Active Record Associations

该文档列出了Rails Active Record Associations的基本知识点.

阅读完该文档后，您将会了解到:

• 为什么使用Active Record Associations
• Active Record Associations主要内容概括.

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Why do we need associations between models? Because they make common operations simpler and easier in your code.

Active Record Associations知识总览

官方文档

为什么使用Associations ?

Associations的类型

• belongs_to
• has_one
• has_many
• has_many :through
• has_one :through
• has_and_belongs_to_many
• 如何判断选择belongs_to还是has_one
• 如何判断选择has_many :through 还是 has_and_belongs_to_many
• Polymorphic Associations（多态关联, :polymorphic, :as）
• Self Joins(自关联)

注意事项

• Controlling caching(控制缓存)
• Avoiding name collisions(避免命名冲突)
• Updating the schema(更新数据库表结构)
  • 为belongs_to创建外键
  • 为has_and_belongs_to_many创建关联表(中间表)
• Controlling association scope(控制关联的scope，必要时需要使用:class_name参数)
• Bi-directional associations(双向关联,多份对象拷贝可能导致数据不同步，:inverse_of参数)

各个关联关系详解

belongs_to详解

• belongs_to 添加了哪些方法
  • association(force_reload = false)
  • association=(associate)
  • build_association(attributes = {})
  • create_association(attributes = {})
  • create_association!(attributes = {})
• belongs_to 有哪些可选参数
  • :autosave
  • :class_name
  • :counter_cache
  • :dependent
  • :foreign_key
  • :inverse_of
  • :polymorphic
  • :touch
  • :validate
• belongs_to 的scopes
  • where
  • includes
  • readonly
  • select
• 判断关联对象是否存在: association.nil? 方法
• 关联对象何时被保存

has_one 详解

• has_one 添加了哪些方法
  • association(force_reload = false)
  • association=(associate)
  • build_association(attributes = {})
  • create_association(attributes = {})
  • create_association!(attributes = {})
• has_one 有哪些可选参数
  • :as
  • :autosave
  • :class_name
  • :dependent
  • :foreign_key
  • :inverse_of
  • :primary_key
  • :source
  • :source_type
  • :through
  • :validate
• 判断关联对象是否存在: association.nil? 方法
• 关联对象何时被保存

has_many 详解

• has_many 添加了哪些方法
  • collection(force_reload = false)
  • collection«(object, …)
  • collection.delete(object, …)
  • collection.destroy(object, …)
  • collection=(objects)
  • collection_singular_ids
  • collection_singular_ids=(ids)
  • collection.clear
  • collection.empty?
  • collection.size
  • collection.find(…)
  • collection.where(…)
  • collection.exists?(…)
  • collection.build(attributes = {}, …)
  • collection.create(attributes = {})
  • collection.create!(attributes = {})
• has_many 有哪些可选参数
  • :as
  • :autosave
  • :class_name
  • :dependent
  • :foreign_key
  • :inverse_of
  • :primary_key
  • :source
  • :source_type
  • :through
  • :validate
• has_many 可用的 scopes
  • where
  • extending
  • group
  • includes
  • limit
  • offset
  • order
  • readonly
  • select
  • uniq
• 关联对象何时被保存

has_and_belongs_to_many 详解

• has_and_belongs_to_many 添加了哪些方法
  • collection(force_reload = false)
  • collection«(object, …)
  • collection.delete(object, …)
  • collection.destroy(object, …)
  • collection=(objects)
  • collection_singular_ids
  • collection_singular_ids=(ids)
  • collection.clear
  • collection.empty?
  • collection.size
  • collection.find(…)
  • collection.where(…)
  • collection.exists?(…)
  • collection.build(attributes = {})
  • collection.create(attributes = {})
  • collection.create!(attributes = {})
• has_many 有哪些可选参数
  • :association_foreign_key
  • :autosave
  • :class_name
  • :foreign_key
  • :join_table
  • :validate
• has_many 可用的 scopes
  • where
  • extending
  • group
  • includes
  • limit
  • offset
  • order
  • readonly
  • select
  • uniq
• 关联对象何时被保存

Association Callbacks(关联Callbacks)

• before_add
• after_add
• before_remove
• after_remove

Association Extensions(关联扩展, extending)