@order.customer = @customer +@book.author = @author ``` ##### `build_association(attributes = {})` @@ -821,8 +825,8 @@ The `association=` method assigns an associated object to this object. Behind th The `build_association` method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through this object's foreign key will be set, but the associated object will _not_ yet be saved. ```ruby -@customer = @order.build_customer(customer_number: 123, - customer_name: "John Doe") +@author = @book.build_author(author_number: 123, + author_name: "John Doe") ``` ##### `create_association(attributes = {})` @@ -830,8 +834,8 @@ The `build_association` method returns a new object of the associated type. This The `create_association` method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through this object's foreign key will be set, and, once it passes all of the validations specified on the associated model, the associated object _will_ be saved. ```ruby -@customer = @order.create_customer(customer_number: 123, - customer_name: "John Doe") +@author = @book.create_author(author_number: 123, + author_name: "John Doe") ``` ##### `create_association!(attributes = {})` @@ -844,8 +848,8 @@ Does the same as `create_association` above, but raises `ActiveRecord::RecordInv While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `belongs_to` association reference. Such customizations can easily be accomplished by passing options and scope blocks when you create the association. For example, this association uses two such options: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, dependent: :destroy, +class Book < ApplicationRecord + belongs_to :author, dependent: :destroy, counter_cache: true end ``` @@ -870,11 +874,11 @@ If you set the `:autosave` option to `true`, Rails will save any loaded members ##### `:class_name` -If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if an order belongs to a customer, but the actual name of the model containing customers is `Patron`, you'd set things up this way: +If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if a book belongs to an author, but the actual name of the model containing authors is `Patron`, you'd set things up this way: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, class_name: "Patron" +class Book < ApplicationRecord + belongs_to :author, class_name: "Patron" end ``` @@ -883,22 +887,22 @@ end The `:counter_cache` option can be used to make finding the number of belonging objects more efficient. Consider these models: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author end -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` -With these declarations, asking for the value of `@customer.orders.size` requires making a call to the database to perform a `COUNT(*)` query. To avoid this call, you can add a counter cache to the _belonging_ model: +With these declarations, asking for the value of `@author.books.size` requires making a call to the database to perform a `COUNT(*)` query. To avoid this call, you can add a counter cache to the _belonging_ model: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, counter_cache: true +class Book < ApplicationRecord + belongs_to :author, counter_cache: true end -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` @@ -907,18 +911,18 @@ With this declaration, Rails will keep the cache value up to date, and then retu Although the `:counter_cache` option is specified on the model that includes the `belongs_to` declaration, the actual column must be added to the _associated_ (`has_many`) model. In the case above, you would need to add a -column named `orders_count` to the `Customer` model. +column named `books_count` to the `Author` model. You can override the default column name by specifying a custom column name in the `counter_cache` declaration instead of `true`. For example, to use -`count_of_orders` instead of `orders_count`: +`count_of_books` instead of `books_count`: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, counter_cache: :count_of_orders +class Book < ApplicationRecord + belongs_to :author, counter_cache: :count_of_books end -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` @@ -945,8 +949,8 @@ WARNING: You should not specify this option on a `belongs_to` association that i By convention, Rails assumes that the column used to hold the foreign key on this model is the name of the association with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, class_name: "Patron", +class Book < ApplicationRecord + belongs_to :author, class_name: "Patron", foreign_key: "patron_id" end ``` @@ -961,11 +965,11 @@ of its tables. The `:primary_key` option allows you to specify a different colum