Rails - Apr 2006 - RANT: belongs_to -> refers_to

Every once in a while, I pipe up and whine that "belongs_to" should
really
be ''refers_to" [http://dev.rubyonrails.org/ticket/2130]. 
It''s time again.

I''ve got a bunch of stuff going onto eBay, so, like any good engineer,
I
don''t just post it manually: I design a Rails-based inventory database
that
creates a semantically-correct XHTML/CSS auction posting with a variable
number of thumbnails (stored with file_column) and Textile fields.

I start out with 

class Item < ActiveRecord::Base
  :has_many :pictures
end class

But I want one of those pictures to be featured as the gallery photo.  So,
naturally, I gravitate towards adding 

class Item < ActiveRecord::Base
  :has_one primary_picture, :class_name => ''Picture''
end

I am encouraged in this futile pursuit by the docs, which I quickly skim to
see things like

  :has_one :last_comment, :class_name => "Comment", :order =>
"posted_on"

Of course, in the light of day, it''s clear that this automagically
creates
the last_comment association by picking the most recent comment.  But at
3am, it just reinforced the tendency to use has_one.  Ditto the many other
has_one examples I found - half of which used the automatic-first-record
feature, and half of which were probably just making the same mistake I
was.

I started getting nil objects, and was mystified until I finally tried
explicitly setting the foreign key:

  :has_one primary_picture, :class_name => ''Picture'', 
    :foreign_key => ''primary_picture_id''

at which point Rails helpfully told me that there was no
Picture.primary_picture_id field.  Then I remembered for the umpteenth time
that has_one is for the record that does NOT have the foreign key, and
belongs_to is for the one that does, and I should not be misled by the
semantics of the actual has_one and belongs_to keywords.

Item :belongs_to primary_picture makes no sense.  Item :refers_to
primary_picture does.  This is way too small for even a plugin - and I
don''t think that type of syntax change belongs in a plugin, anyway.  I
have
yet to hear anyone who''s opposed to it... can''t we PLEASE add
this alias
and consider deprecating belongs_to over time?  It''s... just... wrong.

Jay Levitt

I''m opposed to it.  I''m not sure if your problem is with
grammar or
with the visual abstraction of what you are trying to do, but the  
semantics are correct.  Think of it this way:

Let''s say I have 50 widgets that I want to sell on eBay.  Each of  
these widgets will have its own auction posting.  However, all the  
widgets look alike, so I have a single picture of the widget.  I  
don''t want to have 50 pictures of my widget, so it would be incorrect  
to say that each widget has_one picture.  Instead, each widget  
belongs_to that picture of a widget.

Now, you are using the term Item, so perhaps you have another model  
to handle postings of the Item on eBay.  This would change the  
argument above slightly, but the semantics remain the same.  If a  
picture is used on only a single Item, but an item might have  
multiple pictures (which is how I interpret this domain), then the  
foreign key should be on the picture, not on the Item, and then you  
would be correctly using the has_one and has_many.

Now, by placing the foreign key on the picture, you can achieve the  
concept of a primary picture by placing a boolean flag on the picture  
tuple akin to IsPrimary.  Then set up your item model with a has_many  
picture and a has_one primary picture (with the constraint that it  
must have the IsPrimary flag set).

OR... you can have a has_may :pictures, belongs_to :primary_picture

OR... you can use acts_as_list on your has_many and maintain that the  
first item in the list is the primary picture.  I kinda like this  
approach.

