mysql - Can't select rows grouping by created_at

Question

I have the table:

+----+----------------------+---------------------+
| id | implemented_features | created_at          |
+----+----------------------+---------------------+
|  1 |                   19 | 2013-07-18 04:10:12 |
|  2 |                    6 | 2013-07-18 04:10:12 |
|  3 |                   26 | 2013-07-19 04:10:12 |
|  4 |                   11 | 2013-07-19 04:10:12 |
|  5 |                    1 | 2013-07-20 04:10:12 |
+----+----------------------+---------------------+

When I query this directly via MySQL it works perfectly as I want

select date(created_at) as date, sum(implemented_features) as sum from summaries group by date(created_at);

But when I try to convert this query to ActiveRecord syntax it returns me nil.

2.0.0p0 :035 > Summary.select("date(created_at) as date, sum(implemented_features)").group("date(created_at)")
  Summary Load (0.5ms)  SELECT date(created_at) as date, sum(implemented_features) FROM `summaries` GROUP BY date(created_at)
 => #<ActiveRecord::Relation [#<Summary id: nil>]> 

As you see, final queries are equal in both examples. Why it don't work in case of ActiveRecord?

Using Rails 4.0.0, Ruby 2.0, mysql db in my rails project.

Answer 1

I think you're just a little confused by the console output.

You're saying this:

Summary.select("date(created_at) as date, sum(implemented_features)")...

so the returned Summary instances (wrapped up in an ActiveRecord::Relation) don't have any of the usual Summary attributes: no id, no created_at, no implemented_featured, etc. When you call inspect on an ActiveRecord object, it wants to show you what's inside the object and that means that it wants to show you the contained database attributes; your Summary instances don't have any of the usual attributes so you see things like <Summary id: nil>.

Fear not, the values you selected really are there. If you say:

Summary.select(...).map(&:date)

you should see the date(created_at) as date values. If you add an alias for the sum(implemented_features) then you can extract the sums by using that alias as a method name.