Rails - Aug 2005 - "constrains" -type conditions for ActiveRecord class

Let''s say I have a table "foo_and_bar" and I want to create a
class
"Foo" that only contains the foo records.  I''d like to do
something like:

  class Foo < ActiveRecord::Base
    set_table_name "foo_and_bar"
    constrain :conditions => "type = ''bar''"
  end

But "constrain" only works for a given block.  How do I make a class 
*always* restrict itself to a subset of the rows in a table?

Off hand I don''t think there''s a way to do this directly in
Rails...
however, the functionality you''re describing is pretty much that of a
view...  Perhaps you could wrap your table in a view?  This is fine if
you don''t need to update it - though you can have updatable views in
PostgreSQL and Oracle IIRC.

Alternatively, you could override ActiveRecord::Base#find - something
like this (untested!!):

class Foo < ActiveRecord::Base
  class << self # Used to override/add class methods
    alias_method :find_without_constraints, :find
    alias_method :find, :find_with_constraints
    
    def find_with_constraints(*args)
      find_without_constraints(args, conditions => ["type =
''bar''"])
    end
  end
end

Totally untested, but I think that''s the jist of it.  Actually, come
to think of it, you''d probably have to merge the *args list with the
conditions => ["type = ''bar''"] in the code above,
but that''s pretty
straightforward.  If you wanted to, you could wrap the code above in a
Module to add this kind of ''view'' functionality to Rails
(complete
with a built in validator to auto-assign new model object with the
constraint column values :).

Hope that helps!

-DF

On 8/15/05, Steve Downey
<sldowney-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org>
wrote:
> Let''s say I have a table "foo_and_bar" and I want to
create a class
> "Foo" that only contains the foo records.  I''d like to
do something like:
> 
>   class Foo < ActiveRecord::Base
>     set_table_name "foo_and_bar"
>     constrain :conditions => "type = ''bar''"
>   end
> 
> But "constrain" only works for a given block.  How do I make a
class
> *always* restrict itself to a subset of the rows in a table?
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

Oh, and sorry to reply to myself - you''ll have to override find_all as
well.

Cheers!

-DF

On 8/15/05, David Felstead
<david.felstead-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Off hand I don''t think there''s a way to do this directly
in Rails...
> however, the functionality you''re describing is pretty much that
of a
> view...  Perhaps you could wrap your table in a view?  This is fine if
> you don''t need to update it - though you can have updatable views
in
> PostgreSQL and Oracle IIRC.
> 
> Alternatively, you could override ActiveRecord::Base#find - something
> like this (untested!!):
> 
> class Foo < ActiveRecord::Base
>   class << self # Used to override/add class methods
>     alias_method :find_without_constraints, :find
>     alias_method :find, :find_with_constraints
> 
>     def find_with_constraints(*args)
>       find_without_constraints(args, conditions => ["type =
''bar''"])
>     end
>   end
> end
> 
> Totally untested, but I think that''s the jist of it.  Actually,
come
> to think of it, you''d probably have to merge the *args list with
the
> conditions => ["type = ''bar''"] in the code
above, but that''s pretty
> straightforward.  If you wanted to, you could wrap the code above in a
> Module to add this kind of ''view'' functionality to Rails
(complete
> with a built in validator to auto-assign new model object with the
> constraint column values :).
> 
> Hope that helps!
> 
> -DF
> 
> On 8/15/05, Steve Downey
<sldowney-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org> wrote:
> > Let''s say I have a table "foo_and_bar" and I want
to create a class
> > "Foo" that only contains the foo records.  I''d like
to do something like:
> >
> >   class Foo < ActiveRecord::Base
> >     set_table_name "foo_and_bar"
> >     constrain :conditions => "type =
''bar''"
> >   end
> >
> > But "constrain" only works for a given block.  How do I make
a class
> > *always* restrict itself to a subset of the rows in a table?
> > _______________________________________________
> > Rails mailing list
> > Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> > http://lists.rubyonrails.org/mailman/listinfo/rails
> >
>

You may want to consider the buit-in Rails STI functionality, whereby you 
can use that type field
to define a class as a subclass of the model that associates with the table. 
For example:

user.rb
class User < ActiveRecord::Base
class Person < User

This work automatically as long as your ''type'' field is named
''type'' and
uses camel case class names as values (Foo, Bar).

Joshua Sierles

On 8/14/05, Steve Downey
<sldowney-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org>
wrote:
> 
> Let''s say I have a table "foo_and_bar" and I want to
create a class
> "Foo" that only contains the foo records. I''d like to do
something like:
> 
> class Foo < ActiveRecord::Base
> set_table_name "foo_and_bar"
> constrain :conditions => "type = ''bar''"
> end
> 
> But "constrain" only works for a given block. How do I make a
class
> *always* restrict itself to a subset of the rows in a table?
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

_______________________________________________
Rails mailing list
Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
http://lists.rubyonrails.org/mailman/listinfo/rails