How to perform COUNT with HABTM and left join? - sql

Question

Considering the following associations:
class Pool < ActiveRecord::Base
has_and_belongs_to_many :participations
end
class Participation < ActiveRecord::Base
has_and_belongs_to_many :pools
end
I want to get the number of participations in each pools (even if there is no participation).
This is what I am expecting (id is pool id):
+----+----------------------------+
| id | count('participations.id') |
+----+----------------------------+
| 1 | 1 |
| 2 | 0 |
| 3 | 0 |
| 4 | 0 |
| 5 | 0 |
| 6 | 0 |
| 7 | 0 |
| 8 | 0 |
+----+----------------------------+
This is what I get:
+----+----------------------------+
| id | count('participations.id') |
+----+----------------------------+
| 1 | 3 |
| 2 | 1 |
| 3 | 1 |
| 4 | 1 |
| 5 | 1 |
| 6 | 1 |
| 7 | 1 |
| 8 | 1 |
+----+----------------------------+
To obtain this result, I do a left join with a group by and a count:
Pool.joins('LEFT JOIN participations_pools ON
participations_pools.pool_id = pools.id LEFT JOIN participations ON
participations.id =
participations_pools.participation_id').select("pools.id,
count('participations.id')").group('pools.id')
I don't know how to get the good result and why I get that?
EDIT:
My answer at my question:
Pool.joins('LEFT JOIN participations_pools ON
participations_pools.pool_id = pools.id LEFT JOIN participations ON
participations.id =
participations_pools.participation_id').select("pools.id,
count(participations.id)").group('pools.id')
The quotes around count are the cause of my troubles

Answer 1

If you don't want to worry about that, write your query using only ActiveRecord methods:
Pool.joins('LEFT JOIN participations_pools ON participations_pools.pool_id = pools.id')
.joins('LEFT JOIN participations ON participations.id = participations_pools.participation_id')
.group('pools.id').count('participations.id')
The result will be a hash having pools.id as a key and count('participations.id') as a value for each row extracted from your database.
More info for count method: http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-count
If count is used with group, it returns a Hash whose keys represent the aggregated column, and the values are the respective amounts: