SELECT COUNT(pets_users.pet_id), users.email
FROM users LEFT JOIN pet_users ON (users.id = pets_users.user_id) group by users.email


## console


 User.find(:all, :include => :pets).collect{|user| {user.email => user.pets.size }}
[{"elad@elad.com"=>646}, {"elad@indomite.com"=>0}, {"momo@momo.com"=>0}, {"koko@koko.com"=>200}, {"jojo@jojo.com"=>0}, {"bobo@bobo.com"=>0}, {"kill@kill.com"=>0}, {"test@test.com"=>0}, {"gum@gum.com"=>30}]

>> Pet.count
=> 1321

## SQL resultset

"pet_count","email"
0,"bobo@bobo.com"
1783,"elad@elad.com"
0,"elad@indomite.com"
30,"gum@gum.com"
0,"jojo@jojo.com"
0,"kill@kill.com"
200,"koko@koko.com"
0,"momo@momo.com"
0,"test@test.com"