Mysql 使用JOIN语句扩展ActiveRecord多态关联？

我的表设置为子表与父表的关系为1:1

它们共享一个主键id：

父项具有id、类型和名称 孩子有身份证和健康 子对象是父对象的多态继承。我的目标是Child.find1应该返回一个响应name和health的子对象。SQL语句假设如下所示：

class Child < ApplicationRecord
  self.table_name = "childs"
  belongs_to :parent, foreign_key: :id, dependent: :destroy
  delegate :name, :type, to: :parent
  delegate :name=, to: :parent, allow_nil: true

  after_initialize do
    self.build_parent(type: "Child") if parent.nil?
  end
end

class Parent < ApplicationRecord
  self.inheritance_column = 'foo' # otherwise, type column will be used for STI
  has_one :child, foreign_key: :id
  delegate :health, to: :child
end

irb(main):003:0> p = Parent.joins("LEFT JOIN childs ON (childs.id = parents.id)").select("parents.id, parents.name, childs.health").find(1)
  Parent Load (0.2ms)  SELECT  parents.id, parents.name, childs.health FROM "parents" LEFT JOIN childs ON (childs.id = parents.id) WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Parent id: 1, name: "Hello">
irb(main):004:0> p.name
=> "Hello"
irb(main):005:0> p.health
  Child Load (0.1ms)  SELECT  "childs".* FROM "childs" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Good"

从parents中选择parents.id、parents.name、childs.health 左键连接childs.id=Parents.id上的childs 其中parents.id=1，parents.type='Child'限制为1

因此，我尝试在ActiveRecord中使用多态继承：

class Parent < ApplicationRecord
class Child < Parent

当我尝试执行Child.find1时，我看到：

从“parents”中选择“parents”。“parents”中键入“Child”和“parents”。“ID”=1限制1 =>

值得注意的是，子表上没有连接，但我收到了一个子对象。这会导致子对象不响应运行状况的意外行为。奇怪的是，如果我将显式表关联添加到子类self.table_name=childs，那么查询模式将更改为：

>c=Child.find1 可获得负载0.3ms从“childs”中选择“childs”。“ID”=2限制1

现在我可以访问运行状况，但不能访问类型或名称

如何正确创建此联接关联，以使加载子对象的尝试成功联接来自父对象的数据

---

编辑：这些表是在ActiveRecord迁移之外创建的，它们也被其他预先存在的非Ruby应用程序访问，因此我无法更改它们的模式。我可以想出一些奇特的元编程方法，比如响应方法_missing，这可能会让我通过连接延迟加载缺少的属性。。。但我担心我最终不得不重新实现一堆ActiveRecord，比如删除、创建等，这将导致bug。因此，我正在寻找一些本地/干净的方法来实现这一点。

这不是这里描述的典型Rails多态关联：

因此，在这种情况下，当其他应用程序较早创建表时，我建议您执行以下操作：

class Child < ApplicationRecord
  self.table_name = "childs"
  belongs_to :parent, foreign_key: :id, dependent: :destroy
  delegate :name, :type, to: :parent
  delegate :name=, to: :parent, allow_nil: true

  after_initialize do
    self.build_parent(type: "Child") if parent.nil?
  end
end

class Parent < ApplicationRecord
  self.inheritance_column = 'foo' # otherwise, type column will be used for STI
  has_one :child, foreign_key: :id
  delegate :health, to: :child
end

irb(main):003:0> p = Parent.joins("LEFT JOIN childs ON (childs.id = parents.id)").select("parents.id, parents.name, childs.health").find(1)
  Parent Load (0.2ms)  SELECT  parents.id, parents.name, childs.health FROM "parents" LEFT JOIN childs ON (childs.id = parents.id) WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Parent id: 1, name: "Hello">
irb(main):004:0> p.name
=> "Hello"
irb(main):005:0> p.health
  Child Load (0.1ms)  SELECT  "childs".* FROM "childs" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Good"

现在，您可以访问运行状况、类型和名称：

> c = Child.joins(:parent).find(1)
  Child Load (0.2ms)  SELECT  "childs".* FROM "childs" INNER JOIN "parents" ON "parents"."id" = "childs"."id" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Child id: 1, health: "Good", created_at: "2016-10-27 21:42:55", updated_at: "2016-10-27 21:44:08">
irb(main):002:0> c.health
=> "Good"
irb(main):003:0> c.type
  Parent Load (0.1ms)  SELECT  "parents".* FROM "parents" WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Child"
irb(main):004:0> c.name
=> "Hello"

父母的情况也类似：

p = Parent.joins(:child).find(1)
  Parent Load (0.1ms)  SELECT  "parents".* FROM "parents" INNER JOIN "childs" ON "childs"."id" = "parents"."id" WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Parent id: 1, type: nil, name: "Hello", created_at: "2016-10-27 21:40:35", updated_at: "2016-10-27 21:40:35">
irb(main):003:0> p.name
=> "Hello"
irb(main):004:0> p.health
  Child Load (0.1ms)  SELECT  "childs".* FROM "childs" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Good"

