Rails - Jan 2005 - Making an association return a default object?

I''d like to be able to have an association method return a default
blank ActiveRecord object rather than ''nil'' if the association
doesn''t
exist in the database yet.  For example:

my_booking = Booking.find(10)
location = my_booking.origin_location  #<-- I want this to never return nil

I''ve set up my model like this (taking out irrelevant stuff):

class Booking < ActiveRecord::Base
  belongs_to :origin_location, :class_name => "Location",
:foreign_key
=> "origin_location_id"
  belongs_to :destination_location, :class_name => "Location",
:foreign_key => "destination_location_id"

  alias :old_origin_location :origin_location
  alias :old_destination_location :destination_location

  def origin_location
    @origin_location ||= (old_origin_location || Location.new)
  end

  def destination_location
    @destination_location ||= (old_destination_location || Location.new)
  end
end

The trouble with this code is that by assigning @origin_location and
@destination_location to Location.new, I''ve short-circuited the
built-in association functions.  By so doing, my new Location object
does not have the same methods as the Location object would have if it
were created through the magical association methods.  My new
Location, for example, does not have the "construct_sql" method which
is necessary for the auto-saving to occur when my_booking.save is
called.

Does anyone know how to solve this problem?  And if not, is there
another way to make an association have a ''default object''
returned?

Cheers,
Duane Johnson

If you''re only dealing with ''created'' objects.  i.e
ones returned by
find.  You could simply use the DDL to handle this:

CREATE TABLE bookings (
 id ...
...
... 
origin_location_id integer DEFAULT 0,

);

Then make the entry for ''0'' be your special ''default
blank one''.

If you want to work with objects that are created too,  you could use
the callbacks:

def after_create
  self.origin_location ||= Location.default_value 
end

All this assumes there''s no magic ActiveRecord incantation for it.  ;)

On Thu, 27 Jan 2005 02:41:42 +0000, Duane Johnson
<duane.johnson-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> I''d like to be able to have an association method return a default
> blank ActiveRecord object rather than ''nil'' if the
association doesn''t
> exist in the database yet.  For example:
> 
> my_booking = Booking.find(10)
> location = my_booking.origin_location  #<-- I want this to never return
nil
> 
> I''ve set up my model like this (taking out irrelevant stuff):
> 
> class Booking < ActiveRecord::Base
>   belongs_to :origin_location, :class_name => "Location",
:foreign_key
> => "origin_location_id"
>   belongs_to :destination_location, :class_name => "Location",
> :foreign_key => "destination_location_id"
> 
>   alias :old_origin_location :origin_location
>   alias :old_destination_location :destination_location
> 
>   def origin_location
>     @origin_location ||= (old_origin_location || Location.new)
>   end
> 
>   def destination_location
>     @destination_location ||= (old_destination_location || Location.new)
>   end
> end
> 
> The trouble with this code is that by assigning @origin_location and
> @destination_location to Location.new, I''ve short-circuited the
> built-in association functions.  By so doing, my new Location object
> does not have the same methods as the Location object would have if it
> were created through the magical association methods.  My new
> Location, for example, does not have the "construct_sql" method
which
> is necessary for the auto-saving to occur when my_booking.save is
> called.
> 
> Does anyone know how to solve this problem?  And if not, is there
> another way to make an association have a ''default
object'' returned?
> 
> Cheers,
> Duane Johnson
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
> 

-- 
Cheers

Koz

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Duane Johnson wrote:
>   alias :old_origin_location :origin_location
> 
>   def origin_location
>     @origin_location ||= (old_origin_location || Location.new)
>   end
def origin_location
  old_origin_location or build_origin_location
end

jeremy
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Thank-you both Jeremy and Michael.  You''ve opened my darkened and
weary eyes :o)  This has literally taken me more than two hours to
figure out!

Jeremy, I tried the code you wrote as follows:

  alias :old_origin_location :origin_location

  def origin_location
    old_origin_location or build_origin_location
  end

  alias :old_destination_location :destination_location

  def destination_location
    old_destination_location or build_destination_location
  end

However, I get the following error:
 NameError in Bookings#new
 undefined local variable or method `build_origin_location'' for 
#<Booking:0x8fde374>

Am I missing something?

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Duane Johnson wrote:
>   alias :old_origin_location :origin_location
> 
>   def origin_location
>     old_origin_location or build_origin_location
>   end
> 
> However, I get the following error:
>  NameError in Bookings#new
>  undefined local variable or method `build_origin_location'' for 
> #<Booking:0x8fde374>
> 
> Am I missing something?
Ah, this will work for has_one (composition), not for belongs_to
(reference).  For belongs_to, you must create the associated Location
before you may reference it:

  def origin_location
    old_origin_location or self.origin_location = Location.create
  end

It works for has_one because the reference (foreign key) is in the other
direction, from the newly associated object to its parent.  Thus you can
set the id on the new object before saving it.

Tim Bates did substantial work removing this restriction so perhaps he
has a better way.  Unfortunately some technical issues prevent being
able to say booking.origin_location.build unless the origin_location
already exists (ironic, eh?)  Perhaps
booking.association(:origin_location).build calling build on the
underlying BelongsToAssociation would be a reasonable compromise.

jeremy
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