Question

I have a table of Logs with the columns name, duration, type, ref_id.

I update the table every so often so perhaps it will look like a col of ['bill', 'bob', 'bob', 'jill'] for names, and [3, 5, 6, 2] for duration, and ['man', boy', 'boy', 'girl'] for type, and [1, 2, 2, 3] for ref_id.

I would like to manipulate my table so that I can add all the durations so that I get a hash or something that looks like this:

{'name' => ['bill', 'bob', 'jill'], 'duration' => [3, 11, 2], 'type' => ['man', 'boy', 'girl'], ref_id => [1, 2, 3]}

How can I do this?

(for more info–currently I’m doing Log.sum(:duration, :group => 'name') which gives me the names themselves as the keys (bill, bob, jill) instead of the column name, with the correct duration sums as their values (3, 11, 2). but then I lose the rest of the data…)

I guess I could manually go through each log and add the name/type/ref_id if it’s not in the hash, then add onto the duration. If so what’s the best way to do that?

If you know of good sources on rails array manipulation/commonly used idioms, that would be great too!

Answer 1

Couple of notes first.

Your table is not properly normalized. You should split this table into (at least) two: users, and durations. You should do this for lots of reasons, that’s relational databases 101.

Also, the hash you want as a result also doesn’t look right, it suggests that you are pre-grouping data to suit your presentation. It’s usually more logical to put these results in an array of hashes, than in a hash of arrays.

Now on to the answer:

With your table, you can simply do GROUP BY:

SELECT name, type, ref_id, SUM(duration) as duration
FROM logs
GROUP BY name, type, ref_id

or, using AR:

durations = Log.find(:all,
  :select => 'name, type, ref_id, SUM(duration) as duration',
  :group => 'name, type, ref_id'
)

In order to convert this to a hash of arrays, you’d use something like:

Hash[
  %w{name, type, ref_id, duration}.map{|f|
    [f, durations.map{|h|
      h.attributes[f]
    }]
  }
]