For example, given we have a `users` table with `guid` as the primary key. If we want a separate `todos` table to hold the foreign key `user_id` in the `guid` column, then we can use `primary_key` to achieve this like so: ```ruby -class User < ActiveRecord::Base +class User < ApplicationRecord self.primary_key = 'guid' # primary key is guid and not id end -class Todo < ActiveRecord::Base +class Todo < ApplicationRecord belongs_to :user, primary_key: 'guid' end ``` @@ -978,12 +982,12 @@ When we execute `@user.todos.create` then the `@todo` record will have its The `:inverse_of` option specifies the name of the `has_many` or `has_one` association that is the inverse of this association. Does not work in combination with the `:polymorphic` options. ```ruby -class Customer < ActiveRecord::Base - has_many :orders, inverse_of: :customer +class Author < ApplicationRecord + has_many :books, inverse_of: :author end -class Order < ActiveRecord::Base - belongs_to :customer, inverse_of: :orders +class Book < ApplicationRecord + belongs_to :author, inverse_of: :books end ``` @@ -996,20 +1000,20 @@ Passing `true` to the `:polymorphic` option indicates that this is a polymorphic If you set the `:touch` option to `true`, then the `updated_at` or `updated_on` timestamp on the associated object will be set to the current time whenever this object is saved or destroyed: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, touch: true +class Book < ApplicationRecord + belongs_to :author, touch: true end -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` -In this case, saving or destroying an order will update the timestamp on the associated customer. You can also specify a particular timestamp attribute to update: +In this case, saving or destroying an book will update the timestamp on the associated author. You can also specify a particular timestamp attribute to update: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, touch: :orders_updated_at +class Book < ApplicationRecord + belongs_to :author, touch: :books_updated_at end ``` @@ -1027,8 +1031,8 @@ object won't be validated. By default, this option is set to `false`. There may be times when you wish to customize the query used by `belongs_to`. Such customizations can be achieved via a scope block. For example: ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, -> { where active: true }, +class Book < ApplicationRecord + belongs_to :author, -> { where active: true }, dependent: :destroy end ``` @@ -1045,8 +1049,8 @@ You can use any of the standard [querying methods](active_record_querying.html) The `where` method lets you specify the conditions that the associated object must meet. ```ruby -class Order < ActiveRecord::Base - belongs_to :customer, -> { where active: true } +class book < ApplicationRecord + belongs_to :author, -> { where active: true } end ``` @@ -1055,38 +1059,38 @@ end You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models: ```ruby -class LineItem < ActiveRecord::Base - belongs_to :order +class LineItem < ApplicationRecord + belongs_to :book end -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author has_many :line_items end -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` -If you frequently retrieve customers directly from line items (`@line_item.order.customer`), then you can make your code somewhat more efficient by including customers in the association from line items to orders: +If you frequently retrieve authors directly from line items (`@line_item.book.author`), then you can make your code somewhat more efficient by including authors in the association from line items to books: ```ruby -class LineItem < ActiveRecord::Base - belongs_to :order, -> { includes :customer } +class LineItem < ApplicationRecord + belongs_to :book, -> { includes :author } end -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author has_many :line_items end -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` -NOTE: There's no need to use `includes` for immediate associations - that is, if you have `Order belongs_to :customer`, then the customer is eager-loaded automatically when it's needed. +NOTE: There's no need to use `includes` for immediate associations - that is, if you have `Book belongs_to :author`, then the author is eager-loaded automatically when it's needed. ##### `readonly` @@ -1103,8 +1107,8 @@ TIP: If you use the `select` method on a `belongs_to` association, you should al You can see if any associated objects exist by using the `association.nil?