Use model association

Bad Smell

class PostsController < ApplicationController
  def create
    @post = Post.new(params[:post])
    @post.user_id = current_user.id
    @post.save
  end
end

In this example, user_id is assigned to @post explicitly. It's not too big problem, but we can save this line by using model association.

Refactor

class PostsController < ApplicationController
  def create
    @post = current_user.posts.build(params[:post])
    @post.save
  end
end

class User < ActiveRecord::Base
  has_many :posts
end

We define the association that user has many posts, then we can just use current_user.posts.build or current_user.posts.create to generate a post, and the current_user's id is assigned to the user_id of the post automatically by activerecord.