graph_data/delta_records_since not working with scope

[fmh]

scope :have_clients_company, -> { joins(:clients).where('clients.cat = 0') } 
scope :have_clients_private, -> { joins(:clients).where('clients.cat =1 ') } 

 def self.graph_data since=30.days.ago
    [
      {
          name: 'Type1',
          pointInterval: point_interval = 1.day * 1000,
          pointStart: start_point = since.to_i * 1000,
          data: MyClass.have_clients_company.delta_records_since(since)
      },
      {
          name: 'Type2',
          pointInterval: point_interval,
          pointStart: start_point,
          data: MyClass.have_clients_private.delta_records_since(since)
      }
    ]
  end

always return a 0 as count