` method: ```ruby -if @order.customer.nil? - @msg = "No customer found for this order" +if @book.author.nil? + @msg = "No author found for this book" end ``` @@ -1129,7 +1133,7 @@ When you declare a `has_one` association, the declaring class automatically gain In all of these methods, `association` is replaced with the symbol passed as the first argument to `has_one`. For example, given the declaration: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account end ``` @@ -1193,7 +1197,7 @@ Does the same as `create_association` above, but raises `ActiveRecord::RecordInv While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `has_one` association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account, class_name: "Billing", dependent: :nullify end ``` @@ -1225,7 +1229,7 @@ If you set the `:autosave` option to `true`, Rails will save any loaded members If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if a supplier has an account, but the actual name of the model containing accounts is `Billing`, you'd set things up this way: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account, class_name: "Billing" end ``` @@ -1251,7 +1255,7 @@ unallowed `NULL` value. By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account, foreign_key: "supp_id" end ``` @@ -1263,11 +1267,11 @@ TIP: In any case, Rails will not create foreign key columns for you. You need to The `:inverse_of` option specifies the name of the `belongs_to` association that is the inverse of this association. Does not work in combination with the `:through` or `:as` options. ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account, inverse_of: :supplier end -class Account < ActiveRecord::Base +class Account < ApplicationRecord belongs_to :supplier, inverse_of: :account end ``` @@ -1297,7 +1301,7 @@ If you set the `:validate` option to `true`, then associated objects will be val There may be times when you wish to customize the query used by `has_one`. Such customizations can be achieved via a scope block. For example: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account, -> { where active: true } end ``` @@ -1314,7 +1318,7 @@ You can use any of the standard [querying methods](active_record_querying.html) The `where` method lets you specify the conditions that the associated object must meet. ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account, -> { where "confirmed = 1" } end ``` @@ -1324,16 +1328,16 @@ end You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account end -class Account < ActiveRecord::Base +class Account < ApplicationRecord belongs_to :supplier belongs_to :representative end -class Representative < ActiveRecord::Base +class Representative < ApplicationRecord has_many :accounts end ``` @@ -1341,16 +1345,16 @@ end If you frequently retrieve representatives directly from suppliers (`@supplier.account.representative`), then you can make your code somewhat more efficient by including representatives in the association from suppliers to accounts: ```ruby -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_one :account, -> { includes :representative } end -class Account < ActiveRecord::Base +class Account < ApplicationRecord belongs_to :supplier belongs_to :representative end -class Representative < ActiveRecord::Base +class Representative < ApplicationRecord has_many :accounts end ``` @@ -1411,30 +1415,30 @@ When you declare a `has_many` association, the declaring class automatically gai In all of these methods, `collection` is replaced with the symbol passed as the first argument to `has_many`, and `collection_singular` is replaced with the singularized version of that symbol. For example, given the declaration: ```ruby -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end ``` -Each instance of the `Customer` model will have these methods: +Each instance of the `Author` model will have these methods: ```ruby -orders -orders<<(object, ...) -orders.delete(object, ...) -orders.destroy(object, ...) -orders=(objects) -order_ids -order_ids=(ids) -orders.clear -orders.empty? -orders.size -orders.find(...) -orders.where(...) -orders.exists?(...) -orders.build(attributes = {}, ...) -orders.create(attributes = {}) -orders.create!(attributes = {}) +books +books<<(object, ...) +books.delete(object, ...) +books.destroy(object, ...) +books=(objects) +book_ids +book_ids=(ids) +books.clear +books.empty? +books.size +books.find(...) +books.where(...) +books.exists?(...) +books.build(attributes = {}, ...) +books.create(attributes = {}) +books.create!(attributes = {}) ``` ##### `collection` @@ -1442,7 +1446,7 @@ orders.create!(attributes = {}) The `collection` method returns an array of all of the associated objects. If there are no associated objects, it returns an empty array. ```ruby -@orders = @customer.orders +@books = @author.books ``` ##### `collection<<(object, ...)