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Michael Schuerig

2009-May-12 00:34 UTC

has_many :through and scopes: how to mutate the set of associated objects?

I have a model layer containing Movie, Person, Role, and RoleType, 
making it possible to express facts such as "Clint Easterbunny is 
director of the movie Gran Milano".

The relevant model and associations look like this

class Movie < ActiveRecord::Base
  has_many :roles, :include => :role_type, :dependent => :destroy

  has_many :participants, :through => :roles, :source => :person do
    def as(role_name)
      self.scoped(
        :joins => ''CROSS JOIN role_types'',
        :conditions => [
          "(roles.role_type_id = role_types.id) +
          " AND role_types.name = ?",
          role_name
        ]
      )
    end
  end
  ...
end

Querying is easy:

m = Movie.find_by_title(''Gran Milano'')
m.participants.as(''director'')

However, changing relations is painful. It''s already bad with has_many 
:through associations when the intermediate model is not completely 
dumb, and my scope trickery doesn''t make it any better.

Now, let''s assume for a moment that participants was a plain has_many 
association. Then it would be possible to write things like

m.participants.clear
m.participants << Person.find_by_name(''Steve McKing'')
m.participant_ids = params[:movie][:participants]

With the given has_many :through, none of these work, as Role object 
won''t validate without a role type. Anyway, what I would like to write 
is

m.participants.as(''actor'').clear
m.participants.as(''actor'') <<
Person.find_by_name(''Steve McKing'')
m.participants.as(''actor'') =
Person.find(params[:movie][:participants])

I''m not sure this is possible with ActiveRecord as it is, but
I''m
looking forward to suggestions.

Michael

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

Matt Jones

2009-May-12 16:48 UTC

head link

Re: has_many :through and scopes: how to mutate the set of associated objects?

Haven''t tried it, but have you considered switching the association
extension to a regular named_scope on Person? For simpler cases, I
know that the named_scope code is smart enough to use the scope
conditions to instantiate objects. Not sure if it will work here...

--Matt Jones

On May 11, 7:34 pm, Michael Schuerig
<mich...-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org>
wrote:> I have a model layer containing Movie, Person, Role, and RoleType,
> making it possible to express facts such as "Clint Easterbunny is
> director of the movie Gran Milano".
>
> The relevant model and associations look like this
>
> class Movie < ActiveRecord::Base
>   has_many :roles, :include => :role_type, :dependent => :destroy
>
>   has_many :participants, :through => :roles, :source => :person do
>     def as(role_name)
>       self.scoped(
>         :joins => ''CROSS JOIN role_types'',
>         :conditions => [
>           "(roles.role_type_id = role_types.id) +
>           " AND role_types.name = ?",
>           role_name
>         ]
>       )
>     end
>   end
>   ...
> end
>
> Querying is easy:
>
> m = Movie.find_by_title(''Gran Milano'')
> m.participants.as(''director'')
>
> However, changing relations is painful. It''s already bad with
has_many
> :through associations when the intermediate model is not completely
> dumb, and my scope trickery doesn''t make it any better.
>
> Now, let''s assume for a moment that participants was a plain
has_many
> association. Then it would be possible to write things like
>
> m.participants.clear
> m.participants << Person.find_by_name(''Steve
McKing'')
> m.participant_ids = params[:movie][:participants]
>
> With the given has_many :through, none of these work, as Role object
> won''t validate without a role type. Anyway, what I would like to
write
> is
>
> m.participants.as(''actor'').clear
> m.participants.as(''actor'') <<
Person.find_by_name(''Steve McKing'')
> m.participants.as(''actor'') =
Person.find(params[:movie][:participants])
>
> I''m not sure this is possible with ActiveRecord as it is, but
I''m
> looking forward to suggestions.
>
> Michael
>
> --
> Michael Schuerig
>
mailto:mich...-q5aiKMLteq5BV9CJdY2HSA@public.gmane.org://www.schuerig.de/michael/

Michael Schuerig

2009-May-12 17:16 UTC

head link

Re: has_many :through and scopes: how to mutate the set of associated objects?