-Derrick Spell


On Apr 2, 2006, at 11:12 AM, Jay Levitt wrote:
> Every once in a while, I pipe up and whine that "belongs_to"
should
> really
> be ''refers_to" [http://dev.rubyonrails.org/ticket/2130]. 
It''s time
> again.
>
> I''ve got a bunch of stuff going onto eBay, so, like any good  
> engineer, I
> don''t just post it manually: I design a Rails-based inventory  
> database that
> creates a semantically-correct XHTML/CSS auction posting with a  
> variable
> number of thumbnails (stored with file_column) and Textile fields.
>
> I start out with
>
> class Item < ActiveRecord::Base
>   :has_many :pictures
> end class
>
> But I want one of those pictures to be featured as the gallery  
> photo.  So,
> naturally, I gravitate towards adding
>
> class Item < ActiveRecord::Base
>   :has_one primary_picture, :class_name => ''Picture''
> end
>
> I am encouraged in this futile pursuit by the docs, which I quickly  
> skim to
> see things like
>
>   :has_one :last_comment, :class_name => "Comment", :order
=>
> "posted_on"
>
> Of course, in the light of day, it''s clear that this automagically
> creates
> the last_comment association by picking the most recent comment.   
> But at
> 3am, it just reinforced the tendency to use has_one.  Ditto the  
> many other
> has_one examples I found - half of which used the automatic-first- 
> record
> feature, and half of which were probably just making the same  
> mistake I
> was.
>
> I started getting nil objects, and was mystified until I finally tried
> explicitly setting the foreign key:
>
>   :has_one primary_picture, :class_name => ''Picture'',
>     :foreign_key => ''primary_picture_id''
>
> at which point Rails helpfully told me that there was no
> Picture.primary_picture_id field.  Then I remembered for the  
> umpteenth time
> that has_one is for the record that does NOT have the foreign key, and
> belongs_to is for the one that does, and I should not be misled by the
> semantics of the actual has_one and belongs_to keywords.
>
> Item :belongs_to primary_picture makes no sense.  Item :refers_to
> primary_picture does.  This is way too small for even a plugin - and I
> don''t think that type of syntax change belongs in a plugin,  
> anyway.  I have
> yet to hear anyone who''s opposed to it... can''t we PLEASE
add this
> alias
> and consider deprecating belongs_to over time?  It''s... just...
wrong.
>
> Jay Levitt
>
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

On Sun, Apr 02, 2006 at 11:12:47AM -0400, Jay Levitt wrote:
} Every once in a while, I pipe up and whine that "belongs_to" should
really
} be ''refers_to" [http://dev.rubyonrails.org/ticket/2130]. 
It''s time again.
[...]
} Item :belongs_to primary_picture makes no sense.  Item :refers_to
} primary_picture does.  This is way too small for even a plugin - and I
} don''t think that type of syntax change belongs in a plugin, anyway. 
I have
} yet to hear anyone who''s opposed to it... can''t we PLEASE
add this alias
} and consider deprecating belongs_to over time?  It''s... just...
wrong.

In some contexts it is wrong. In others it is right, such as has_many.
Perhaps this would make your life easier:

class ActiveRecord::Base
  class << self
    alias :refers_to :belongs_to
  end
end

} Jay Levitt
--Greg

If you really want this, just place this in your environment.rb

module ActiveRecord::Associations::ClassMethods
  alias :refers_to :belongs_to
end

And there you have your refers_to method.

Improve your code

class Picture < AR:B
   acts_as_list
end

class Item < ActiveRecord::Base
  has_one :primary_picture, :class_name => ''Picture'',
:order => ''position ASC''
end

