How to model a mutual friendship in Rails

Question

I know this question has been asked before on Stack Overflow, but the answers aren\'t doing it for me in ways I can explain. My general approach was inspired by this tutoria

Answer 1

This is also achievable with a single has_many :through association and some query fiddling:

# app/models/friendship.rb

class Friendship < ApplicationRecord
  belongs_to :user
  belongs_to :friend, class_name: 'User'
end

 

# app/models/user.rb

class User < ApplicationRecord
  has_many :friendships,
    ->(user) { FriendshipsQuery.both_ways(user_id: user.id) },
    inverse_of: :user,
    dependent: :destroy

  has_many :friends,
    ->(user) { UsersQuery.friends(user_id: user.id, scope: true) },
    through: :friendships
end

 

# app/queries/friendships_query.rb

module FriendshipsQuery
  extend self

  def both_ways(user_id:)
    relation.unscope(where: :user_id)
      .where(user_id: user_id)
      .or(relation.where(friend_id: user_id))
  end

  private

  def relation
    @relation ||= Friendship.all
  end
end

 

# app/queries/users_query.rb

module UsersQuery
  extend self

  def friends(user_id:, scope: false)
    query = relation.joins(sql(scope: scope)).where.not(id: user_id)

    query.where(friendships: { user_id: user_id })
      .or(query.where(friendships: { friend_id: user_id }))
  end

  private

  def relation
    @relation ||= User.all
  end

  def sql(scope: false)
    if scope
      <<~SQL
        OR users.id = friendships.user_id
      SQL
    else
      <<~SQL
        INNER JOIN friendships
          ON users.id = friendships.friend_id
          OR users.id = friendships.user_id
      SQL
    end
  end
end

It may not be the simplest of them all but it's certainly the DRYest. It does not use any callbacks, additional records and associations, and keeps the association methods intact, including implicit association creation:

user.friends << new_friend

via gist.github.com