Postgres加入Rails查询

Question

I have researched a lot and I can't find the answer for my question. I have the following setup on my Rails app:

class Group < ActiveRecord::Base
  has_many :people
  # ...
end

class City < ActiveRecord::Base
  has_many :people
  # ...
end

class Person < ActiveRecord::Base
  belongs_to :city
  belongs_to :group
  # ...
end

The people have the column :role that is 0 or 1 .

I want to get all the groups that have at least one person with role == 0 and one person with the role == 1 .

Any idea? I'm using Postgres by the way.

Answer 1

Here's a query I just tested on my SQLite3 database (should work on Postgres too I believe):

 Group.select("groups.*").joins("LEFT JOIN people on groups.id = people.group_id").where("people.role==0 OR people.role==1").group("id")

Here I assume you've already added the foreign key group_id in your people migration. Hope this helps.