01_models.rb

class User
  has_many :topics
  has_many :posts, through: topics
end

class Topic
  belongs_to :user
  has_many :posts
end

class Post
  belongs_to :topic
  belongs_to :user, deferred_from: topic
end

02_example.rb

@user = User.find(42)
Post.create(user: @user)

@post = @topic.posts.create(content: 'foooo')
@post.user # => #<User id: 42>
@post.user_id # => 42

# Of course, you can override it:
@post = @topic.posts.create(user: User.find(24), content: 'foooo')
@post.user # => #<User id: 24>