Using Rails' build() method through a renamed association

Question

I have a Message model containing

belongs_to :from, class_name: "User"
belongs_to :to, class_name: "User"

When I do something like

user.messages.build(text: "my message")

how can I ensure the user put into the "from" association is the building user? How does Rails know whether to put it into "from" or "to"?

Answer 1

I think the correct way to set up the message model would be like this:

belongs_to :sender, :foreign_key => :sender_id, class_name: 'User'
belongs_to :recipient, :foreign_key => :recipient_id, class_name: 'User'

Your migration would be like

create_table :messages do |t|
  t.integer :sender_id
  t.integer :recipient_id
  t.timestamps
end

Then, in the user model:

has_many :messages, :foreign_key => :sender_id

Then, in the message form you could set the sender like this:

<%= f.hidden_field :sender_id, value: current_user.id %>
<%= f.hidden_field :recipient_id, value: @user.id %>

You could also set the parameters like in your example in the question like this:

Message.create(sender_id: 1, recipient_id: 3)

This should let you do things like:

@message.sender.username
@message.recipient.username