` @@ -1450,7 +1454,7 @@ The `collection` method returns an array of all of the associated objects. If th The `collection<<` method adds one or more objects to the collection by setting their foreign keys to the primary key of the calling model. ```ruby -@customer.orders << @order1 +@author.books << @book1 ``` ##### `collection.delete(object, ...)` @@ -1458,7 +1462,7 @@ The `collection<<` method adds one or more objects to the collection by setting The `collection.delete` method removes one or more objects from the collection by setting their foreign keys to `NULL`. ```ruby -@customer.orders.delete(@order1) +@author.books.delete(@book1) ``` WARNING: Additionally, objects will be destroyed if they're associated with `dependent: :destroy`, and deleted if they're associated with `dependent: :delete_all`. @@ -1468,7 +1472,7 @@ WARNING: Additionally, objects will be destroyed if they're associated with `dep The `collection.destroy` method removes one or more objects from the collection by running `destroy` on each object. ```ruby -@customer.orders.destroy(@order1) +@author.books.destroy(@book1) ``` WARNING: Objects will _always_ be removed from the database, ignoring the `:dependent` option. @@ -1482,7 +1486,7 @@ The `collection=` method makes the collection contain only the supplied objects, The `collection_singular_ids` method returns an array of the ids of the objects in the collection. ```ruby -@order_ids = @customer.order_ids +@book_ids = @author.book_ids ``` ##### `collection_singular_ids=(ids)` @@ -1494,7 +1498,7 @@ The `collection_singular_ids=` method makes the collection contain only the obje The `collection.clear` method removes all objects from the collection according to the strategy specified by the `dependent` option. If no option is given, it follows the default strategy. The default strategy for `has_many :through` associations is `delete_all`, and for `has_many` associations is to set the foreign keys to `NULL`. ```ruby -@customer.orders.clear +@author.books.clear ``` WARNING: Objects will be deleted if they're associated with `dependent: :destroy`, @@ -1505,8 +1509,8 @@ just like `dependent: :delete_all`. The `collection.empty?` method returns `true` if the collection does not contain any associated objects. ```erb -<% if @customer.orders.empty? %> - No Orders Found +<% if @author.books.empty? %> + No Books Found <% end %> ``` @@ -1515,7 +1519,7 @@ The `collection.empty?` method returns `true` if the collection does not contain The `collection.size` method returns the number of objects in the collection. ```ruby -@order_count = @customer.orders.size +@book_count = @author.books.size ``` ##### `collection.find(...)` @@ -1523,7 +1527,7 @@ The `collection.size` method returns the number of objects in the collection. The `collection.find` method finds objects within the collection. It uses the same syntax and options as `ActiveRecord::Base.find`. ```ruby -@open_orders = @customer.orders.find(1) +@available_books = @author.books.find(1) ``` ##### `collection.where(...)` @@ -1531,8 +1535,8 @@ The `collection.find` method finds objects within the collection. It uses the sa The `collection.where` method finds objects within the collection based on the conditions supplied but the objects are loaded lazily meaning that the database is queried only when the object(s) are accessed. ```ruby -@open_orders = @customer.orders.where(open: true) # No query yet -@open_order = @open_orders.first # Now the database will be queried +@available_books = @author.books.where(available: true) # No query yet +@available_book = @available_books.first # Now the database will be queried ``` ##### `collection.exists?(...)` @@ -1546,12 +1550,12 @@ conditions exists in the collection. It uses the same syntax and options as The `collection.build` method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, and the link through their foreign key will be created, but the associated objects will _not_ yet be saved. ```ruby -@order = @customer.orders.build(order_date: Time.now, - order_number: "A12345") +@book = @author.books.build(published_at: Time.now, + book_number: "A12345") -@orders = @customer.orders.build([ - { order_date: Time.now, order_number: "A12346" }, - { order_date: Time.now, order_number: "A12347" } +@books = @author.books.build([ + { published_at: Time.now, book_number: "A12346" }, + { published_at: Time.now, book_number: "A12347" } ]) ``` @@ -1560,12 +1564,12 @@ The `collection.build` method returns a single or array of new objects of the as The `collection.create` method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, the link through its foreign key will be created, and, once it passes all of the validations