On Tuesday 12 May 2009, Matt Jones wrote:> Haven''t tried it, but have you considered switching the
association
> extension to a regular named_scope on Person? For simpler cases, I
> know that the named_scope code is smart enough to use the scope
> conditions to instantiate objects. Not sure if it will work here...
If I understand you correctly, I''ve already considered that case.

class Person < ActiveRecord::Base
  named_scope :actors, ... # add a condition picking out the actors
end

Then, with 

movie.participants.actors

I''d get the people who are participating in movie and who are actors. 
However, what I want are the people participating in movie *as* actors.

It might be possible to get this to work as intended, but I tend to 
think it''s not. Dealing with the joins involved is already tricky. 
ActiveRecord (almost) doesn''t have an abstract model of queries, it
more
or less concatenates strings. There''s no support for expressing, on the
one hand, that a specific join is needed (without duplicating it), and 
on the other, that you want another, independent join.

Michael
> On May 11, 7:34 pm, Michael Schuerig
<mich...-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org> wrote:
> > I have a model layer containing Movie, Person, Role, and RoleType,
> > making it possible to express facts such as "Clint Easterbunny is
> > director of the movie Gran Milano".
> >
> > The relevant model and associations look like this
> >
> > class Movie < ActiveRecord::Base
> >   has_many :roles, :include => :role_type, :dependent =>
:destroy
> >
> >   has_many :participants, :through => :roles, :source => :person
do
> >     def as(role_name)
> >       self.scoped(
> >         :joins => ''CROSS JOIN role_types'',
> >         :conditions => [
> >           "(roles.role_type_id = role_types.id) +
> >           " AND role_types.name = ?",
> >           role_name
> >         ]
> >       )
> >     end
> >   end
> >   ...
> > end
> >
> > Querying is easy:
> >
> > m = Movie.find_by_title(''Gran Milano'')
> > m.participants.as(''director'')
> >
> > However, changing relations is painful. It''s already bad with
> > has_many
> >
> > :through associations when the intermediate model is not completely
> >
> > dumb, and my scope trickery doesn''t make it any better.
> >
> > Now, let''s assume for a moment that participants was a plain
> > has_many association. Then it would be possible to write things
> > like
> >
> > m.participants.clear
> > m.participants << Person.find_by_name(''Steve
McKing'')
> > m.participant_ids = params[:movie][:participants]
> >
> > With the given has_many :through, none of these work, as Role
> > object won''t validate without a role type. Anyway, what I
would
> > like to write is
> >
> > m.participants.as(''actor'').clear
> > m.participants.as(''actor'') <<
Person.find_by_name(''Steve McKing'')
> > m.participants.as(''actor'') > >
Person.find(params[:movie][:participants])
> >
> > I''m not sure this is possible with ActiveRecord as it is, but
I''m
> > looking forward to suggestions.
> >
> > Michael


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

Matt Jones

2009-May-13 17:27 UTC

head link

Re: has_many :through and scopes: how to mutate the set of associated objects?

Try it in a named_scope, thus:

class Person < AR::Base
  named_scope :as, lambda { |role_name| { :joins => ''CROSS JOIN
role_types'', :conditions => ["(roles.role_type_id =
role_types.id) AND
role_types.name = ?", role_name] } }
end

But I''m *almost* positive that that still won''t be able to
trigger the
named_scope :create_scope magic. The other thought would be to return
a custom subclass of AssociationCollection from your ''as''
association
extension. You''d probably need to override a few methods (<<
especially) to take into account the source of the request (the
argument passed to as).

