Get records based on other table

Question

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.

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.

Answer 1

Yet another option is to use includes:

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

It will make a single query to database.

Answer 2

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

A.joins(:B)

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

A.where(B: nil)