This adds namespace lookup to serializer_for (#1968)

* This adds namespace lookup to serializer_for

* address rubocop issue

* address @bf4's feedback

* add docs

* update docs, add more tests

* apparently rails master doesn't have before filter

* try to address serializer cache issue between tests

* update cache for serializer lookup to include namespace in the key, and fix the tests for explicit namespace

* update docs, and use better cache key creation method

* update docs [ci skip]

* update docs [ci skip]

* add to changelog [ci skip]

test/action_controller/namespace_lookup_test.rb

@@ -0,0 +1,149 @@
+require 'test_helper'
+
+module ActionController
+  module Serialization
+    class NamespaceLookupTest < ActionController::TestCase
+      class Book < ::Model; end
+      class Page < ::Model; end
+      class Writer < ::Model; end
+
+      module Api
+        module V2
+          class BookSerializer < ActiveModel::Serializer
+            attributes :title
+          end
+        end
+
+        module V3
+          class BookSerializer < ActiveModel::Serializer
+            attributes :title, :body
+
+            belongs_to :writer
+          end
+
+          class WriterSerializer < ActiveModel::Serializer
+            attributes :name
+          end
+
+          class LookupTestController < ActionController::Base
+            before_action only: [:namespace_set_in_before_filter] do
+              self.namespace_for_serializer = Api::V2
+            end
+
+            def implicit_namespaced_serializer
+              writer = Writer.new(name: 'Bob')
+              book = Book.new(title: 'New Post', body: 'Body', writer: writer)
+
+              render json: book
+            end
+
+            def explicit_namespace_as_module
+              book = Book.new(title: 'New Post', body: 'Body')
+
+              render json: book, namespace: Api::V2
+            end
+
+            def explicit_namespace_as_string
+              book = Book.new(title: 'New Post', body: 'Body')
+
+              # because this is a string, ruby can't auto-lookup the constant, so otherwise
+              # the looku things we mean ::Api::V2
+              render json: book, namespace: 'ActionController::Serialization::NamespaceLookupTest::Api::V2'
+            end
+
+            def explicit_namespace_as_symbol
+              book = Book.new(title: 'New Post', body: 'Body')
+
+              # because this is a string, ruby can't auto-lookup the constant, so otherwise
+              # the looku things we mean ::Api::V2
+              render json: book, namespace: :'ActionController::Serialization::NamespaceLookupTest::Api::V2'
+            end
+
+            def invalid_namespace
+              book = Book.new(title: 'New Post', body: 'Body')
+
+              render json: book, namespace: :api_v2
+            end
+
+            def namespace_set_in_before_filter
+              book = Book.new(title: 'New Post', body: 'Body')
+              render json: book
+            end
+          end
+        end
+      end
+
+      tests Api::V3::LookupTestController
+
+      setup do
+        @test_namespace = self.class.parent
+      end
+
+      test 'implicitly uses namespaced serializer' do
+        get :implicit_namespaced_serializer
+
+        assert_serializer Api::V3::BookSerializer
+
+        expected = { 'title' => 'New Post', 'body' => 'Body', 'writer' => { 'name' => 'Bob' } }
+        actual = JSON.parse(@response.body)
+
+        assert_equal expected, actual
+      end
+
+      test 'explicit namespace as module' do
+        get :explicit_namespace_as_module
+
+        assert_serializer Api::V2::BookSerializer
+
+        expected = { 'title' => 'New Post' }
+        actual = JSON.parse(@response.body)
+
+        assert_equal expected, actual
+      end
+
+      test 'explicit namespace as string' do
+        get :explicit_namespace_as_string
+
+        assert_serializer Api::V2::BookSerializer
+
+        expected = { 'title' => 'New Post' }
+        actual = JSON.parse(@response.body)
+
+        assert_equal expected, actual
+      end
+
+      test 'explicit namespace as symbol' do
+        get :explicit_namespace_as_symbol
+
+        assert_serializer Api::V2::BookSerializer
+
+        expected = { 'title' => 'New Post' }
+        actual = JSON.parse(@response.body)
+
+        assert_equal expected, actual
+      end
+
+      test 'invalid namespace' do
+        get :invalid_namespace
+
+        assert_serializer ActiveModel::Serializer::Null
+
+        expected = { 'title' => 'New Post', 'body' => 'Body' }
+        actual = JSON.parse(@response.body)
+
+        assert_equal expected, actual
+      end
+
+      test 'namespace set in before filter' do
+        get :namespace_set_in_before_filter
+
+        assert_serializer Api::V2::BookSerializer
+
+        expected = { 'title' => 'New Post' }
+        actual = JSON.parse(@response.body)
+
+        assert_equal expected, actual
+      end
+    end
+  end
+end