Question

Given

User:

class User < ActiveRecord::Base
   has_many :discussions
   has_many :posts
end

Discussions:

class Discussion < ActiveRecord::Base
    belongs_to :user
    has_many :posts
end

Posts:

class Post < ActiveRecord::Base
    belongs_to :user
    belongs_to :discussion 
end

I am currently initializing Posts in the controller via

@post = current_user.posts.build(params[:post])

My question is, how do I set/save/edit the @post model such that the relationship between the post and the discussion is also set?

Answer 1

Save and edit discussions along with post

Existing Discussion

To associate the post you’re building with an existing discussion, just merge the id into the post params

@post = current_user.posts.build(
          params[:post].merge(
            :discussion_id => existing_discussion.id
        ) 

You will have to have a hidden input for discussion id in the form for @post so the association gets saved.

New Discussion

If you want to build a new discussion along with every post and manage its attributes via the form, use accepts_nested_attributes

class Post < ActiveRecord::Base
  belongs_to :user
  belongs_to :discussion
  accepts_nested_attributes_for :discussion
end

You then have to build the discussion in the controller with build_discussion after you built the post

@post.build_discussion

And in your form, you can include nested fields for discussions

form_for @post do |f|
  f.fields_for :discussion do |df|
    ...etc

This will create a discussion along with the post. For more on nested attributes, watch this excellent railscast

Better Relations

Furthermore, you can use the :through option of the has_many association for a more consistent relational setup:

class User < ActiveRecord::Base
  has_many :posts
  has_many :discussions, :through => :posts, :source => :discussion
end

class Discussion < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user
  belongs_to :discussion 
end

Like this, the relation of the user to the discussion is maintained only in the Post model, and not in two places.