GrapQL Ruby: Custom Relay connection class for ActiveRecord::Relation objects

custom_ar_relation_relay_connection.rb

# This custom relay connection class exists because the built-in connection
# class is broken when max_page_size is used.
#
# See: https://github.com/rmosolgo/graphql-ruby/issues/1109
class CustomArRelationRelayConnection < GraphQL::Relay::BaseConnection
  def cursor_from_node(item)
    cursor_col = item.class.implicit_order_column
    encode(item.send(cursor_col).to_s)
  end

  def has_next_page
    paged_nodes && @will_have_next_page
  end

  def has_previous_page
    paged_nodes && @will_have_prev_page
  end

  private

  def paged_nodes
    return @computed_paged_nodes if defined? @computed_paged_nodes

    @will_have_next_page = false
    @will_have_prev_page = false

    page =
      ActiveRecord::Base.transaction do
        rel = sliced_nodes

        if first
          if rel.count > first
            @will_have_next_page = true
          end
          rel = rel.limit(first)
        end

        if last
          skip = rel.count - last
          if skip > 0
            rel = rel.offset(skip)
            @will_have_prev_page = true
          end
        end

        if max_page_size
          if rel.count > max_page_size
            rel = rel.limit(max_page_size)
            @will_have_next_page = true
          end
        end

        rel.to_a
      end

    @computed_paged_nodes = page
    @computed_paged_nodes
  end

  def sliced_nodes
    ActiveRecord::Base.transaction do
      cursor_col = nodes.klass.implicit_order_column

      rel = nodes

      if after
        after_val = decode(after)

        rel = rel.where(rel.arel_table[cursor_col].gt(after_val))

        if nodes.where(nodes.arel_table[cursor_col].lteq(after_val)).exists?
          @will_have_prev_page = true
        end
      end

      if before
        before_val = decode(before)

        rel = rel.where(rel.arel_table[cursor_col].lt(before_val))

        if nodes.where(nodes.arel_table[cursor_col].gteq(before_val)).exists?
          @will_have_next_page = true
        end
      end

      rel
    end
  end
end

This implementation assumes:

• you are using Postgres
• the relation you are paging is ordered ascending by the ID column (override by changing implicit_order_column value on the model
  class)
• you are trying to solve this issue: rmosolgo/graphql-ruby#1109

To use this:

class ApplicationSchema < GraphQL::Schema
  # ... other code here

  GraphQL::Relay::BaseConnection.register_connection_implementation(ActiveRecord::Relation, CustomArRelationRelayConnection)

  # ... other code here
end