gistfile1.md

class Post < ActiveRecord::Base
  belongs_to :author, :class_name => 'User'
end

class User < ActiveRecord::Base
  has_many :posts, :foreign_key => 'author_id'
end

So to build a post for current_user we can do the following:

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

But what if the @post has to be built using multiple steps like:

@post.author = author
@post.editor = editor
@post.category = category

There are two problems I see with the above:

1. It's possible to forget one of the objects and you can have inconsistent data. You can of course use validations to get rid of this problem.
2. You need to repeat those steps every time you want to build and save a @post. It also looks brittle in a controller imho.

So would the following make sense

@post = Post.build(params[:post], current_user, editor, category)

i.e define a .build method as follows

class Post < ActiveRecord::Base
  def self.build(params, author, editor, category)
    post = Post.new(params[:post])
    post.author = author
    post.editor = editor
    post.category = category

    post
  end
end

This isolates the steps required to build a @post and thus changes can be done in one place (but also update the callers but that's a smaller less error prone change compared to prev case). Also you can write unit tests for .build method.

What do you think?

Yes I missed that. There is no build method on the klass itself.