使用 Rails 包括对儿童的条件 [英] Using Rails includes with conditions on children

问题描述

我有一个模型 Parent 有很多孩子 Child.我想获取所有 Parent 模型并显示 Parent 的每个 Child .据我所知，这是 Rails 的 includes 方法的经典用例.

I have a model Parent that has many children Child. I want to get all Parent models and show every Child of the Parent as well. This is a classic use case for Rails' includes method, as far as I can tell.

但是，如果不将父模型限制为有子模型的模型，我无法让 Rails 向子模型添加条件.

However, I can't get Rails to add conditions to the child models without limiting the Parent models to those that have children.

例如，这仅输出有孩子的父母:

For example, this only outputs parents that have children:

Parent.includes(:children).where(children: {age: 10}).each do |parent|
  # output parent info
  parent.children.where("age = 10").each do |child|
   #output child info
  end
end

我查看了 Rails 包含条件，但似乎我有与问题的 OP 相同的麻烦，接受的答案的任何部分都不能解决它(它要么只有一些父母，要么求助于多个查询).

I've looked at Rails includes with conditions but it seems like I'm having the same trouble as the question's OP and neither part of the accepted answer doesn't solve it (it either has only some parents, or resorts to multiple queries).

推荐答案

您需要使用<代码>左加入.

Parent.joins("LEFT JOIN children ON parent.id = children.parent_id")
      .where("parent.age = 10 AND children.age = 10")
      .select("parent.*, children.*")

如果您想从 parent 表中选择在 children 表中可能有也可能没有相应行的行，您可以使用 LEFT JOIN 子句.如果 children 表中没有匹配的行，children 表中列的值将替换为 NULL 值.

If you want to select rows from the parent table which may or may not have corresponding rows in the children table, you use the LEFT JOIN clause. In case there is no matching row in the children table, the values of the columns in the children table are substituted by the NULL values.

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