diff --git a/CHANGELOG.md b/CHANGELOG.md
index 92fd7a5..d7b824b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased
+- [378](https://github.com/JsonApiClient/json_api_client/pull/378) - Add the ability to create a subclass of JsonApiClient::Resource to have a modified id method
 
 ## 1.21.0
 - [#395](https://github.com/JsonApiClient/json_api_client/pull/395) - relaxing faraday dependency to anything less than 2.0
diff --git a/lib/json_api_client/included_data.rb b/lib/json_api_client/included_data.rb
index 8cf689c..3018f75 100644
--- a/lib/json_api_client/included_data.rb
+++ b/lib/json_api_client/included_data.rb
@@ -24,7 +24,7 @@ def initialize(result_set, data)
       end
 
       grouped_included_set.each do |type, resources|
-        grouped_included_set[type] = resources.index_by(&:id)
+        grouped_included_set[type] = resources.index_by { |resource| resource.attributes[:id] }
       end
 
       @data = grouped_included_set
diff --git a/test/unit/integer_id_test.rb b/test/unit/integer_id_test.rb
new file mode 100644
index 0000000..8c8bf04
--- /dev/null
+++ b/test/unit/integer_id_test.rb
@@ -0,0 +1,183 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class BaseResource < JsonApiClient::Resource
+  self.site = 'http://example.com/'
+  def id
+    attributes[:id].to_i if attributes[:id].present?
+  end
+end
+
+class Actor < BaseResource
+  has_many :movies
+end
+
+class Movie < BaseResource
+  belongs_to :actor, shallow_path: true
+  has_one :director, shallow_path: true
+end
+
+class Director < BaseResource
+  has_many :movies
+end
+
+NUMERIC_ASSERTION = Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.4') ? Fixnum : Integer
+
+class IntegerIdTestAssociationTest < MiniTest::Test
+
+  def test_included_document_test_id_from_method_as_integer
+    stub_request(:get, 'http://example.com/movies/1?include=actor')
+      .to_return(headers: { content_type: 'application/vnd.api+json',
+                            accept: 'application/vnd.api+json' },
+                 body: {
+                   data: {
+                     id: '1',
+                     type: 'movie',
+                     attributes: {
+                       actor_id: 1,
+                       director_id: 1,
+                       created_at: '2021-04-20T17:27:06-07:00',
+                       updated_at: '2021-04-20T17:27:07-07:00'
+                     },
+                     relationships: {
+                       actor: {
+                         data: {
+                           id: '1',
+                           type: 'actor'
+                         }
+                       },
+                       director: {
+                         data: {
+                           id: '1',
+                           type: 'director'
+                         }
+                       }
+                     }
+                   },
+                   included: [
+                     {
+                       id: '1',
+                       type: 'actor',
+                       attributes: {
+                         name: 'Keanu',
+                         updated_at: '2021-04-22T13:50:19-07:00',
+                         created_at: '2021-04-19T16:20:13-07:00'
+                       }
+                     }
+                   ]
+                 }.to_json)
+    movie = Movie.includes(:actor).find(1).last
+    assert_equal(NUMERIC_ASSERTION, movie.id.class)
+    assert_equal(1, movie.id)
+    assert_equal(String, movie.attributes[:id].class)
+    assert_equal('1', movie.attributes[:id])
+    assert_equal(Actor, movie.actor.class)
+    assert_equal(NUMERIC_ASSERTION, movie.actor.id.class)
+    assert_equal(1, movie.actor.id)
+    assert_equal('1', movie.actor.attributes[:id])
+    assert_equal(movie.actor_id, movie.actor.id)
+  end
+
+  def test_not_included_data_document
+    stub_request(:get, 'http://example.com/movies/1')
+      .to_return(headers: { content_type: 'application/vnd.api+json',
+                            accept: 'application/vnd.api+json' },
+                 body: {
+                   data: {
+                     id: '1',
+                     type: 'movie',
+                     attributes: {
+                       actor_id: 1,
+                       created_at: '2021-04-20T17:27:06-07:00',
+                       updated_at: '2021-04-20T17:27:07-07:00'
+                     },
+                     relationships: {
+                       actor: {
+                         data: {
+                           id: '1',
+                           type: 'actor'
+                         },
+                         director: {
+                           data: {
+                             id: '1',
+                             type: 'director'
+                           }
+                         }
+                       }
+                     }
+                   }
+                 }.to_json)
+    movie = Movie.find(1).last
+    assert_equal(NUMERIC_ASSERTION, movie.id.class)
+    assert_equal(1, movie.id)
+    assert_equal(String, movie.attributes[:id].class)
+    assert_equal('1', movie.attributes[:id])
+    assert_nil(movie.actor)
+  end
+
+  def test_not_included_data_document_with_relationships_links
+    stub_request(:get, 'http://example.com/movies/1')
+      .to_return(headers: { content_type: 'application/vnd.api+json',
+                            accept: 'application/vnd.api+json' },
+                 body: {
+                   data: {
+                     id: '1',
+                     type: 'movie',
+                     attributes: {
+                       actor_id: 1,
+                       created_at: '2021-04-20T17:27:06-07:00',
+                       updated_at: '2021-04-20T17:27:07-07:00'
+                     },
+                     relationships: {
+                       actor: {
+                         links: {
+                           self: '/movies/1',
+                           related: '/actors/1'
+                         }
+                       },
+                       director: {
+                         links: {
+                           self: '/movies/1',
+                           related: '/directors/1'
+                         }
+                       }
+                     }
+                   }
+                 }.to_json)
+    stub_request(:get, 'http://example.com/directors/1')
+      .to_return(headers: { content_type: 'application/vnd.api+json',
+                            accept: 'application/vnd.api+json' },
+                 body: {
+                   data: {
+                     id: '1',
+                     type: 'movie',
+                     attributes: {
+                       actor_id: 1,
+                       created_at: '2021-04-20T17:27:06-07:00',
+                       updated_at: '2021-04-20T17:27:07-07:00'
+                     },
+                     relationships: {
+                       actor: {
+                         links: {
+                           self: '/movies/1',
+                           related: '/actors/1'
+                         }
+                       },
+                       director: {
+                         links: {
+                           self: '/movies/1',
+                           related: '/directors/1'
+                         }
+                       }
+                     }
+                   }
+                 }.to_json)
+    movie = Movie.find(1).last
+    assert_equal(NUMERIC_ASSERTION, movie.id.class)
+    assert_equal(1, movie.id)
+    assert_equal(String, movie.attributes[:id].class)
+    assert_equal('1', movie.attributes[:id])
+    assert_equal(Director, movie.director.class)
+  end
+end