Rails - Feb 2006 - Using :include with has_one vs. has_many

While I''m happy to continue talking about the vagaries of Rails
deployments, I
also need to actually build apps. :)

I may have discovered a bit of a problem with :include as it applies to has_one 
associations. First, let me say that I''m a big fan of :include for
reducing the
number of SQL calls. There are times when I wish it could go more than 1 step 
away from the parent object, but I kind of understand why it can''t.
Now, on to
my "discovery"...

Completely contrived example follows:


I have the following Models:

class Author < ActiveRecord::Base
   has_many :posts
end

class Post < ActiveRecord::Base
   belongs_to :author
   has_one :category
end

class Category < ActiveRecord::Base
   belongs_to :post
end

There are many authors in the database that no posts, and there are several 
posts in the database that have no category.

If I do:

   @authors = Author.find(:all, :include => :posts)

Every entry in the @authors array will have a posts list attached to it. This 
list will be empty if there are no posts for that author, which is great for 
handling associated posts in a view.

However, if I do:

   @posts = Post.find(:all, :include => :category)

We actually see something different (and I think, inconsistent). Only those post
objects that have an associated category will have a category entry in their 
hash. Those without, don''t have a category entry.

Sorry for the length... here''s my actual question. :)

Why doesn''t :include result in an empty object in those posts without
an
associated category record? If I loop over @posts and make a call to:

post.category.nil?

ActiveRecord ends up making additional SQL calls for every case where that 
statement evals to true. Why doesn''t the :include "work" for
these cases?

OK, so I really had 2 questions...

-Brian

I think you have your Category associations backwards.  belongs_to goes 
in the model with the foreign key.
> class Author < ActiveRecord::Base
>    has_many :posts
> end
> 
> class Post < ActiveRecord::Base
>    belongs_to :author
>    has_one :category
> end
> 
> class Category < ActiveRecord::Base
>    belongs_to :post
> end
A Category would have many posts, not belong to only one. Try this:

class Post < ActiveRecord::Base
  belongs_to :author
  belongs_to :category
end

class Category < ActiveRecord::Base
  has_many :posts
end


--josh

-- 
Posted via http://www.ruby-forum.com/.

Joshua Susser wrote:
> I think you have your Category associations backwards.  belongs_to goes 
> in the model with the foreign key.
> 
>> class Author < ActiveRecord::Base
>>    has_many :posts
>> end
>>
>> class Post < ActiveRecord::Base
>>    belongs_to :author
>>    has_one :category
>> end
>>
>> class Category < ActiveRecord::Base
>>    belongs_to :post
>> end
> 
> A Category would have many posts, not belong to only one. Try this:
> 
> class Post < ActiveRecord::Base
>   belongs_to :author
>   belongs_to :category
> end
> 
> class Category < ActiveRecord::Base
>   has_many :posts
> end
Nope... I typed exactly what I meant. But, remember, my example was deliberately
contrived to show what I''m seeing. Yes, a category could have multiple
posts,
but in this case they don''t.

Also, I''m not looking for help in making "proper"
associations (I''m already
pretty good at data normalization ;). I''m trying to understand a
behavior I''m
seeing with has_one associations being used with :include vs. the behavior of 
has_many associations.

-Brian

Yep, as far as I can see through the code, if there is no
corresponding record in the database for a has_one association Rails
leaves this association uninitialized. Meaning that the first time you
try to access it, it''ll try to load it again.

Submit the request for improvement.

Kent

On 2/17/06, Brian V. Hughes <brianvh@alum.dartmouth.org>
wrote:
> Joshua Susser wrote:
> > I think you have your Category associations backwards.  belongs_to
goes
> > in the model with the foreign key.
> >
> >> class Author < ActiveRecord::Base
> >>    has_many :posts
> >> end
> >>
> >> class Post < ActiveRecord::Base
> >>    belongs_to :author
> >>    has_one :category
> >> end
> >>
> >> class Category < ActiveRecord::Base
> >>    belongs_to :post
> >> end
> >
> > A Category would have many posts, not belong to only one. Try this:
> >
> > class Post < ActiveRecord::Base
> >   belongs_to :author
> >   belongs_to :category
> > end
> >
> > class Category < ActiveRecord::Base
> >   has_many :posts
> > end
>
> Nope... I typed exactly what I meant. But, remember, my example was
deliberately
> contrived to show what I''m seeing. Yes, a category could have
multiple posts,
> but in this case they don''t.
>
> Also, I''m not looking for help in making "proper"
associations (I''m already
> pretty good at data normalization ;). I''m trying to understand a
behavior I''m
> seeing with has_one associations being used with :include vs. the behavior
of
> has_many associations.
>
> -Brian
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

--
Kent
---
http://www.datanoise.com