Rails belongs_to issue in the views

Im having problems with an association in rails:

Currently I have Post and User models, and the relationship is set this way:

class User < ActiveRecord::Base
  attr_accessible :username, :name, :lastname
  has_many :posts
end

class Post < ActiveRecord::Base
  attr_accessible :title, :body
  belongs_to :开发者_运维百科user
end

However, in my app/views/posts/index.html.haml when Im trying to access the username for the post I get this error:

undefined method `name' for nil:NilClass

This is my view:

- title "Posts"

%table
  %tr
    %th Title
    %th Body
    %th Author
  - for post in @posts
    %tr
    %td= h post.title
    %td= h post.body
    %td= h post.user.name
    %td= link_to 'Show', post
    %td= link_to 'Edit', edit_post_path(post)
    %td= link_to 'Destroy', post, :confirm => 'Are you sure?', :method => :delete

%p= link_to "New Post", new_post_path

Any thoughts of what Im doing wrong will be appretiated

First of all check if posts table has a column named user_id (you can lookup this table in db/schema.rb)

If not, you probably need to create a migration:
$ ruby script/generate migration AddUserIdToPosts

class AddUserIdToPosts < ActiveRecord::Migration
  def self.up
    add_column :posts, :user_id, :integer
  end

  def self.down
    remove_column :posts, :user_id
  end
end

And then apply changes to the database and schema.rb
$ rake db:migrate

Based on your comments, it looks like you are not associating the actual User data with the Post data. You have the associations set up, and after Andrew's answer you seem to now have the database schema set up, but you are not saving the data anywhere.

To do so, update your create method to be something like this:

def PostsController < ActionController::Base
  def create
    # Assumes @user is already set to whatever user should be associated with the post
    post = @user.posts.build(params[:post]) # Where params[:post] are the post inputs from your form
    # ...
    if post.save
      flash[:notice] = "Successfully created post."
      redirect_to post
    else
      render :action => 'new'
    end
  end
end

@user.posts.build tells Rails to create a new Post (in memory) and associate it to the @user User model. An equivalent (but lengthier) way of expressing this is:

post = Post.new(post[:params])
post.user = @user

See the documentation for ActiveRecord::Associations::ClassMethods for more information about what you can do with associations.