Question

My last post was on the best way to write a SQL query with conditions on a LEFT OUTER JOIN:
LEFT OUTER JOIN with conditions (where, order by)?

Now, I need to convert that good piece of SQL into a sweet Active Record Query (Rails 3). 😉

I have 2 Models:

Training has_many :training_histories

TrainingHistory belongs_to :training

How can I write a scope to get the results the SQL below retrieve ?

SELECT tc.id, tc.name, tc.order, 
th.id as history_id, th.finished_at, th.score
FROM trainings tc
LEFT OUTER JOIN training_histories th ON th.training_id = tc.id 
    and th.id =
    (SELECT th1.id from training_histories th1 where th1.training_id = tc.id
     and th1.finished_at is not null
     order by th1.finished_at desc limit 1)
WHERE tc.id > 4
AND tc.id < 8
GROUP BY tc.id
ORDER BY tc.order_by ASC, tc.id ASC

I want to retrieve all the records of TRAININGS and add the last valid (aka finished) associated record of TRAINING_HISTORIES.

Any suggestions ?

Thank you

Answer 1

For rails 3, try this:

Training.select("trainings.id, trainings.name, trainings.order, 
                 trainings.id AS history_id, training_histories.finished_at,
                 training_histories.score").
         joins("LEFT OUTER JOIN training_histories 
                ON training_histories.training_id = trainings.id 
                AND training_histories.id = (SELECT th1.id FROM training_histories th1          
                                             WHERE th1.training_id = tc.id
                                             AND th1.finished_at IS NOT NULL
                                             ORDER BY th1.finished_at DESC LIMIT 1)").
         where("trainings.id > 4 AND trainings.id < 8").
         group("trainings.id").
         order("trainings.order_by ASC, trainings.id ASC")

Basically, you’re just converting your pre-written query into Rails 3 finder methods.