Association between two models rails 3 - ruby-on-rails

Question

This should be somewhat simple but cant seem to grasp the association.
I am using nested_form and paperclip. I have a model called photo to store all images and a post model. I am trying to show the photos associated to the relevant post but am getting 'undefined method avatar' when rendering the view.
class Post < ActiveRecord::Base
has_many :photos, :dependent => :destroy
accepts_nested_attributes_for :photos
attr_accessible :title, :comments, :photo_id, :avatar, :photos_attributes
end
Class Photo < ActiveRecord::Base
belongs_to :post
attr_accessible :avatar, :post_id
has_attached_file :avatar, :styles => { :medium => "300x300>", :thumb => "100x100>" }
end
Controller
def new
#post = Post.new
#post.photos.build
end
So i am under the impression that when a post gets built an association between the Post and Photo model is made? is that right?
So when i call this in the view i get the undefined method, can anyone advise where I am going wrong please
<% #posts.each do |f| %>
<ul>
<li><%= f.title %></li>
<li><%= f.department.name %></li>
<li><%= image_tag f.avatar.url(:thumb) %></li>
<li><%= link_to "Delete Post", post_path(f.id), :confirm => "Are you sure?", :method => :delete %></li>
</ul>
<% end %>
I have tried
<%= image_tag f.photo.avatar.url(:thumb) %>
but that doesnt work either

Answer 1

May be you are creating photo wrong.
Here you can see how the form looks: Nested form using paperclip
And also Post has_many :photos, so it must be somth. like
<% #posts.each do |f| %>
....
<% f.photos.each do |photo| %>
<%= image_tag photo.avatar.url(:thumb) %>
<% end %>
...
<% end %>

Answer 2

When I work with nested attributes I follow three steps. First, in the parent model you can use accepts_nested_attributes_for:
Class Post
has_many :photos, dependent: :destroy
accepts_nested_attributes_for :photos
attr_accessible :photos_attributes
end
Second, you can incorporate a nested form for photos so you can set the attributes of photos for that particular post:
<%= form_for(#post) do |f| %>
<%= f.fields_for :photos do |p| %>
...rest of form here...
Third, you can create a photo through the new action in the post model:
Class UserController
def new
#user = User.new(photos: Photo.new)
end
end
This last step is important. If you don't do this, you would not see the photo fields in the user form otherwise. If you follow these steps you should be able to set all the attributes from both photos and users in the users form.

Answer 3

I think in your controller you should first define which is the post object you are associating to :
def new
#post = Post.find(params[:post_id]
#photo = #post.photos.build
....
end
The same is in the create action of the PhotosController .