Rails - Jan 2008 - Updating Child Records

I have a 3 level hierarchical data structure, for this example lets
say it''s Bike (has many) Wheels (has many) Spokes.

Here''s what I want to do, update a bunch of Spokes that all belong to
one Bike but do it all with one Save! method call.

I''ve tried loading the Bike, accessing, changing, and adding Spokes
through the Bike.wheels.spokes collections then doing Bike.save! but
it doesn''t work (the database remains unchanged).  It DOES work
however when I create a new bike record, add the wheels and spokes
then save that.


So is there any way to have all the Spoke records saved with one call
to save?  I''m attempting to limit the number of individual SQL calls,
cause there are a lot of spoke records and it will happen often.

Thanks in advance,
Danny
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The problem with bike.wheels.spokes is because bike has many wheels, and
*each wheel* has many spokes. With bike.wheels.spokes, you''re trying to
call
spokes on the collection of all wheels for that bike, so it isn''t going
to
work! What you need to do is to find a single wheel, something a simple as @
bike.wheels.first will do, or do @wheel = Wheel.find(id) and just find the
one wheel.

To add spokes:

@bike.wheels.first.spokes.build(:spoke_field_1 => "blah")

To remove a spoke:

@bike.wheels.first.spokes - Spoke.find(spoke_id)

To edit a spoke:

@bike.wheels.first.spokes.find(id).update_attributes(params[:spoke])

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Sorry, I should have been more clear, I''m aware of how the collections
work, that was pseudo-code.  I''m doing it properly in my real code.

#Pseudo code
b = bike.find(id)
then I loop through b.wheels
   loop through wheel.spokes
       spoke.thing = "newvalue"
   end
end

b.save!

On Jan 6, 4:58 pm, "Ryan Bigg"
<radarliste...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> The problem with bike.wheels.spokes is because bike has many wheels, and
> *each wheel* has many spokes. With bike.wheels.spokes, you''re
trying to call
> spokes on the collection of all wheels for that bike, so it isn''t
going to
> work! What you need to do is to find a single wheel, something a simple as
@
> bike.wheels.first will do, or do @wheel = Wheel.find(id) and just find the
> one wheel.
>
> To add spokes:
>
> @bike.wheels.first.spokes.build(:spoke_field_1 => "blah")
>
> To remove a spoke:
>
> @bike.wheels.first.spokes - Spoke.find(spoke_id)
>
> To edit a spoke:
>
> @bike.wheels.first.spokes.find(id).update_attributes(params[:spoke])
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I wouldn''t think that would work, because you''re not changing
the
associations themselves (i.e. adding/removing records), but you''re
editing
instances of the single records.

I can''t think of another way to do this, sorry. If it''s one
value you could
loop through and do update_attribute("field","value"), and
update_attributes
which I guess you know about already.

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On Jan 6, 3:14 pm, Daniel Beardsley
<dbear...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> Here''s what I want to do, update a bunch of Spokes that all belong
to
> one Bike but do it all with one Save! method call.
I recommend (as I have in another thread) that you not do this.

Third normal form states that every column in a table should be
dependent on the primary key. Well, in this case, one of the columns
in the spokes table appears to be dependent not on its primary key,
but on its associated bike''s primary key.

Just my $.02.

///ark
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It was a hypothetical example, things in my real DB are all correct
and don''t break any of the normal forms, one pk and one foreign key to
link each record to it''s parent.

According to Ryan''s reply above, Save''s are only propagated
(to
descendents) in Ruby model objects when there are insertions or
deletions, not when the fields have been changed.

Thanks,
Danny

On Jan 6, 6:57 pm, Mark Wilden
<m...-OCn100epQuBBDgjK7y7TUQ@public.gmane.org>
wrote:
> On Jan 6, 3:14 pm, Daniel Beardsley
<dbear...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
>
>
> > Here''s what I want to do, update a bunch of Spokes that all
belong to
> > one Bike but do it all with one Save! method call.
>
> I recommend (as I have in another thread) that you not do this.
>
> Third normal form states that every column in a table should be
> dependent on the primary key. Well, in this case, one of the columns
> in the spokes table appears to be dependent not on its primary key,
> but on its associated bike''s primary key.
>
> Just my $.02.
>
> ///ark
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It''s true - they''re done automatically on create, but not on
update.

