Rails - Nov 2008 - How to make named_scope aware of association context?

Given these models

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

  class CategoryAssignment < ActiveRecord::Base
    belongs_to :post
    belongs_to :category
    # Has boolean column ''featured''
  end

  class Post < ActiveRecord::Base
    has_many :category_assignments
    has_many :categories, :through => :category_assignments
  end

I want to add a named_scope to Post that will return all featured
posts. But the semantics of this named_scope vary depending upon
whether I am calling the named scope on Post or on category.posts:
  Post.featured
is simple enough. The definition
  named_scope :featured,
              :joins => :category_assignments,
              :conditions => {:category_assignments.featured => true }
will return all posts for which any associated category_assignment
record has featured = true.

But when I take a category instance and call category.posts.featured I
want to get all of the posts which are featured *in that category*,
which requires restricting by category_id. To do this properly the
named_scope code needs to be able to determine whether it''s being
called in a context in which category_assignments has already been
joined (and/or whether a category_id is specified in the conditions).
I have not figured out how to do this.

If I can detect whether or not the named_scope is being called within
the context of an association then I can do something like the
following, which changes named_scope semantics based on whether or not
a category _argument_ is explicitly provided:

  named_scope :featured, lambda { |*args|
    if (category = args.first)
      { :joins => :category_assignments,
        :conditions => {:category_id => category.id,
                        :featured    => true} }
    else
      { :joins => :category_assignments,
        :conditions => {:featured    => true} }
    end
  }

Is there some way to inspect a joins or conditions hash to determine
association state from within a named_scope? And if so, is it reliable
regardless of the order of named_scope stacking (i.e. will it work for
either category.posts.other.named.scopes.featured or
category.posts.featured.other.named.scopes)?

Thanks,

Sven
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On Nov 24, 5:43 pm, Sven
<Sven....-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Given these models
>
>   class Category < ActiveRecord
>     has_many :category_assignments
>     has_many :posts, :through => :category_assignments
>   end
>
>   class CategoryAssignment < ActiveRecord::Base
>     belongs_to :post
>     belongs_to :category
>     # Has boolean column ''featured''
>   end
>
>   class Post < ActiveRecord::Base
>     has_many :category_assignments
>     has_many :categories, :through => :category_assignments
>   end
>
> I want to add a named_scope to Post that will return all featured
> posts. But the semantics of this named_scope vary depending upon
> whether I am calling the named scope on Post or on category.posts:
>   Post.featured
> is simple enough. The definition
>   named_scope :featured,
>               :joins => :category_assignments,
>               :conditions => {:category_assignments.featured => true
}
> will return all posts for which any associated category_assignment
> record has featured = true.
>
> But when I take a category instance and call category.posts.featured I
> want to get all of the posts which are featured *in that category*,
> which requires restricting by category_id. To do this properly the
It should do that by itself. You shouldn''t need to do anything. Have
you tried it?

Fred

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On Mon, Nov 24, 2008 at 12:49 PM, Frederick Cheung <
frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> It should do that by itself. You shouldn''t need to do anything.
Have
> you tried it?
>
> Fred
>
Yes, with code like this:

  named_scope :featured, :joins => :category_assignments,
                       :conditions =>
[''category_assignments.featured = ?'',
true ]

When I call

  Post.featured

it works fine. But when I call, category.posts.featured I get

  ActiveRecord::StatementInvalid: SQLite3::SQLException: ambiguous column
name: category_assignments.post_id

because Rails generates this query:

  SELECT "posts".* FROM "posts"
  INNER JOIN "category_assignments" ON category_assignments.post_id
post.id
  INNER JOIN category_assignments ON posts.id category_assignments.post_id
  WHERE (("category_assignments".category_id = 1))
  AND   ((category_assignments.featured = ''t''))

As you can see, category_assignments is being joined twice. The named_scope
needs to omit the category_assignments join when it gets called on a
Category instance.

I''m running this on Rails 2.1.1, by the way.

-Sven

PS: there was an error in my initial post. I specified
  :conditions => {:category_assignments.featured => true }
where I ought to have written
  :conditions => [''category_assignments.featured = ?'',
true]

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On 24 Nov 2008, at 18:07, Sven Aas wrote:
> On Mon, Nov 24, 2008 at 12:49 PM, Frederick Cheung
<frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
> > wrote:
> It should do that by itself. You shouldn''t need to do anything.
Have
> you tried it?
>
> Fred
>
> Yes, with code like this:
>
>   named_scope :featured, :joins => :category_assignments,
>                        :conditions =>  
> [''category_assignments.featured = ?'', true ]
>
> When I call
>
>   Post.featured
>
> it works fine. But when I call, category.posts.featured I get
>
>   ActiveRecord::StatementInvalid: SQLite3::SQLException: ambiguous  
> column name: category_assignments.post_id
>
> because Rails generates this query:
>
>   SELECT "posts".* FROM "posts"
>   INNER JOIN "category_assignments" ON
category_assignments.post_id
> = post.id
>   INNER JOIN category_assignments ON posts.id =  
> category_assignments.post_id
>   WHERE (("category_assignments".category_id = 1))
>   AND   ((category_assignments.featured = ''t''))
>
> As you can see, category_assignments is being joined twice. The  
> named_scope needs to omit the category_assignments join when it gets  
> called on a Category instance.
>
Oops. I skimmed over the definition of your scope and didn''t notice  
that there was a join. I''m going out on a limb here but from inside a  
procedural named scope then if there is a @proxy_owner instance  
variable or if it responds to proxy_owner/proxy_target then it;s an  
association. You could also  check the current scope and see what  
joins are in there.

Fred
> I''m running this on Rails 2.1.1, by the way.
>
> -Sven
>
> PS: there was an error in my initial post. I specified
>   :conditions => {:category_assignments.featured => true }
> where I ought to have written
>   :conditions => [''category_assignments.featured = ?'',
true]
>
> >

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That does seem promising, but I haven''t gotten it to work yet. Both
proxy_owner and proxy_target remain undefined (on self) and
@proxy_owner remains null regardless of which way I invoke the
named_scope lambda. You also suggested checking the current scope for
joins but that''s precisely the thing I don''t know how to do,
which
prompted my initial post. I''m still working on it, though.

-Sven
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