Rails - Jun 2010 - Can i eager-load associations onto an already-loaded object?

Let''s say that i have a User object in @user, already loaded.   I want
to constantly refer to @user.role on a particular page (where User
belongs_to :role), so i want to ''eager load'' the role
association.

For the sake of argument, let''s say i wasn''t able to do this
in the
normal way (ie with the :include option to a find call) when i loaded
the user out of the database in the first place, but need to do it
subsequent to that.  Is that possible?
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Max Williams wrote:
> Let''s say that i have a User object in @user, already loaded.   I
want
> to constantly refer to @user.role on a particular page (where User
> belongs_to :role), so i want to ''eager load'' the role
association.
> 
> For the sake of argument, let''s say i wasn''t able to do
this in the
> normal way (ie with the :include option to a find call) when i loaded
> the user out of the database in the first place, but need to do it
> subsequent to that.  Is that possible?
Just to answer my own question, sort of, i think that i can do

@user[:role] = @user.role

and that sets the instance variable which is used for subsequent calls 
to @user.role, effectively eager loading that association.  is that 
correct?  Is this the same thing as the eager loading would do?  eg, is 
it precisely as prone to weird side effects as the regular eager 
loading?
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On Jun 8, 11:25 am, Max Williams
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> Max Williams wrote:
> > Let''s say that i have a User object in @user, already loaded.
  I want
> > to constantly refer to @user.role on a particular page (where User
> > belongs_to :role), so i want to ''eager load'' the
role association.
>
> > For the sake of argument, let''s say i wasn''t able to
do this in the
> > normal way (ie with the :include option to a find call) when i loaded
> > the user out of the database in the first place, but need to do it
> > subsequent to that.  Is that possible?
>
> Just to answer my own question, sort of, i think that i can do
>
> @user[:role] = @user.role
>
> and that sets the instance variable which is used for subsequent calls
> to @user.role, effectively eager loading that association.  is that
> correct?  Is this the same thing as the eager loading would do?  eg, is
> it precisely as prone to weird side effects as the regular eager
> loading?
It''s not at all what normal eager loading would do.

if you do

class User < ActiveRecord::Base
  class << self
     public :preload_associations
  end
end

then you can do

User.preload_associations [user1, user2, user3], :role

(at least in rails 2.x - might very easily not work in 3.x)

Fred



Fred
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On Jun 8, 11:41 am, Frederick Cheung
<frederick.che...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> On Jun 8, 11:25 am, Max Williams
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
> then you can do
>
> User.preload_associations [user1, user2, user3], :role
>
One addition - it''s pointless doing this unless you have more than 1
user object - it won''t be any faster

Fred

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Frederick Cheung wrote:
> One addition - it''s pointless doing this unless you have more than
1
> user object - it won''t be any faster
> 
> Fred
Thanks fred - that works.  Faster than what, though?  Than my approach?
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On Jun 8, 12:09 pm, Max Williams
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> Frederick Cheung wrote:
> > One addition - it''s pointless doing this unless you have more
than 1
> > user object - it won''t be any faster
>
> > Fred
>
> Thanks fred - that works.  Faster than what, though?  Than my approach?
compared to not eagerloading it at all.

Fred
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Frederick Cheung wrote:
> On Jun 8, 12:09�pm, Max Williams
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
>> Frederick Cheung wrote:
>> > One addition - it''s pointless doing this unless you have
more than 1
>> > user object - it won''t be any faster
>>
>> > Fred
>>
>> Thanks fred - that works. �Faster than what, though? �Than my approach?
> 
> compared to not eagerloading it at all.
> 
> Fred
In this case, i have a page which loads a partial twenty times.  Each 
partial calls a method on the user object (current_user as it happens) 
which inspects its associated role object. So, if i don''t eager load, 
then the role gets loaded twenty times.  I might be misunderstanding 
your point though.
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On 8 June 2010 14:21, Max Williams
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
>
> In this case, i have a page which loads a partial twenty times.  Each
> partial calls a method on the user object (current_user as it happens)
> which inspects its associated role object. So, if i don''t eager
load,
> then the role gets loaded twenty times.  I might be misunderstanding
> your point though.
Memoize the role if necessary (although I thought AR would do that for
you after the first load - not got time to check it out to be sure at
the moment)

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On Jun 8, 2:21 pm, Max Williams
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
> > Fred
>
> In this case, i have a page which loads a partial twenty times.  Each
> partial calls a method on the user object (current_user as it happens)
> which inspects its associated role object. So, if i don''t eager
load,
> then the role gets loaded twenty times.  I might be misunderstanding
> your point though.
If it''s the same user object throughout active record will cache the
loaded role object for you - no need for eager loading here.

Fred
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Frederick Cheung wrote:
> On Jun 8, 2:21�pm, Max Williams
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
> 
>> > Fred
>>
>> In this case, i have a page which loads a partial twenty times. �Each
>> partial calls a method on the user object (current_user as it happens)
>> which inspects its associated role object. So, if i don''t
eager load,
>> then the role gets loaded twenty times. �I might be misunderstanding
>> your point though.
> 
> If it''s the same user object throughout active record will cache
the
> loaded role object for you - no need for eager loading here.
> 
> Fred
ah i see, i knew mysql cached the repeated queries but didn''t know AR 
cached the retrieved association as well.

thanks! max
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