Generic page number / per-page pagination with GraphQL-Ruby

page_example.rb

# This is a full-blown offset-based pagination system modelled after GraphQL-Ruby's
# built-in connections. It has a few different elements:
#
# - `::Page`, a plain ol' Ruby class for modeling _pages_ of things.
#   This class handles applying pagination arguments to lists (Arrays and AR::Relations)
#   and provides metadata about pagination. (Similar to `will_paginate`.)
# - `Schema::BasePage` is a generic GraphQL-Ruby object type. It's never used directly,
#   but it can generate subclasses which wrap _specific_ object types in the schema.
# - `Schema::BaseObject.page_type` is a convenience method for generating page types
#   from your object types. You could leave this out and make subclasses with plain ol'
#   Ruby instead (using `class BookPage < BasePage ...`). It's a lot _simpler_ since
#   there's no dynamically generated classes, but it's more verbose. A tradeoff.
# - `Schema::BaseField::PageWrapperExtension` applies the pagination arguments & wrapper
#   to lists of items. This is just for convenience, like connections in GraphQL-Ruby.
#   If you left this off, you could easily add arguments and `::Page.new(...)` setup
#   to each field that return a `.page_type`.
#
# Finally, there's an example schema and some queries to demonstrate the behavior.

require "graphql"
require "active_record"

# Some example data
BOOKS = [
  { title: "Ruby Under a Microscope" },
  { title: "Patterns of Enterprise Application Architecture" },
  { title: "ReWork" },
  { title: "Land of Lisp" },
  { title: "The C Programming Language" },
  { title: "Smalltalk-80: The Language and its Implementation" },
  { title: "Domain-Driven Design Distilled" },
]

# This object represents a generic page of items.
# `all_nodes` may be an ActiveRecord::Relation or an Array.
# (Other implementations may be added below.)
#
# It implements the fields of the `BasePage` type defined below.
class Page
  DEFAULT_PAGE_SIZE = 20
  MAX_PAGE_SIZE = 50
  def initialize(all_nodes, page:, per_page:)
    @all_nodes = all_nodes
    # Normalize pagination arguments
    @page = if page.nil? || page < 1
      1
    else
      page
    end

    @per_page = if per_page.nil? || per_page < 1
      DEFAULT_PAGE_SIZE
    elsif per_page > MAX_PAGE_SIZE
      MAX_PAGE_SIZE
    else
      per_page
    end
  end

  # True if there are items on the list after this page of items
  def has_next_page
    nodes_count > (@page * @per_page)
  end

  # True if there are items on the list before this page of items
  def has_previous_page
    @page > 1
  end

  # The total number of items in the list
  def nodes_count
    @nodes_count ||= case @all_nodes
    when ActiveRecord::Relation
      # Remove `ORDER BY` for better performance
      @all_nodes.unscope(:order).count
    when Array
      @all_nodes.count
    else
      # TODO: implement other counts here
      raise "`nodes_count` not implemented for #{@all_nodes.class} (#{@all_nodes.inspect})"
    end
  end

  # The total number of pages for this page size
  def pages_count
    (nodes_count / @per_page.to_f).ceil
  end

  # The items in this page
  def nodes
    @nodes ||= case @all_nodes
    when ActiveRecord::Relation
      offset = (@page - 1) * @per_page
      @all_nodes.offset(offset).limit(@per_page)
    when Array
      offset = (@page - 1) * @per_page
      @all_nodes[offset, @per_page] || [] # return empty if out-of-bounds
    else
      # TODO: implement other counts here
      raise "`nodes_count` not implemented for #{@all_nodes.class} (#{@all_nodes.inspect})"
    end
  end
end

class Schema < GraphQL::Schema
  class BaseField < GraphQL::Schema::Field
    def initialize(**kwargs, &block)
      # Do all the normal field setup:
      super
      # Add pagination args if this is a `Page` field
      return_type = kwargs[:type]
      if return_type.is_a?(Class) && return_type < BasePage
        self.extension(PageWrapperExtension)
      end
    end

    # Like the built-in ConnectionExtension, this adds arguments
    # and automatic argument handling for page fields
    # @see https://graphql-ruby.org/type_definitions/field_extensions.html
    class PageWrapperExtension < GraphQL::Schema::FieldExtension
      # Add the arguments to the field
      def apply
        field.argument(:page, Integer, required: false)
        field.argument(:per_page, Integer, required: false)
      end

      def resolve(object:, arguments:, **rest)
        # Remove pagination arguments
        cleaned_arguments = arguments.dup
        page = cleaned_arguments.delete(:page)
        per_page = cleaned_arguments.delete(:per_page)
        # Call the underlying resolver (without pagination args)
        resolved_object = yield(object, cleaned_arguments)
        # Then, apply the wrapper and return it
        ::Page.new(resolved_object, page: page, per_page: per_page)
      end
    end
  end

  class BaseObject < GraphQL::Schema::Object
    field_class BaseField
    # Generate a page type for this object,
    # or use an already-cached one.
    def self.page_type
      @page_type ||= BasePage.create(self)
    end
  end

  # A generic page type
  class BasePage < BaseObject
    def self.create(node_class)
      Class.new(self) do
        # Override the name so it reflects the node class
        graphql_name("#{node_class.graphql_name}Page")
        # Add the nodes field which reflects the node class
        field :nodes, [node_class], null: false
      end
    end

    field :has_previous_page, Boolean, null: false
    field :has_next_page, Boolean, null: false
    field :pages_count, Integer, null: false
    field :nodes_count, Integer, null: false
  end

  class Book < BaseObject
    field :title, String, null: false
  end

  class Query < BaseObject
    field :books, Book.page_type, null: false

    def books
      BOOKS
    end
  end

  query(Query)
end


query_string = <<-GRAPHQL
query($page: Int, $perPage: Int) {
  books(page: $page, perPage: $perPage) {
    hasNextPage
    hasPreviousPage
    pagesCount
    nodesCount
    nodes {
      title
    }
  }
}
GRAPHQL


# Using the default page size
pp Schema.execute(query_string)["data"]["books"]
# {"hasNextPage"=>false,
#  "hasPreviousPage"=>false,
#  "pagesCount"=>1,
#  "nodesCount"=>7,
#  "nodes"=>
#   [{"title"=>"Ruby Under a Microscope"},
#    {"title"=>"Patterns of Enterprise Application Architecture"},
#    {"title"=>"ReWork"},
#    {"title"=>"Land of Lisp"},
#    {"title"=>"The C Programming Language"},
#    {"title"=>"Smalltalk-80: The Language and its Implementation"},
#    {"title"=>"Domain-Driven Design Distilled"}]}

# Out-of-bounds page number
pp Schema.execute(query_string, variables: { page: 2 })["data"]["books"]
# {"hasNextPage"=>false,
#  "hasPreviousPage"=>true,
#  "pagesCount"=>1,
#  "nodesCount"=>7,
#  "nodes"=>[]}

# Using a custom page size
pp Schema.execute(query_string, variables: { page: 3, perPage: 2 })["data"]["books"]
# {"hasNextPage"=>true,
#  "hasPreviousPage"=>true,
#  "pagesCount"=>4,
#  "nodesCount"=>7,
#  "nodes"=>
#   [{"title"=>"The C Programming Language"},
#    {"title"=>"Smalltalk-80: The Language and its Implementation"}]}