Rails - Feb 2008 - Using group in with find in an ActiveRecord appropriately

I''m playing around with the Practical Rails Projects code, and was
attempting to make a change for graphing data differently using
groupings.

Basically the code went from:
        total_weight = @exercise.activities.collect {|e| e.repetitions
* e.resistance}
        workout_dates = @exercise.activities.collect {|e|
e.workout.date.to_s}

To:
        total_weight = @exercise.activities.find(
                                                 :all,
                                                 :select =>
"sum( repetitions * resistance ) AS resistance",
                                                 :group =>
"workout_id",
                                                 :order =>
"workout_id"
        ).collect{ |e| e.resistance }
        workout_dates = @exercise.activities.find(
                                                  :all,
                                                  :group =>
"workout_id",
                                                  :order =>
"workout_id"
        ).collect {|e| e.workout.date.to_s}

Now, the total_weight array works fine in the new implementation, but
the generation of it is much less than optimal.  I''m renaming the sum
as an existing column so I can collect it, which is just a nasty hack.

How can this be done better?  I understand I can use
Enumerable#group_by, but I want the db to do this one.
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Rob wrote:
> 
> How can this be done better?  I understand I can use
> Enumerable#group_by, but I want the db to do this one.
Rob,

If you want the db to do this one then just do the optimal SQL query 
yourself and load it up, for one, it will probably be easier to read 
than the complex meta you currently have.

I am never afraid to get my hands dirty with SQL as I am already 
committed to a particular vendor and sometimes I can fine tune the query 
much better than rails.

hth

ilan




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On Feb 25, 9:28 pm, Ilan Berci
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
> Rob wrote:
>
> > How can this be done better?  I understand I can use
> > Enumerable#group_by, but I want the db to do this one.
>
> Rob,
>
> If you want the db to do this one then just do the optimal SQL query
> yourself and load it up, for one, it will probably be easier to read
> than the complex meta you currently have.
Thanks.  I now have:

        total_weight = Exercise.find_by_sql( ["
          SELECT sum( repetitions * resistance ) AS total_weight
          FROM exercises
            JOIN workouts ON( activities.workout_id = workouts.id )
            JOIN activities ON( activities.exercise_id exercises.id )
          WHERE exercises.id = ?
            AND exercises.user_id = ?
          GROUP BY workout_id
          ORDER BY workout_id
        ", @exercise.id, @exercise.user_id] ).collect{ |e|
e.total_weight }

Cheers,
Rob
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Rob wrote:
> 
> Thanks.  I now have:
> 
>         total_weight = Exercise.find_by_sql( ["
>           SELECT sum( repetitions * resistance ) AS total_weight
>           FROM exercises
>             JOIN workouts ON( activities.workout_id = workouts.id )
>             JOIN activities ON( activities.exercise_id > exercises.id )
>           WHERE exercises.id = ?
>             AND exercises.user_id = ?
>           GROUP BY workout_id
>           ORDER BY workout_id
>         ", @exercise.id, @exercise.user_id] ).collect{ |e|
> e.total_weight }
> 
> Cheers,
> Rob
Another little trick if you are using MySQL is to use the undocumented 
method all_hashes in the mysql adapter which converts the unwieldy 
result set into a regular hash which is a lot more convenient to work 
with..

ilan


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On Mar 1, 2:16 pm, Ilan Berci
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
>
> Another little trick if you are using MySQL is to use the undocumented
> method all_hashes in the mysql adapter which converts the unwieldy
> result set into a regular hash which is a lot more convenient to work
> with..
Nah, I''m a SQLite and PostgreSQL guy...  Which meant my sql was bad
anyway.  So now it''s:

          SELECT sum( repetitions * resistance ) AS total_weight,
                 MIN( date ) AS date
          FROM activities
            JOIN workouts ON( activities.workout_id = workouts.id )
            JOIN exercises ON( activities.exercise_id = exercises.id )
          WHERE exercises.id = ?
            AND exercises.user_id = ?
          GROUP BY workout_id
          ORDER BY workout_id

Cheers,
Rob

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