Rails一对多关联查询

Question

Models:

class User < ActiveRecord::Base
  has_may :friends
end

class Friend < ActiveRecord::Base
  belongs_to :user
end

Tables:

To Query:

users which both has:

friends.name = "Lily" 
# and 
friends.name = "Steven"

I want by Model Where Query like:

User.joins(xxx).where(xxx)

Is there any way to do?

Answer 1

I think

User.joins(:friends).where("friends.name = 'Lily' OR friends.name = 'Steven'")

will do what you are looking for

Answer 2

SELECT users.id, users.name 
FROM users 
INNER JOIN friends ON users.id = friends.user_id 
WHERE friends.name IN ('Lily', 'Steven') 
GROUP BY users.id 
HAVING COUNT(*) = 2;

ActiveRecord Query

names = ['Lily', 'Steven']

User.select('users.id, users.name').joins(:friends).where(friends: { name: names }).group('users.id').having("COUNT(*) = #{names.count}")

SQLFiddle

The query above will return you only the users who are friends with "Lily" and "Steven"