如果愿意，可以按如下方式指定左连接：

class Child < ApplicationRecord
  self.table_name = "childs"
  belongs_to :parent, foreign_key: :id, dependent: :destroy
  delegate :name, :type, to: :parent
  delegate :name=, to: :parent, allow_nil: true

  after_initialize do
    self.build_parent(type: "Child") if parent.nil?
  end
end

class Parent < ApplicationRecord
  self.inheritance_column = 'foo' # otherwise, type column will be used for STI
  has_one :child, foreign_key: :id
  delegate :health, to: :child
end

irb(main):003:0> p = Parent.joins("LEFT JOIN childs ON (childs.id = parents.id)").select("parents.id, parents.name, childs.health").find(1)
  Parent Load (0.2ms)  SELECT  parents.id, parents.name, childs.health FROM "parents" LEFT JOIN childs ON (childs.id = parents.id) WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Parent id: 1, name: "Hello">
irb(main):004:0> p.name
=> "Hello"
irb(main):005:0> p.health
  Child Load (0.1ms)  SELECT  "childs".* FROM "childs" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Good"

---

这不是一个典型的Rails多态关联，如下所述：

因此，在这种情况下，当其他应用程序较早创建表时，我建议您执行以下操作：

class Child < ApplicationRecord
  self.table_name = "childs"
  belongs_to :parent, foreign_key: :id, dependent: :destroy
  delegate :name, :type, to: :parent
  delegate :name=, to: :parent, allow_nil: true

  after_initialize do
    self.build_parent(type: "Child") if parent.nil?
  end
end

class Parent < ApplicationRecord
  self.inheritance_column = 'foo' # otherwise, type column will be used for STI
  has_one :child, foreign_key: :id
  delegate :health, to: :child
end

irb(main):003:0> p = Parent.joins("LEFT JOIN childs ON (childs.id = parents.id)").select("parents.id, parents.name, childs.health").find(1)
  Parent Load (0.2ms)  SELECT  parents.id, parents.name, childs.health FROM "parents" LEFT JOIN childs ON (childs.id = parents.id) WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Parent id: 1, name: "Hello">
irb(main):004:0> p.name
=> "Hello"
irb(main):005:0> p.health
  Child Load (0.1ms)  SELECT  "childs".* FROM "childs" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Good"

现在，您可以访问运行状况、类型和名称：

> c = Child.joins(:parent).find(1)
  Child Load (0.2ms)  SELECT  "childs".* FROM "childs" INNER JOIN "parents" ON "parents"."id" = "childs"."id" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Child id: 1, health: "Good", created_at: "2016-10-27 21:42:55", updated_at: "2016-10-27 21:44:08">
irb(main):002:0> c.health
=> "Good"
irb(main):003:0> c.type
  Parent Load (0.1ms)  SELECT  "parents".* FROM "parents" WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Child"
irb(main):004:0> c.name
=> "Hello"

父母的情况也类似：

p = Parent.joins(:child).find(1)
  Parent Load (0.1ms)  SELECT  "parents".* FROM "parents" INNER JOIN "childs" ON "childs"."id" = "parents"."id" WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Parent id: 1, type: nil, name: "Hello", created_at: "2016-10-27 21:40:35", updated_at: "2016-10-27 21:40:35">
irb(main):003:0> p.name
=> "Hello"
irb(main):004:0> p.health
  Child Load (0.1ms)  SELECT  "childs".* FROM "childs" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Good"

如果愿意，可以按如下方式指定左连接：

class Child < ApplicationRecord
  self.table_name = "childs"
  belongs_to :parent, foreign_key: :id, dependent: :destroy
  delegate :name, :type, to: :parent
  delegate :name=, to: :parent, allow_nil: true

  after_initialize do
    self.build_parent(type: "Child") if parent.nil?
  end
end

class Parent < ApplicationRecord
  self.inheritance_column = 'foo' # otherwise, type column will be used for STI
  has_one :child, foreign_key: :id
  delegate :health, to: :child
end

irb(main):003:0> p = Parent.joins("LEFT JOIN childs ON (childs.id = parents.id)").select("parents.id, parents.name, childs.health").find(1)
  Parent Load (0.2ms)  SELECT  parents.id, parents.name, childs.health FROM "parents" LEFT JOIN childs ON (childs.id = parents.id) WHERE "parents"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> #<Parent id: 1, name: "Hello">
irb(main):004:0> p.name
=> "Hello"
irb(main):005:0> p.health
  Child Load (0.1ms)  SELECT  "childs".* FROM "childs" WHERE "childs"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "Good"

---

杰出的非标准问题的绝佳解决方案。但快速跟进：Child.new.save当前不会创建父对象，同样，delete也不会清理联接的表。我计划自己使用before_*filters来实现簿记，但我很好奇你是否有更好的主意。我编辑了子类，这样它会自动创建和删除父记录，例如c=Child.new，然后c.name=Foo c.save和删除c.destroy注意：这绝对不是典型的Rails方式，但它可以解决这个非标准问题。太棒了！非标准问题的绝佳解决方案。但快速跟进：Child.new.save当前不会创建父对象，同样，delete也不会清理联接的表。我计划自己使用before_*filters来实现簿记，但我很好奇你是否有更好的主意。我编辑了子类，这样它会自动创建和删除父记录，例如c=Child.new，然后c.name=Foo c.save和删除c.destroy注意：这绝对不是典型的Rails方式，但它可以解决这个非标准问题。