Question

I have models User and Message. Every user can send messages to other users. In User there are no fields related to the Message. In Message there are the following: from_id, to_id, content.

I was trying to work it myself, but got confused after a few minutes.

What combination of has_many has_one, belongs_to should I use in each model? There should be the following methods available:

msg.from => author of the message
msg.to => receiver
usr.sent_messages => array of sent messages
usr.received_messages => array of received messages

Moreover I want the message to be destroyed when either sender or receiver is destroyed. So :dependent=>:destroy is needed somewhere

This are my attempts, which of course don’t work:

User:

has_many :sent_messages, :source => :message, :dependent => :destroy
has_many :received_messages, :source => :message, :dependent => :destroy

Message:

has_one :from, :source=>:user
has_one :to, :source=>:user

Thanks in advance,

Bye

Answer 1

You could do it like this:

## User
  has_many :received_messages, :foreign_key => :to_id, :class_name => 'Message', :dependent => :destroy
  has_many :sent_messages, :foreign_key => :from_id, :class_name => 'Message', :dependent => :destroy

## Message
  belongs_to :from, :class_name => 'User', :foreign_key => :from_id
  belongs_to :to, :class_name => 'User', :foreign_key => :to_id

Just remember that when doing associations, when you do a belongs_to it means that the object that belongs is a weak entity, so it has to contain the foreign key, when you say has_one or has_many, the object that has is the strong entity so it does not contain the foreign key.

Think about it by the existence idea, if an object can exist without it’s association, it’s a strong entity, if it can’t exist without it, it’s a weak one.