Question

I have two models: Group & GroupMember

I would like to return Groups, sorted by the highest GroupMember Count? How do you setup this type of join/query count in Rails?

Answer 1

Life is so much easier with counter_cache:

$ rails g migration add_group_member_counts_to_groups

# migration
def change
  add_column :groups, :group_members_count, :integer, default: 0
end

class GroupMember < ActiveRecord::Base
  belongs_to :group, counter_cache: true
end

class Group < ActiveRecord::Base
  has_many :group_members
  scope :order_by_group_members_count, order('group_members_count DESC')
end

Group.order_by_group_members_count # => What you're looking for

Only one query, only on the groups table.

edit

If you need to reset the group_members_count:

ids = Group.select(:id).map &:id
ids.each do |id|
  Group.reset_counters(id, :group_members)
end

(because reset_counters doesn’t take an array of ids)

In future Rails 3.2.0, you will write:

ids = Group.pluck(:id)

I can’t wait 😉