specified on the associated model, the associated object _will_ be saved. ```ruby -@order = @customer.orders.create(order_date: Time.now, - order_number: "A12345") +@book = @author.books.create(published_at: Time.now, + book_number: "A12345") -@orders = @customer.orders.create([ - { order_date: Time.now, order_number: "A12346" }, - { order_date: Time.now, order_number: "A12347" } +@books = @author.books.create([ + { published_at: Time.now, book_number: "A12346" }, + { published_at: Time.now, book_number: "A12347" } ]) ``` @@ -1578,8 +1582,8 @@ Does the same as `collection.create` above, but raises `ActiveRecord::RecordInva While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `has_many` association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, dependent: :delete_all, validate: false +class Author < ApplicationRecord + has_many :books, dependent: :delete_all, validate: false end ``` @@ -1608,11 +1612,11 @@ If you set the `:autosave` option to `true`, Rails will save any loaded members ##### `:class_name` -If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if a customer has many orders, but the actual name of the model containing orders is `Transaction`, you'd set things up this way: +If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if an author has many books, but the actual name of the model containing books is `Transaction`, you'd set things up this way: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, class_name: "Transaction" +class Author < ApplicationRecord + has_many :books, class_name: "Transaction" end ``` @@ -1635,8 +1639,8 @@ Controls what happens to the associated objects when their owner is destroyed: By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, foreign_key: "cust_id" +class Author < ApplicationRecord + has_many :books, foreign_key: "cust_id" end ``` @@ -1647,12 +1651,12 @@ TIP: In any case, Rails will not create foreign key columns for you. You need to The `:inverse_of` option specifies the name of the `belongs_to` association that is the inverse of this association. Does not work in combination with the `:through` or `:as` options. ```ruby -class Customer < ActiveRecord::Base - has_many :orders, inverse_of: :customer +class Author < ApplicationRecord + has_many :books, inverse_of: :author end -class Order < ActiveRecord::Base - belongs_to :customer, inverse_of: :orders +class Book < ApplicationRecord + belongs_to :author, inverse_of: :books end ``` @@ -1666,7 +1670,7 @@ hold the `guid` column value as the foreign key and not `id` value. This can be achieved like this: ```ruby -class User < ActiveRecord::Base +class User < ApplicationRecord has_many :todos, primary_key: :guid end ``` @@ -1696,8 +1700,8 @@ If you set the `:validate` option to `false`, then associated objects will not b There may be times when you wish to customize the query used by `has_many`. Such customizations can be achieved via a scope block. For example: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, -> { where processed: true } +class Author < ApplicationRecord + has_many :books, -> { where processed: true } end ``` @@ -1719,22 +1723,22 @@ You can use any of the standard [querying methods](active_record_querying.html) The `where` method lets you specify the conditions that the associated object must meet. ```ruby -class Customer < ActiveRecord::Base - has_many :confirmed_orders, -> { where "confirmed = 1" }, - class_name: "Order" +class Author < ApplicationRecord + has_many :confirmed_books, -> { where "confirmed = 1" }, + class_name: "Book" end ``` You can also set conditions via a hash: ```ruby -class Customer < ActiveRecord::Base - has_many :confirmed_orders, -> { where confirmed: true }, - class_name: "Order" +class Author < ApplicationRecord + has_many :confirmed_books, -> { where confirmed: true }, + class_name: "Book" end ``` -If you use a hash-style `where` option, then record creation via this association will be automatically scoped using the hash. In this case, using `@customer.confirmed_orders.create` or `@customer.confirmed_orders.build` will create orders where the confirmed column has the value `true`. +If you use a hash-style `where` option, then record creation via this association will be automatically scoped using the hash. In this case, using `@author.confirmed_books.create` or `@author.confirmed_books.build` will create books where the confirmed column has the value `true`. ##### `extending` @@ -1745,9 +1749,9 @@ The `extending` method specifies a named module to extend the association proxy. The `group` method supplies an attribute name to group the result set by, using a `GROUP BY` clause in the finder SQL. ```ruby -class Customer < ActiveRecord::Base - has_many :line_items, -> { group 'orders.id' }, - through: :orders +class Author < ApplicationRecord + has_many :line_items, -> { group 'books.id' }, + through: :books end ``` @@ -1756,34 +1760,34 @@ end You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models: ```ruby -class Customer < ActiveRecord::Base - has_many :orders +class Author < ApplicationRecord + has_many :books end -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author has_many :line_items end -class LineItem < ActiveRecord::Base - belongs_to :order +class LineItem < ApplicationRecord + belongs_to :book end ``` -If you frequently retrieve line items directly from customers (`@customer.orders.line_items`), then you can make your code somewhat more efficient by including line items in the association from customers to orders: +If you frequently retrieve line items directly from authors (`@author.books.line_items`), then you can make your code somewhat more efficient by including line items in the association from authors to books: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, -> { includes :line_items } +class Author < ApplicationRecord + has_many :books, -> { includes :line_items } end -class Order < ActiveRecord::Base - belongs_to :customer +class Book < ApplicationRecord + belongs_to :author has_many :line_items end -class LineItem < ActiveRecord::Base - belongs_to :order +class LineItem < ApplicationRecord + belongs_to :book end ``` @@ -1792,10 +1796,10 @@ end The `limit` method lets you restrict the total number of objects that will be fetched through an association. ```ruby -class Customer < ActiveRecord::Base - has_many :recent_orders, - -> { order('order_date desc').limit(100) }, - class_name: "Order", +class Author < ApplicationRecord + has_many :recent_books, + -> { order('published_at desc').limit(100) }, + class_name: "Book", end ``` @@ -1808,8 +1812,8 @@ The `offset` method lets you specify the starting offset for fetching objects vi The `order` method dictates the order in which associated objects will be received (in the syntax used by an SQL `ORDER BY` clause). ```ruby -class Customer < ActiveRecord::Base - has_many :orders, -> { order "date_confirmed DESC" } +class Author < ApplicationRecord + has_many :books, -> { order "date_confirmed DESC" } end ``` @@ -1829,7 +1833,7 @@ Use the `distinct` method to keep the collection free of duplicates. This is mostly useful together with the `:through` option. ```ruby -class Person < ActiveRecord::Base +class Person < ApplicationRecord has_many :readings has_many :articles, through: :readings end @@ -1868,11 +1872,21 @@ If you want to make sure that, upon insertion, all of the records in the persisted association are distinct (so that you can be sure that when you inspect the association that you will never find duplicate records), you should add a unique index on the table itself. For example, if you have a table named -`person_articles` and you want to make sure all the articles are unique, you could -add the following in a migration: +`readings` and you want to make sure the articles can only be added to a person once, +you could add the following in a migration: ```ruby -add_index :person_articles, :article, unique: true +add_index :readings, [:person_id, :article_id], unique: true +``` + +Once you have this unique index, attempting to add the article to a person twice +will raise an `ActiveRecord::RecordNotUnique` error: + +```ruby +person = Person.create(name: 'Honda') +article = Article.create(name: 'a1') +person.articles << article +person.articles << article # => ActiveRecord::RecordNotUnique ``` Note that checking for uniqueness using something like `include?` is subject @@ -1923,7 +1937,7 @@ When you declare a `has_and_belongs_to_many` association, the declaring class au In all of these methods, `collection` is replaced with the symbol passed as the first argument to `has_and_belongs_to_many`, and `collection_singular` is replaced with the singularized version of that symbol. For example, given the declaration: ```ruby -class Part < ActiveRecord::Base +class Part < ApplicationRecord has_and_belongs_to_many :assemblies end ``` @@ -2077,7 +2091,7 @@ Does the same as `collection.create`, but raises `ActiveRecord::RecordInvalid` i While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `has_and_belongs_to_many` association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options: ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, -> { readonly }, autosave: true end @@ -2099,7 +2113,7 @@ By convention, Rails assumes that the column in the join table used to hold the TIP: The `:foreign_key` and `:association_foreign_key` options are useful when