[Ruby on Rails] having or missing associations with filter

having or missing assoc.rb

module HavingAssoc
  extend ActiveSupport::Concern

  class_methods do
    # @example
    #   Child.missing_assoc(:lessons)
    #   Child.missing_assoc(:lessons).size                      # => children's count
    #   Child.missing_assoc(:lessons, filter: Lesson.finished)  # => [children]
    def missing_assoc(assoc_name, filter: nil)
      if filter.nil?
        where.missing(assoc_name)
      else
        having_assoc(assoc_name, count: "= 0", filter:)
      end
    end

    # @example
    #   Child.having_assoc(:lessons, count: "> 10")                  # => [children]
    #   Child.having_assoc(:lessons, count: "> 10").size             # => { id: count }
    #   Child.having_assoc(:lessons, count: "> 10",
    #     filter: Lesson.where("date > ?", 1.year.ago))              # => [children]
    #   Child.having_assoc(:lessons, count: "> 10", filter: ..).size # => { id: 0 }
    #
    # @param [Symbol] count_as must be `:count_all` if want to use `.size`
    def having_assoc(assoc_name, count:, filter: nil, count_as: :count_all)
      assoc_table_name = reflect_on_all_associations.find { _1.name == assoc_name.to_sym }.table_name

      if filter.nil?
        left_outer_joins(assoc_name)
          .select("#{table_name}.*, COUNT(*) FILTER (WHERE #{assoc_table_name}.id IS NOT NULL) AS #{count_as}")
          .having("count(#{assoc_table_name}) #{count}")
      else
        filter_sql = filter.to_sql.split(" WHERE ").last
        left_outer_joins(assoc_name)
          .select("#{table_name}.*, COUNT(*) FILTER (WHERE #{filter_sql}) AS #{count_as}")
          .having("count(#{assoc_table_name}) FILTER (WHERE #{filter_sql}) #{count}")
      end.group(:id)
    end
  end
end