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Michael Schuerig

2007-Jan-03 14:38 UTC

Self-referential habtm with condition is broken

Here''s a simplistic model class.

  class Person < ActiveRecord::Base
    has_and_belongs_to_many :relatives,
      :class_name => ''Person'', :join_table =>
''relatives'',
      :foreign_key => ''from_id'', :association_foreign_key
=> ''to_id''

    has_and_belongs_to_many :brothers,
      :class_name => ''Person'', :join_table =>
''relatives'',
      :foreign_key => ''from_id'', :association_foreign_key
=> ''to_id'',
      :conditions => "gender = ''m''"
    #                 ^ plain reference to column

    has_and_belongs_to_many :sisters,
      :class_name => ''Person'', :join_table =>
''relatives'',
      :foreign_key => ''from_id'', :association_foreign_key
=> ''to_id'',
      :conditions => "sisters_people.gender =
''f''"
    #                 ^ qualified reference to column

  end

The noteworthy things about it are that its habtm associations are 
self-referential and that two of them have conditions. Let''s see how to
use this. Also, the brothers and sisters associations use different 
ways to refer to the column on which their respective condition 
depends.

Lets put this class through its paces.

  >> Person.find(:first, :include => :sisters)
  -> works
  >> Person.find(:first, :include => :brothers)
  -> exception; invalid SQL, reference to column ''gender''
is ambiguous

  >> p = Person.find(:first)
  >> p.brothers.map(&:first_name)
  -> works
  >> p.sisters.map(&:first_name)
  -> exception; invalid SQL, no alias defined for sisters_people

I may be missing something, but from what it looks like, ActiveRecord 
doesn''t handle this case as it should.

Michael

-- 
Michael Schuerig
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org
http://www.schuerig.de/michael/

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Ilan Berci

2007-Jan-03 15:35 UTC

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Re: Self-referential habtm with condition is broken

Michael Schuerig wrote:> Here''s a simplistic model class.
> 
>   class Person < ActiveRecord::Base
>     has_and_belongs_to_many :relatives,
>       :class_name => ''Person'', :join_table =>
''relatives'',
>       :foreign_key => ''from_id'',
:association_foreign_key => ''to_id''
> 
> 
> Michael
> 
Could you please provide the schema that this code is running against. 
As a guess, it looks as though gender is defined in both the People? 
table and it''s join  (Relatives?) which would lead to an ambiguous
query
unless it was explicitly qualified as it was in the sister''s case.

ilan



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Kad Kerforn

2007-Jan-03 16:51 UTC

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Re: Self-referential habtm with condition is broken

Michael Schuerig wrote:
> 
> The noteworthy things about it are that its habtm associations are
> self-referential and that two of them have conditions. Let''s see
how to
> use this. Also, the brothers and sisters associations use different
> ways to refer to the column on which their respective condition
> depends.
Hi Michael

that''s exactly what I am trying to solve using a :through statement...
if you solve this problem, please let me know... I''ll appreciate
cloning
your definitions....

Yves

mailto:ydj.dufour-39ZsbGIQGT5GWvitb5QawA@public.gmane.org

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Michael Schuerig

2007-Jan-03 17:54 UTC

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Re: Self-referential habtm with condition is broken

On Wednesday 03 January 2007 16:35, Ilan Berci wrote:> Michael Schuerig wrote:
> > Here''s a simplistic model class.
> >
> >   class Person < ActiveRecord::Base
> >     has_and_belongs_to_many :relatives,
> >
> >       :class_name => ''Person'', :join_table =>
''relatives'',
> >       :foreign_key => ''from_id'',
:association_foreign_key =>
> >       : ''to_id''
> Could you please provide the schema that this code is running
> against. As a guess, it looks as though gender is defined in both the
> People? table and it''s join  (Relatives?) which would lead to an
> ambiguous query unless it was explicitly qualified as it was in the
> sister''s case.
ActiveRecord::Schema.define(:version => 10) do
  create_table "people", :force => true do |t|
    t.column "first_name", :string,  :limit => 40, :null =>
false
    t.column "last_name",  :string,  :limit => 40, :null =>
false
    t.column "gender",     :string,  :limit => 1,  :null =>  
end

  create_table "relatives", :id => false, :force => true do |t|
    t.column "from_id", :integer, :null => false
    t.column "to_id",   :integer, :null => false
  end
end

In case it wasn''t clear from my original message, the point
I''m trying
to make is that apparently it is not possible to specify a condition on 
a habtm association in such a way that both fetching the associated 
objects and finding with the association :include''d are possible.

Michael

-- 
Michael Schuerig
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org
http://www.schuerig.de/michael/

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