Understanding associations in rails 3 - ruby-on-rails

Question

Seems I need to brush up on my associations in rails. At present I am trying to display all posts that have the department name as staff.
two models exist at present, posts and departments
class Post < ActiveRecord::Base
belongs_to :department
attr_accessible :title, :comments, :department_id
end
class Department < ActiveRecord::Base
has_many :posts
attr_accessible :name, :post_id
#Scopes
scope :staff_posts, where(:name => "Staff")
end
So i want to display all posts that have the department name staff
to do this i have put this in my controller
class PublicPagesController < ApplicationController
def staffnews
#staffpost = Department.staff_posts
end
end
In my view i am trying to display all these posts like so
<% #staffpost.each do |t| %>
<h2><%= t.title %>
<h2><%= t.comments %></h2>
<% end %>
Clearly going wrong somewhere as i get undefined method nil, even though i have 3 posts with the name 'Staff'
Can someone please explain where i am misunderstanding the association as would love to get this right
EDIT
Routes
scope :controller => :public_pages do
get "our_news"
match "our_news/staffnews" => "public_pages#staffnews"

Answer 1

In controller it returns department with name staff. And you are using title and comments on on department objects thats why its giving nil method error.
Use like this:
def staffnews
#dept_staff = Department.staff_posts
end
<% #dept_staff.each do |ds| %>
<% ds.posts.each do |p| %>
<h2><%= p.title %></h2>
<h2><%= p.comments %></h2>
<% end %>
<% end %>
or
In post model create named_scope
class Post < ActiveRecord::Base
belongs_to :department
attr_accessible :title, :comments, :department_id
scope :staff_posts, :include => :department, :conditions => {"departments.name" => "Staff"}
end
class Department < ActiveRecord::Base
has_many :posts
attr_accessible :name, :post_id
end
Controller:
def staffnews
#staffpost = Post.staff_posts
end
View: #No change
<% #staffpost.each do |t| %>
<h2><%= t.title %></h2>
<h2><%= t.comments %></h2>
<% end %>

Answer 2

Your staff_posts scope is only selecting the Departments with the name "Staff". Assuming you will have one and only one department named staff, you have a few ways to handle this.
This will find all departments with the name staff, and eager load the posts that go along with it:
#department = Department.where(name: "Staff").include(:posts).first
Since you are trying to scope Post, however, this belongs in Post. Here's an example using a method as scope:
class Post < ActiveRecord::Base
belongs_to :department
attr_accessible :title, :comments, :department_id
def self.staff
where(department_id: staff_department_id)
end
def staff_department_id
Department.find_by_name!("Staff").id
end
end
This way, you can use #staff_posts = Post.staff and iterate over that collection (Note: I don't recommend getting staff_department_id this way permanently. This could be set to a constant when the app boots up, or some other more robust solution).

Answer 3

You can find the all the posts that have the department name staff by following changes:
class PublicPagesController < ApplicationController
def staffnews
#get all the department which have name is staff
departments = Department.where("name=?","staff")
#get all the ids
department_ids = departments.map(&:id)
#retrieve post that department name is staff
#staffpost = Post.find_by_department_id(department_ids)
end
end