setting up a many-to-many self-join. For example: ```ruby -class User < ActiveRecord::Base +class User < ApplicationRecord has_and_belongs_to_many :friends, class_name: "User", foreign_key: "this_user_id", @@ -2116,7 +2130,7 @@ If you set the `:autosave` option to `true`, Rails will save any loaded members If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if a part has many assemblies, but the actual name of the model containing assemblies is `Gadget`, you'd set things up this way: ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, class_name: "Gadget" end ``` @@ -2126,7 +2140,7 @@ end By convention, Rails assumes that the column in the join table used to hold the foreign key pointing to this model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly: ```ruby -class User < ActiveRecord::Base +class User < ApplicationRecord has_and_belongs_to_many :friends, class_name: "User", foreign_key: "this_user_id", @@ -2147,7 +2161,7 @@ If you set the `:validate` option to `false`, then associated objects will not b There may be times when you wish to customize the query used by `has_and_belongs_to_many`. Such customizations can be achieved via a scope block. For example: ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, -> { where active: true } end ``` @@ -2170,7 +2184,7 @@ You can use any of the standard [querying methods](active_record_querying.html) The `where` method lets you specify the conditions that the associated object must meet. ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, -> { where "factory = 'Seattle'" } end @@ -2179,7 +2193,7 @@ end You can also set conditions via a hash: ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, -> { where factory: 'Seattle' } end @@ -2196,7 +2210,7 @@ The `extending` method specifies a named module to extend the association proxy. The `group` method supplies an attribute name to group the result set by, using a `GROUP BY` clause in the finder SQL. ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, -> { group "factory" } end ``` @@ -2210,7 +2224,7 @@ You can use the `includes` method to specify second-order associations that shou The `limit` method lets you restrict the total number of objects that will be fetched through an association. ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, -> { order("created_at DESC").limit(50) } end @@ -2225,7 +2239,7 @@ The `offset` method lets you specify the starting offset for fetching objects vi The `order` method dictates the order in which associated objects will be received (in the syntax used by an SQL `ORDER BY` clause). ```ruby -class Parts < ActiveRecord::Base +class Parts < ApplicationRecord has_and_belongs_to_many :assemblies, -> { order "assembly_name ASC" } end @@ -2267,10 +2281,10 @@ Association callbacks are similar to normal callbacks, but they are triggered by You define association callbacks by adding options to the association declaration. For example: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, before_add: :check_credit_limit +class Author < ApplicationRecord + has_many :books, before_add: :check_credit_limit - def check_credit_limit(order) + def check_credit_limit(book) ... end end @@ -2281,15 +2295,15 @@ Rails passes the object being added or removed to the callback. You can stack callbacks on a single event by passing them as an array: ```ruby -class Customer < ActiveRecord::Base - has_many :orders, +class Author < ApplicationRecord + has_many :books, before_add: [:check_credit_limit, :calculate_shipping_charges] - def check_credit_limit(order) + def check_credit_limit(book) ... end - def calculate_shipping_charges(order) + def calculate_shipping_charges(book) ... end end @@ -2302,10 +2316,10 @@ If a `before_add` callback throws an exception, the object does not get added to You're not limited to the functionality that Rails automatically builds into association proxy objects. You can also extend these objects through anonymous modules, adding new finders, creators, or other methods. For example: ```ruby -class Customer < ActiveRecord::Base - has_many :orders do - def find_by_order_prefix(order_number) - find_by(region_id: order_number[0..2]) +class Author < ApplicationRecord + has_many :books do + def find_by_book_prefix(book_number) + find_by(category_id: book_number[0..2]) end end end @@ -2320,11 +2334,11 @@ module FindRecentExtension end end -class Customer < ActiveRecord::Base - has_many :orders, -> { extending FindRecentExtension } +class Author < ApplicationRecord + has_many :books, -> { extending FindRecentExtension } end -class Supplier < ActiveRecord::Base +class Supplier < ApplicationRecord has_many :deliveries, -> { extending FindRecentExtension } end ``` |