根据其他表获取记录

I'm very new to Ruby and have no idea to do this. I need to create a select based on 2 tables: A and B. Both are ActiveRecord.

我是Ruby的新手，不知道怎么做。我需要基于两个表创建一个select: a和b，它们都是ActiveRecord。

class A < ActiveRecord::Base
  belongs_to :b
end

class B < ActiveRecord::Base
  has_one :a
end

There are A records with no B records associated. I would like to get those records with this problem.

有一个记录没有关联的B记录。我想把这些记录和这个问题联系起来。

4 个解决方案

#1

In Rails, the belongs_to association will mean that table a has a field called b_id. This means that the A table has direct access to it's association to B.

在Rails中，belongs_to关联意味着表a有一个名为b_id的字段。这意味着A表可以直接访问它与B的关联。

To get the A records without any B attached, run A.where(b_id: nil), this will return an array of all A records without a B

要获得没有任何B的A记录，运行A。在哪里(b_id: nil)，它将返回一个数组，其中包含所有没有B的A记录

Edit: As coorasse stated, you can also just run A.where(b: nil), which Rails will interpret as A.where(b_id: nil).

编辑:正如coorasse所说，你也可以运行A。其中(b: nil)， Rails将把它解释为A。在哪里(b_id:nil)。

#2

You can retrieve A records with a B record associated with:

您可以检索与以下相关的B记录:

A.joins(:B)

if you need all A records which don't have an association with B you can execute:

如果你需要所有与B没有关联的A记录，你可以执行:

A.where(B: nil)

#3

Yet another option is to use includes:

另一个选择是使用包括:

A.includes(:b).where(bs: { id: nil })

It will make a single query to database.

它将对数据库执行单个查询。

#4

Hhhmm..you can do something like,

Hhhmm . .你可以这样做，

unassociated_ids = A.pluck(:id) - B.pluck(:a_id).uniq
A.where(id: unassociated_ids)

Hope it helps..

希望它能帮助. .

#1