Rails SQL query for mutual friends

Question

My User model looks like this:

has_many :relationships, foreign_key: "follower_id", dependent: :destroy
has_many :followed_users, through: :relationships, source: :followed

has_many :reverse_relationships, foreign_key: "followed_id",
                                 class_name:  "Relationship",
                                 dependent:   :destroy
has_many :followers, through: :reverse_relationships, source: :follower

What I want is the raw SQL query to get users that a given user follows and is followed back by.

Currently I've only gotten as far as getting the ids of users I'm following:

followed_user_ids = "SELECT followed_id FROM relationships
                             WHERE follower_id = #{user.id}"

I have a rails methods that returns mutual friends, but I want the SQL equivalent:

def mutual_friends
    # interesect both arrays to find similar elements
  self.followed_users & self.followers
end

Answer 1

Something like this should work

SELECT followed_id
FROM relationships
WHERE follower_id = #{user.id} AND followed_id IN (
    SELECT follower_id FROM relationships WHERE followed_id = #{user.id} 
)

Answer 2

Please use it like this, let me know if it works for you.

has_many :company_friendships, autosave: true
has_many :company_friends, through: :company_friendships, autosave: true
has_many :inverse_company_friendships, class_name: "CompanyFriendship", foreign_key: "company_friend_id", autosave: true
has_many :inverse_company_friends, through: :inverse_company_friendships, source: :company, autosave: true  

def mutual_company_friends
  Company.where(id: (company_friends | inverse_company_friends).map(&:id))
end