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Diffstat (limited to 'guides/source/association_basics.md')
| -rw-r--r-- | guides/source/association_basics.md | 157 |
1 files changed, 106 insertions, 51 deletions
diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md index 3837cda553..b5bd24d027 100644 --- a/guides/source/association_basics.md +++ b/guides/source/association_basics.md @@ -154,7 +154,7 @@ case, the column definition might look like this: ```ruby create_table :accounts do |t| - t.belongs_to :supplier, index: true, unique: true, foreign_key: true + t.belongs_to :supplier, index: { unique: true }, foreign_key: true # ... end ``` @@ -582,14 +582,30 @@ class CreateBooks < ActiveRecord::Migration[5.0] t.string :book_number t.integer :author_id end - - add_index :books, :author_id end end ``` If you create an association some time after you build the underlying model, you need to remember to create an `add_column` migration to provide the necessary foreign key. +It's a good practice to add an index on the foreign key to improve queries +performance and a foreign key constraint to ensure referential data integrity: + +```ruby +class CreateBooks < ActiveRecord::Migration[5.0] + def change + create_table :books do |t| + t.datetime :published_at + t.string :book_number + t.integer :author_id + end + + add_index :books, :author_id + add_foreign_key :books, :authors + end +end +``` + #### 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 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. @@ -647,11 +663,11 @@ By default, associations look for objects only within the current module's scope module MyApplication module Business class Supplier < ApplicationRecord - has_one :account + has_one :account end class Account < ApplicationRecord - belongs_to :supplier + belongs_to :supplier end end end @@ -663,13 +679,13 @@ This will work fine, because both the `Supplier` and the `Account` class are def module MyApplication module Business class Supplier < ApplicationRecord - has_one :account + has_one :account end end module Billing class Account < ApplicationRecord - belongs_to :supplier + belongs_to :supplier end end end @@ -681,14 +697,14 @@ To associate a model with a model in a different namespace, you must specify the module MyApplication module Business class Supplier < ApplicationRecord - has_one :account, + has_one :account, class_name: "MyApplication::Billing::Account" end end module Billing class Account < ApplicationRecord - belongs_to :supplier, + belongs_to :supplier, class_name: "MyApplication::Business::Supplier" end end @@ -709,55 +725,73 @@ class Book < ApplicationRecord 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: +Active Record will attempt to automatically identify that these two models share a bi-directional association based on the association name. In this way, Active Record will only load one copy of the `Author` object, making your application more efficient and preventing inconsistent data: ```ruby 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 +a.first_name = 'David' +a.first_name == b.author.first_name # => true ``` -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: +Active Record supports automatic identification for most associations with standard names. However, Active Record will not automatically identify bi-directional associations that contain any of the following options: + +* `:conditions` +* `:through` +* `:polymorphic` +* `:class_name` +* `:foreign_key` + +For example, consider the following model declarations: ```ruby class Author < ApplicationRecord - has_many :books, inverse_of: :author + has_many :books end class Book < ApplicationRecord - belongs_to :author, inverse_of: :books + belongs_to :writer, class_name: 'Author', foreign_key: 'author_id' end ``` -With these changes, Active Record will only load one copy of the author object, preventing inconsistencies and making your application more efficient: +Active Record will no longer automatically recognize the bi-directional association: ```ruby 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 +a.first_name == b.writer.first_name # => true +a.first_name = 'David' +a.first_name == b.writer.first_name # => false ``` -There are a few limitations to `inverse_of` support: +Active Record provides the `:inverse_of` option so you can explicitly declare bi-directional associations: + +```ruby +class Author < ApplicationRecord + has_many :books, inverse_of: 'writer' +end + +class Book < ApplicationRecord + belongs_to :writer, class_name: 'Author', foreign_key: 'author_id' +end +``` + +By including the `:inverse_of` option in the `has_many` association declaration, Active Record will now recognize the bi-directional association: + +```ruby +a = Author.first +b = a.books.first +a.first_name == b.writer.first_name # => true +a.first_name = 'David' +a.first_name == b.writer.first_name # => true +``` + +There are a few limitations to `:inverse_of` support: * They do not work with `:through` associations. * They do not work with `:polymorphic` associations. * They do not work with `:as` associations. -* For `belongs_to` associations, `has_many` inverse associations are ignored. - -Every association will attempt to automatically find the inverse association -and set the `:inverse_of` option heuristically (based on the association name). -Most associations with standard names will be supported. However, associations -that contain the following options will not have their inverses set -automatically: - -* `:conditions` -* `:through` -* `:polymorphic` -* `:foreign_key` Detailed Association Reference ------------------------------ @@ -777,6 +811,7 @@ When you declare a `belongs_to` association, the declaring class automatically g * `build_association(attributes = {})` * `create_association(attributes = {})` * `create_association!