count

# File activerecord/lib/active_record/associations/collection_association.rb, line 179
      def count(column_name = nil, count_options = {})
        column_name, count_options = nil, column_name if column_name.is_a?(Hash)

        if options[:counter_sql] || options[:finder_sql]
          unless count_options.blank?
            raise ArgumentError, "If finder_sql/counter_sql is used then options cannot be passed"
          end

          reflection.klass.count_by_sql(custom_counter_sql)
        else
          relation = scope
          if association_scope.distinct_value
            # This is needed because 'SELECT count(DISTINCT *)..' is not valid SQL.
            column_name ||= reflection.klass.primary_key
            relation = relation.distinct
          end

          value = relation.count(column_name)

          limit  = options[:limit]
          offset = options[:offset]

          if limit || offset
            [ [value - offset.to_i, 0].max, limit.to_i ].min
          else
            value
          end
        end
      end