Counting and grouping at the same time

Question

I have a Mail model with the following schema:

t.string   "mail"
t.integer  "country"
t.boolean  "validated"
t.datetime "created_at"
t.datetime "updated_at"

And I want to find the top 5 countries in the database, so i go ahead and type

@top5 = Mail.find(:all,:group =>  'country',:conditions => [ "validated = ?" , "t" ], :limit => 5 )

This will tell me the groups(i need an order by i dont know how to write)

@top5 = Mail.count(:all,:group =>  'country',:conditions => [ "validated = ?" , "t" ], :limit => 5 )

This will tell me how many mails are in each group

Im wondering if i can group and count in just one go

Answer 1

Try:

Mail.count(:group => 'country', :conditions => ['validated = ?', 't'])

I'm not sure count accepts :limit though.

EDIT:

I think this is more readable:

Mail.count(:group => :country, :conditions => {:validated => true})

Answer 2

With Rails 3 you can simplify it further:

Mail.where(validated: true).count(group: :country)

You can order by fields in the group - in this case only :country would be valid:

Mail.where(validated: true)
    .order(:country)
    .count(group: :country)

You can also order by the count, using "count_all":

Mail.where(validated: true)
    .order("count_all desc")
    .count(group: :country)

You can also limit the number of groups returned. To do this you must call limit before calling count (because #count returns ActiveSupport::OrderedHash):

Mail.where(validated: true)
    .order("count_all desc")
    .limit(5)
    .count(group: :country)

Updated syntax for Rails 4:

Mail.where(validated: true)
    .group(:country)
    .count