Question

Message has_many user_messages. It has two.

1 UserMessage will have the sender as user_id

The other UserMessage will have the receiver as user_id

Knowing one user, how can I, for any message, find the other?

Here’s what I tried as a method to the Message class, but it fails:

 32   def other_user(one_user)      
 33     um = self.user_messages
 34     um.each do |user_message|
 35       
 36       output_user = User.find(user_message.user_id) unless user_message.user_id == one_user.id
 37     end
 38     
 39     return output_user
 40   end

Answer 1

This could be implemented as an association enhancement. This will be efficient for high-volume of messages as it pushes processing to DB.

class User < ActiveRecord::Base

    has_many :sent_messages, :class_name => "Message", :foreign_key => "sender_id"
    has_many :receivers, :through => :sent_messages, :source => :receiver do 
        def for_message(message_id)
            where("messages.id = ?", message_id)
        end
    end

    # Another association mapping for received_messages

end


class Message < ActiveRecord::Base

    belongs_to :sender, :class_name => "User", :foreign_key => "sender_id"
    belongs_to :receiver, :class_name => "User", :foreign_key => "receiver_id"

end


#Usage
User.first.sent_messages

#all users received messages from first user
User.first.receivers

#all users received messages from first user for message of message_id
User.first.receivers.for_message(message_id)