--Matt Jones


On May 12, 12:16 pm, Michael Schuerig
<mich...-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org>
wrote:> On Tuesday 12 May 2009, Matt Jones wrote:
>
> > Haven''t tried it, but have you considered switching the
association
> > extension to a regular named_scope on Person? For simpler cases, I
> > know that the named_scope code is smart enough to use the scope
> > conditions to instantiate objects. Not sure if it will work here...
>
> If I understand you correctly, I''ve already considered that case.
>
> class Person < ActiveRecord::Base
>   named_scope :actors, ... # add a condition picking out the actors
> end
>
> Then, with
>
> movie.participants.actors
>
> I''d get the people who are participating in movie and who are
actors.
> However, what I want are the people participating in movie *as* actors.
>
> It might be possible to get this to work as intended, but I tend to
> think it''s not. Dealing with the joins involved is already tricky.
> ActiveRecord (almost) doesn''t have an abstract model of queries,
it more
> or less concatenates strings. There''s no support for expressing,
on the
> one hand, that a specific join is needed (without duplicating it), and
> on the other, that you want another, independent join.
>
> Michael
>
>
>
> > On May 11, 7:34 pm, Michael Schuerig
<mich...-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org> wrote:
> > > I have a model layer containing Movie, Person, Role, and
RoleType,
> > > making it possible to express facts such as "Clint
Easterbunny is
> > > director of the movie Gran Milano".
>
> > > The relevant model and associations look like this
>
> > > class Movie < ActiveRecord::Base
> > >   has_many :roles, :include => :role_type, :dependent =>
:destroy
>
> > >   has_many :participants, :through => :roles, :source =>
:person do
> > >     def as(role_name)
> > >       self.scoped(
> > >         :joins => ''CROSS JOIN role_types'',
> > >         :conditions => [
> > >           "(roles.role_type_id = role_types.id) +
> > >           " AND role_types.name = ?",
> > >           role_name
> > >         ]
> > >       )
> > >     end
> > >   end
> > >   ...
> > > end
>
> > > Querying is easy:
>
> > > m = Movie.find_by_title(''Gran Milano'')
> > > m.participants.as(''director'')
>
> > > However, changing relations is painful. It''s already bad
with
> > > has_many
>
> > > :through associations when the intermediate model is not
completely
>
> > > dumb, and my scope trickery doesn''t make it any better.
>
> > > Now, let''s assume for a moment that participants was a
plain
> > > has_many association. Then it would be possible to write things
> > > like
>
> > > m.participants.clear
> > > m.participants << Person.find_by_name(''Steve
McKing'')
> > > m.participant_ids = params[:movie][:participants]
>
> > > With the given has_many :through, none of these work, as Role
> > > object won''t validate without a role type. Anyway, what
I would
> > > like to write is
>
> > > m.participants.as(''actor'').clear
> > > m.participants.as(''actor'') <<
Person.find_by_name(''Steve McKing'')
> > > m.participants.as(''actor'') > > >
Person.find(params[:movie][:participants])
>
> > > I''m not sure this is possible with ActiveRecord as it
is, but I''m
> > > looking forward to suggestions.
>
> > > Michael
>
> --
> Michael Schuerig
>
mailto:mich...-q5aiKMLteq5BV9CJdY2HSA@public.gmane.org://www.schuerig.de/michael/

Michael Schuerig

2009-May-13 18:47 UTC

head link

Re: has_many :through and scopes: how to mutate the set of associated objects?

On Wednesday 13 May 2009, Matt Jones wrote:> Try it in a named_scope, thus:
>
> class Person < AR::Base
>   named_scope :as, lambda { |role_name| { :joins => ''CROSS JOIN
> role_types'', :conditions => ["(roles.role_type_id =
role_types.id)
> AND role_types.name = ?", role_name] } }
> end
That works for queries, but AFAICT it is equivalent to what I''m already
doing. The drawback is that

Person.as(''actor'') doesn''t work because the necessary
join with roles is
missing. If I add that, movie.participants.as(''actor'') blows
up because
there the added join is a duplicate.
> But I''m *almost* positive that that still won''t be able
to trigger
> the named_scope :create_scope magic.
No, it won''t, simply because ActiveRecord has no way to figure out what
additional parameters to use to build the through model (Role).

As I''m staring at this stuff for some time now, I''m still
surprised that
there doesn''t seem to be a fairly generic way to add elements to a 
has_many :through association with a non-trivial through-model. For me, 
the point of has_many :through, as opposed to habtm, is that the 
intervening model carries some weight apart from relating two other 
models with each other.
> The other thought would be to
> return a custom subclass of AssociationCollection from your
''as''
> association extension. You''d probably need to override a few
methods
> (<< especially) to take into account the source of the request (the
> argument passed to as).
I was thinking of returning a subclass of AR::NamedScope::Scope. I
can''t
say which would be better.

Michael

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

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