Ryan Bates covers this in his screencast on managing multiple models
with 1 form (http://railscasts.com/episodes/75). It''s relatively
simple to iterate through the child records using something like
def save_wheels
    wheels.each do |w|
        w.save(false)
    end
  end

It assumes of course that you have updated their attributes and have a
has_many :wheels in the Bike model.

and calling this method (of the Bike model) with a callback in the
Bike model
after_update :save_wheels

This way, the children are automatically saved when the parent is
saved.

MC


On Jan 7, 7:12 pm, Daniel Beardsley
<dbear...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> It was a hypothetical example, things in my real DB are all correct
> and don''t break any of the normal forms, one pk and one foreign
key to
> link each record to it''s parent.
>
> According to Ryan''s reply above, Save''s are only
propagated (to
> descendents) in Ruby model objects when there are insertions or
> deletions, not when the fields have been changed.
>
> Thanks,
> Danny
>
> On Jan 6, 6:57 pm, Mark Wilden
<m...-OCn100epQuBBDgjK7y7TUQ@public.gmane.org> wrote:
>
> > On Jan 6, 3:14 pm, Daniel Beardsley
<dbear...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > > Here''s what I want to do, update a bunch of Spokes that
all belong to
> > > one Bike but do it all with one Save! method call.
>
> > I recommend (as I have in another thread) that you not do this.
>
> > Third normal form states that every column in a table should be
> > dependent on the primary key. Well, in this case, one of the columns
> > in the spokes table appears to be dependent not on its primary key,
> > but on its associated bike''s primary key.
>
> > Just my $.02.
>
> > ///ark
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eeew @ .save(false)

Take this as a warning to be very careful when using .save(false), passing
in false skips all validations and you might not get desired results.

On Jan 8, 2008 9:15 AM, mylescarrick-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
<mylescarrick-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> It''s true - they''re done automatically on create, but not
on update.
>
> Ryan Bates covers this in his screencast on managing multiple models
> with 1 form (http://railscasts.com/episodes/75). It''s relatively
> simple to iterate through the child records using something like
> def save_wheels
>    wheels.each do |w|
>        w.save(false)
>    end
>  end
>
> It assumes of course that you have updated their attributes and have a
> has_many :wheels in the Bike model.
>
> and calling this method (of the Bike model) with a callback in the
> Bike model
> after_update :save_wheels
>
> This way, the children are automatically saved when the parent is
> saved.
>
> MC
>
>
> On Jan 7, 7:12 pm, Daniel Beardsley
<dbear...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > It was a hypothetical example, things in my real DB are all correct
> > and don''t break any of the normal forms, one pk and one
foreign key to
> > link each record to it''s parent.
> >
> > According to Ryan''s reply above, Save''s are only
propagated (to
> > descendents) in Ruby model objects when there are insertions or
> > deletions, not when the fields have been changed.
> >
> > Thanks,
> > Danny
> >
> > On Jan 6, 6:57 pm, Mark Wilden
<m...-OCn100epQuBBDgjK7y7TUQ@public.gmane.org> wrote:
> >
> > > On Jan 6, 3:14 pm, Daniel Beardsley
<dbear...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >
> > > > Here''s what I want to do, update a bunch of Spokes
that all belong
> to
> > > > one Bike but do it all with one Save! method call.
> >
> > > I recommend (as I have in another thread) that you not do this.
> >
> > > Third normal form states that every column in a table should be
> > > dependent on the primary key. Well, in this case, one of the
columns
> > > in the spokes table appears to be dependent not on its primary
key,
> > > but on its associated bike''s primary key.
> >
> > > Just my $.02.
> >
> > > ///ark
> >
>

-- 
Ryan Bigg
http://www.frozenplague.net
Feel free to add me to MSN and/or GTalk as this email.

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