(attributes = {})` +* `reload_association` In all of these methods, `association` is replaced with the symbol passed as the first argument to `belongs_to`. For example, given the declaration: @@ -794,6 +829,7 @@ author= build_author create_author create_author! +reload_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. @@ -806,10 +842,10 @@ The `association` method returns the associated object, if any. If no associated @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. +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_association` on the parent object. ```ruby -@author = @book.reload.author +@author = @book.reload_author ``` ##### `association=(associate)` @@ -926,19 +962,18 @@ class Author < ApplicationRecord end ``` -NOTE: You only need to specify the :counter_cache option on the `belongs_to` +NOTE: You only need to specify the `:counter_cache` option on the `belongs_to` side of the association. Counter cache columns are added to the containing model's list of read-only attributes through `attr_readonly`. ##### `:dependent` -Controls what happens to associated objects when their owner is destroyed: +If you set the `:dependent` option to: -* `:destroy` causes the associated objects to also be destroyed. -* `:delete_all` causes the associated objects to be deleted directly from the database (callbacks are not executed). -* `:nullify` causes the foreign keys to be set to `NULL` (callbacks are not executed). -* `:restrict_with_exception` causes an exception to be raised if there are associated records. -* `:restrict_with_error` causes an error to be added to the owner if there are associated objects. +* `:destroy`, when the object is destroyed, `destroy` will be called on its +associated objects. +* `:delete`, when the object is destroyed, all its associated objects will be +deleted directly from the database without calling their `destroy` method. WARNING: You should not specify this option on a `belongs_to` association that is connected with a `has_many` association on the other class. Doing so can lead to orphaned records in your database. @@ -1127,6 +1162,7 @@ When you declare a `has_one` association, the declaring class automatically gain * `build_association(attributes = {})` * `create_association(attributes = {})` * `create_association!(attributes = {})` +* `reload_association` In all of these methods, `association` is replaced with the symbol passed as the first argument to `has_one`. For example, given the declaration: @@ -1144,6 +1180,7 @@ account= build_account create_account create_account! +reload_account ``` 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. @@ -1156,10 +1193,10 @@ The `association` method returns the associated object, if any. If no associated @account = @supplier.account ``` -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. +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_association` on the parent object. ```ruby -@account = @supplier.reload.account +@account = @supplier.reload_account ``` ##### `association=(associate)` @@ -1383,7 +1420,7 @@ If either of these saves fails due to validation errors, then the assignment sta If the parent object (the one declaring the `has_one` association) is unsaved (that is, `new_record?` returns `true`) then the child objects are not saved. They will automatically when the parent object is saved. -If you want to assign an object to a `has_one` association without saving the object, use the `association.build` method. +If you want to assign an object to a `has_one` association without saving the object, use the `build_association` method. ### `has_many` Association Reference @@ -1409,6 +1446,7 @@ When you declare a `has_many` association, the declaring class automatically gai * `collection.build(attributes = {}, ...)` * `collection.create(attributes = {})` * `collection.create!(attributes = {})` +* `collection.reload` 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: @@ -1437,11 +1475,12 @@ books.exists?(...) books.build(attributes = {}, ...) books.create(attributes = {}) books.create!(attributes = {}) +books.reload ``` ##### `collection` -The `collection` method returns an array of all of the associated objects. If there are no associated objects, it returns an empty array. +The `collection` method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation. ```ruby @books = @author.books @@ -1525,7 +1564,7 @@ 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 -@available_books = @author.books.find(1) +@available_book = @author.books.find(1) ``` ##### `collection.where(...)` @@ -1575,6 +1614,14 @@ The `collection.create` method returns a single or array of new objects of the a Does the same as `collection.create` above, but raises `ActiveRecord::RecordInvalid` if the record is invalid. +##### `collection.reload` + +The `collection.reload` method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation. + +```ruby +@books = @author.books.reload +``` + #### Options for `has_many` 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: @@ -1797,7 +1844,7 @@ The `limit` method lets you restrict the total number of objects that will be fe class Author < ApplicationRecord has_many :recent_books, -> { order('published_at desc').limit(100) }, - class_name: "Book", + class_name: "Book" end ``` @@ -1931,6 +1978,7 @@ When you declare a `has_and_belongs_to_many` association, the declaring class au * `collection.build(attributes = {})` * `collection.create(attributes = {})` * `collection.create!(attributes = {})` +* `collection.reload` 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: @@ -1959,6 +2007,7 @@ assemblies.exists?(...) assemblies.build(attributes = {}, ...) assemblies.create(attributes = {}) assemblies.create!(attributes = {}) +assemblies.reload ``` ##### Additional Column Methods @@ -1970,7 +2019,7 @@ WARNING: The use of extra attributes on the join table in a `has_and_belongs_to_ ##### `collection` -The `collection` method returns an array of all of the associated objects. If there are no associated objects, it returns an empty array. +The `collection` method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation. ```ruby @assemblies = @part.assemblies @@ -1994,11 +2043,9 @@ The `collection.delete` method removes one or more objects from the collection b @part.assemblies.delete(@assembly1) ``` -WARNING: This does not trigger callbacks on the join records. - ##### `collection.destroy(object, ...)` -The `collection.destroy` method removes one or more objects from the collection by running `destroy` on each record in the join table, including running callbacks. This does not destroy the objects. +The `collection.destroy` method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects. ```ruby @part.assemblies.destroy(@assembly1) @@ -2084,6 +2131,14 @@ The `collection.create` method returns a new object of the associated type. This Does the same as `collection.create`, but raises `ActiveRecord::RecordInvalid` if the record is invalid. +##### `collection.reload` + +The `collection.reload` method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation. + +```ruby +@assemblies = @part.assemblies.reload +``` + #### Options for `has_and_belongs_to_many` 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: |