eager_cursor.rb

dataset_module do
  # User.one_to_one(:auth)
  # User.eager_cursor(1000, :auth) do |user|
  #   user.auth # no additional query
  # end
  def eager_cursor(rows_per_fetch = 1000, *associations)
    cursor = use_cursor(rows_per_fetch: rows_per_fetch)
    cursor.each_slice(rows_per_fetch) do |records|
      associations.each do |assoc|
        refl = model.association_reflection(assoc)
        case refl[:type]
        when :one_to_one
          EagerCursorOneToMany
        when :many_to_one
          EagerCursorManyToOne
        else
          fail "Cannot handle: #{refl[:type]}"
        end.new(records, refl).load

        records.each do |r|
          yield(r)
        end
      end
    end
  end

  EagerCursor = Struct.new(:records, :reflection) do
    def load
      associations = reflection.associated_class.where(association_key => records.map(&record_key)).all

      records.each do |r|
        if (i = associations.find_index(&method(:find).curry[r]))
          a = associations.delete_at(i)
          r.associations[reflection[:name]] = a
        end
      end

      records
    end

    private

    def find(record, association)
      record.public_send(record_key) == association.public_send(association_key)
    end
  end

  class EagerCursorOneToMany < EagerCursor
    private

    def association_key
      reflection[:key]
    end

    def record_key
      reflection.primary_key
    end
  end

  class EagerCursorManyToOne < EagerCursor
    private

    def association_key
      reflection.primary_key
    end

    def record_key
      reflection[:key]
    end
  end
end