Rails multiple belongs_to assignment

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?