null user_id has_many :through

gistfile1.rb

#### User Model ####
class User < ActiveRecord::Base

  devise :database_authenticatable, :registerable, :confirmable,
         :recoverable, :rememberable, :trackable, :validatable

  attr_accessible :email, :password, :password_confirmation, 
                  :remember_me, :username, :profile_image, 
                  :first_name, :last_name

  has_many :publications
  has_many :posts, :through => :publications

end

#### Posts Model ####
class Post < ActiveRecord::Base
  attr_accessible :description, :name, :tag_list, :thumbnail_image, :status, :post_type, :subtitle, :summary

  has_one :publication
  has_one :user, :through => :publication

end


#### Controller ####
def create
  @user = User.find(params[:user_id])
  @post = @user.posts.new(params[:post])
  @post.build_publication(user_id: @user)


  if @post.save
    redirect_to post_wizard_path(:id => "step_two", :post_id => @post.id)
  else
    redirect_to browse_path
  end

end


#### View ####
= form_for [@user, @post] do |f|
  - if @post.errors.any?
    #error_explanation
      %h2= "#{pluralize(@post.errors.count, "error")} prohibited this post from being saved:"
      %ul
        - @post.errors.full_messages.each do |msg|
          %li= msg

  .field
    %span Title:
    = f.text_field :name

  .actions
    = f.submit 'Submit'   
    
  
  #### Log ####
  Started POST "/users/3/posts" for 127.0.0.1 at 2013-06-30 08:04:23 -0600
Processing by PostsController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"WPg02ogJnTnwu4+GV0daLr/xxpLhGioaCqlIOJIyHZ8=", "post"=>{"post_type"=>"standard"}, "commit"=>"Continue", "user_id"=>"3"}
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 3 LIMIT 1
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", "3"]]
  CACHE (0.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", "3"]]
   (0.3ms)  BEGIN
  SQL (0.6ms)  INSERT INTO "posts" ("created_at", "description", "post_image_id", "post_type", "name", "slug", "status", "subtitle", "summary", "thumbnail_image", "updated_at", "user_id") VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12) RETURNING "id"  [["created_at", Sun, 30 Jun 2013 14:04:23 UTC +00:00], ["description", nil], ["post_image_id", nil], ["post_type", "standard"], ["name", nil], ["slug", nil], ["status", nil], ["subtitle", nil], ["summary", nil], ["thumbnail_image", nil], ["updated_at", Sun, 30 Jun 2013 14:04:23 UTC +00:00], ["user_id", nil]]
  SQL (0.6ms)  INSERT INTO "publications" ("created_at", "post_id", "updated_at", "user_id") VALUES ($1, $2, $3, $4) RETURNING "id"  [["created_at", Sun, 30 Jun 2013 14:04:23 UTC +00:00], ["post_id", 176], ["updated_at", Sun, 30 Jun 2013 14:04:23 UTC +00:00], ["user_id", 3]]
   (1.9ms)  COMMIT
  Publication Load (0.6ms)  SELECT "publications".* FROM "publications" WHERE "publications"."post_id" = 176 LIMIT 1
   (0.1ms)  BEGIN
   (0.4ms)  UPDATE "publications" SET "post_id" = NULL, "updated_at" = '2013-06-30 14:04:23.710774' WHERE "publications"."id" = 43
   (0.3ms)  COMMIT
   (0.1ms)  BEGIN
  SQL (0.3ms)  INSERT INTO "publications" ("created_at", "post_id", "updated_at", "user_id") VALUES ($1, $2, $3, $4) RETURNING "id"  [["created_at", Sun, 30 Jun 2013 14:04:23 UTC +00:00], ["post_id", 176], ["updated_at", Sun, 30 Jun 2013 14:04:23 UTC +00:00], ["user_id", 1]]
   (0.4ms)  COMMIT
Redirected to http://localhost:3000/posts/176/wizard/step_two
Completed 302 Found in 29ms (ActiveRecord: 7.4ms)