On 4/2/06, Jay Levitt <jay+news@jay.fm> wrote:
> Every once in a while, I pipe up and whine that "belongs_to"
should really
> be ''refers_to" [http://dev.rubyonrails.org/ticket/2130]. 
It''s time again.
>
> I''ve got a bunch of stuff going onto eBay, so, like any good
engineer, I
> don''t just post it manually: I design a Rails-based inventory
database that
> creates a semantically-correct XHTML/CSS auction posting with a variable
> number of thumbnails (stored with file_column) and Textile fields.
>
> I start out with
>
> class Item < ActiveRecord::Base
>   :has_many :pictures
> end class
>
> But I want one of those pictures to be featured as the gallery photo.  So,
> naturally, I gravitate towards adding
>
> class Item < ActiveRecord::Base
>   :has_one primary_picture, :class_name => ''Picture''
> end
>
> I am encouraged in this futile pursuit by the docs, which I quickly skim to
> see things like
>
>   :has_one :last_comment, :class_name => "Comment", :order
=> "posted_on"
>
> Of course, in the light of day, it''s clear that this automagically
creates
> the last_comment association by picking the most recent comment.  But at
> 3am, it just reinforced the tendency to use has_one.  Ditto the many other
> has_one examples I found - half of which used the automatic-first-record
> feature, and half of which were probably just making the same mistake I
> was.
>
> I started getting nil objects, and was mystified until I finally tried
> explicitly setting the foreign key:
>
>   :has_one primary_picture, :class_name => ''Picture'',
>     :foreign_key => ''primary_picture_id''
>
> at which point Rails helpfully told me that there was no
> Picture.primary_picture_id field.  Then I remembered for the umpteenth time
> that has_one is for the record that does NOT have the foreign key, and
> belongs_to is for the one that does, and I should not be misled by the
> semantics of the actual has_one and belongs_to keywords.
>
> Item :belongs_to primary_picture makes no sense.  Item :refers_to
> primary_picture does.  This is way too small for even a plugin - and I
> don''t think that type of syntax change belongs in a plugin,
anyway.  I have
> yet to hear anyone who''s opposed to it... can''t we PLEASE
add this alias
> and consider deprecating belongs_to over time?  It''s... just...
wrong.
>
> Jay Levitt
>
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

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http://blog.leetsoft.com - Technical weblog

On Sun, 2 Apr 2006 12:50:22 -0400, Derrick Spell wrote:
> Now, you are using the term Item, so perhaps you have another model  
> to handle postings of the Item on eBay.  This would change the  
> argument above slightly, but the semantics remain the same.  If a  
> picture is used on only a single Item, but an item might have  
> multiple pictures (which is how I interpret this domain), then the  
> foreign key should be on the picture, not on the Item, and then you  
> would be correctly using the has_one and has_many.
Your second guess is right - I''m not actually posting to eBay with this
model (yet), and the items are one-of-a-kind, so there''s no habtm
relationship; each picture is associated with one and only one item, but
each item has_many pictures.
> 
> Now, by placing the foreign key on the picture, you can achieve the  
> concept of a primary picture by placing a boolean flag on the picture  
> tuple akin to IsPrimary.  Then set up your item model with a has_many  
> picture and a has_one primary picture (with the constraint that it  
> must have the IsPrimary flag set).
The problem with that is that there''s no way to enforce "one and
only one
primary picture" if the boolean''s in the picture.
> 
> OR... you can have a has_may :pictures, belongs_to :primary_picture
Which is what I''m doing now, but semantically, in this context, an item
doesn''t belong to a picture - it''s the other way around. 
Don''t you think?
> 
> OR... you can use acts_as_list on your has_many and maintain that the  
> first item in the list is the primary picture.  I kinda like this  
> approach.
Yep, I hadn''t thought of that, but it makes the most sense, and
probably
would have fallen out once I started thinking about being able to re-order
the pictures!  That one''s nice.

Jay

On Sun, 2 Apr 2006 12:51:54 -0400, Gregory Seidman wrote:
> In some contexts it is wrong. In others it is right, such as has_many.
> Perhaps this would make your life easier:
> 
> class ActiveRecord::Base
>   class << self
>     alias :refers_to :belongs_to
>   end
> end
Yep, I keep swearing I''m gonna do that.  I just think it might help
other
programmers as well - I often see belongs_to confusion here, especially
among novices.  

Jay

Jay,

Just wanted to say that I pretty much agree with you.  I was in 
basically the exact same situation you were the other day, with a 
listing of Items where each Item has_many :images, and I thought I could 
do "has_one :main_image", like you described.  Of course I realized
the
same thing you did and it still feels kinda funny, since semantically, 
my Item doesn''t belong to its main picture, the picture belongs to the 
item.

They even take time in Agile Web Dev to say "we know it seems funny to 
say ''belongs_to'', so think of it as
''refers_to''".  Not sure I understand
why it was coded that way in the first place, since so many of Rails 
conventions are based on easily translating coding semantics into 
natural language semantics.

Jeff

-- 
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