Group By Year, Month & then Count in ActiveRecord 3 - sql

Question

I'm trying to pull a count of all users created in a year, and month but the following doesn't seem to work as expected.
User.group("YEAR(created_AT), MONTH(created_at)").
count("DISTINCT(id), YEAR(created_at), MONTH(created_at)")
i'm looking for something like
{2011 => {1 => 222, 2 => 333, 4 => 444, 5 => 667 ... }}
but i'm getting
{1 => 222, 2 => 333, 4 => 444, 5 => 667 ... }
Am i missing something, or can ActiveRecord not give me this result in one query?

Answer 1

The count method doesn't work like you think it does. You end up doing this:
select count(distinct(id), year(created_at), month(created_at))
from users
group by year(created_at), month(created_at)
That SELECT clause is pretty dodgy but MySQL will, in its usual sloppy manner, let it through. I think you want this query:
select count(distinct(id)), year(created_at), month(created_at)
from users
group by year(created_at), month(created_at)
I'd probably go straight to select_all like this:
a = User.connection.select_all(%q{
select count(distinct(id)) as c, year(created_at) as y, month(created_at) as m
from users
group by y, m
})
Or you could do it like this:
a = User.connection.select_all(
User.select('count(distinct(id)) as c, year(created_at) as y, month(created_at) as m').
group('y, m')
)
Those will give you an array, a, of Hashes with c, y, and m keys like this:
a = [
{ 'c' => '23', 'y' => '2010', 'm' => '11' },
{ 'c' => '1', 'y' => '2011', 'm' => '1' },
{ 'c' => '5', 'y' => '2011', 'm' => '3' },
{ 'c' => '2', 'y' => '2011', 'm' => '4' },
{ 'c' => '11', 'y' => '2011', 'm' => '8' }
]
Then a bit of data wrangling is all you need to finish the job:
h = a.group_by { |x| x['y'] }.each_with_object({}) do |(y,v), h|
h[y.to_i] = Hash[v.map { |e| [e['m'].to_i, e['c'].to_i] }]
end
# {2010 => {11 => 23}, 2011 => {1 => 1, 3 => 5, 4 => 2, 8 => 11}}