Rails - Oct 2008 - Revising named_scope semantics in associated classes

Suppose I have the following models:

  class Post < ActiveRecord::Base
    has_and_belongs_to_many :categories
    named_scope :active, :conditions => {:active => true}
  end

  class Category < ActiveRecord::Base
    has_and_belongs_to_many :posts
  end

I can call Post.active to get the set of all active Post objects. And
I can call some_category.posts.active to get all active Post objects
associated with some_category.

But what if I want slightly different semantics for the active posts
within a category? Perhaps there is an additional condition that
should apply in this case, as suggested by the new method shown here:

  class Category < ActiveRecord::Base
    has_and_belongs_to_many :posts

    def self.active_posts
      channels.active.all :conditions => {:show_in_category => true}
    end
  end

Instead of introducing a new active_posts method, is there any way to
define conditions that should be merged into the named_scope when it
is reference from the associated class?

-Sven
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I''m a little confused by your example. Do you mean posts instead of
channels? :show_in_category is on Post, correct?

Assuming posts and :show_in_category on Post, couldn''t you just add a
second
named_scope to Post, show_in_category (for lack of a better name)?

class Post < ActiveRecord::Base
  named_scope :active, :conditions => { :active => true }
  named_scope :show_in_category, :conditions => { :show_in_category =>
true
}
  ...
end

Then you could do some_category.posts.active or
some_category.posts.active.show_in_category as you see fit.

Thoughts?

Regards,
Craig

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Sorry, I made some mistakes when I tried to simplify my project code.
I''m trying to amend the semantics of an existing named_scope when
it''s
called from an association, not add an additional one. Here''s a more
complete example (which I hope I''ve gotten right):

  class Post < ActiveRecord::Base
    has_many :category_assignments
    has_many :categories, :through => :category_assignments
    named_scope :active, :conditions => {:active => true}
  end

  class Category < ActiveRecord::Base
    has_many :category_assignments
    has_many :posts, :through => :category_assignments
  end

  class CategoryAssignment < ActiveRecord::Base
    belongs_to :post
    belongs_to :category
    # has attribute show_in_category which allows
    # showing/hiding posts on a per-category basis
  end

I want to be able to do this

  some_category.posts.active

And get the semantics of the active_posts method below:

  class Category < ActiveRecord::Base
    has_many :category_assignments
    has_many :posts, :through => :category_assignment

    def active_posts
      posts.active.all :conditions =>
[''category_assignment.show_in_category = ?'', true]
    end
  end

So when I call

  Post.active

I get the set of all active posts. But when I call

  some_category.posts.active

I get the set of all active posts in some_category where
show_in_category is true (like calling my active_posts method, but
using a named_scope instead).

One possibility is to do something like this:

  class Category < ActiveRecord::Base
    has_many :category_assignments
    has_many :posts, :through => :category_assignment
    has_many :visible_posts, :class_name => ''Post'',
:through
=> :category_assignment,
             :conditions => [''category_assignment.show_in_category
= ?'', true]
  end

And then remember to call

  some_category.visible_posts.active

This lets me use the named scope instead of a method but the semantics
aren''t right. I''m trying to express the notion that active
category.posts are subject to an additional restriction relative to
active posts. I''m not trying to divide _posts_ into two groups.
I''m
just trying to restrict _active_ posts within a category.

-Sven
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On Wed, Oct 22, 2008 at 1:43 PM, Sven
<Sven.Aas-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> ...
> This lets me use the named scope instead of a method but the semantics
> aren''t right. I''m trying to express the notion that
active
> category.posts are subject to an additional restriction relative to
> active posts. I''m not trying to divide _posts_ into two groups.
I''m
> just trying to restrict _active_ posts within a category.

Restricting is dividing your set into those items you want and those you
don''t. Since you have attributes across two models, Post#active and
CategoryAssignment#show_in_category, though, I don''t think you can
combine
them into one named_scope. I think you''ll have to do something like you
suggested at the end of your last message, or you''ll have to wrap
everything
in a class method to have the semantics that you want.

Regards,
Craig

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