How can I find records by “count” of association using rails and mongoid?

Question

With these models:

class Week
  has_many :proofs
end
class Proof
  belongs_to :week
end

I want to do something like:

Week.where         


        

Answer 1

With rails (and without counter_cache), you could do:

class Week < ActiveRecord::Base
  has_many :proofs

  def self.by_proofs_size
    sort_by { |week| week.proofs.size }
  end

  def self.with_at_least_n_proofs(n = 1)
    select { |week| week.proofs.size >= n }
  end
end

Even though each of those operations produces 2 queries, this is far from ideal.

The pair of queries is repeated (=> 4 queries for each operation) with scopes (bug?):

scope :with_at_least_n_proofs, -> (n = 1) { select { |w| w.proofs.size >= n } }
scope :by_proofs_size, -> { sort_by { |w| w.proofs.size } }

The ideal is probably to use counter_cache

scope :with_at_least_n_proofs, -> (n = 1) { where('proofs_count >= ?', n) }
scope :by_proofs_size, -> { order(proofs_count: :desc) }

Answer 2

I don't know if this is the best solution, as it maps it through a array, but this does the job: (the other solutions mentioned here gives me exceptions)

class Week < ActiveRecord::Base

  scope :has_proofs, -> { any_in(:_id => includes(:proofs).select{ |w| w.proofs.size > 0 }.map{ |r| r.id }) }

end

Answer 3

Pardon me if I'm way off - but would you be able to use a simple counter_cache in the weeks table? Then you could do something like week.proofs_count.