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/serializers/serializer_for_with_namespace_test.rb

@@ -0,0 +1,87 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class SerializerForWithNamespaceTest < ActiveSupport::TestCase
+      class Book < ::Model; end
+      class Page < ::Model; end
+      class Publisher < ::Model; end
+
+      module Api
+        module V3
+          class BookSerializer < ActiveModel::Serializer
+            attributes :title, :author_name
+
+            has_many :pages
+            belongs_to :publisher
+          end
+
+          class PageSerializer < ActiveModel::Serializer
+            attributes :number, :text
+
+            belongs_to :book
+          end
+
+          class PublisherSerializer < ActiveModel::Serializer
+            attributes :name
+          end
+        end
+      end
+
+      class BookSerializer < ActiveModel::Serializer
+        attributes :title, :author_name
+      end
+      test 'resource without a namespace' do
+        book = Book.new(title: 'A Post', author_name: 'hello')
+
+        # TODO: this should be able to pull up this serializer without explicitly specifying the serializer
+        #       currently, with no options, it still uses the Api::V3 serializer
+        result = ActiveModelSerializers::SerializableResource.new(book, serializer: BookSerializer).serializable_hash
+
+        expected = { title: 'A Post', author_name: 'hello' }
+        assert_equal expected, result
+      end
+
+      test 'resource with namespace' do
+        book = Book.new(title: 'A Post', author_name: 'hi')
+
+        result = ActiveModelSerializers::SerializableResource.new(book, namespace: Api::V3).serializable_hash
+
+        expected = { title: 'A Post', author_name: 'hi', pages: nil, publisher: nil }
+        assert_equal expected, result
+      end
+
+      test 'has_many with nested serializer under the namespace' do
+        page = Page.new(number: 1, text: 'hello')
+        book = Book.new(title: 'A Post', author_name: 'hi', pages: [page])
+
+        result = ActiveModelSerializers::SerializableResource.new(book, namespace: Api::V3).serializable_hash
+
+        expected = {
+          title: 'A Post', author_name: 'hi',
+          publisher: nil,
+          pages: [{
+            number: 1, text: 'hello'
+          }]
+        }
+        assert_equal expected, result
+      end
+
+      test 'belongs_to with nested serializer under the namespace' do
+        publisher = Publisher.new(name: 'Disney')
+        book = Book.new(title: 'A Post', author_name: 'hi', publisher: publisher)
+
+        result = ActiveModelSerializers::SerializableResource.new(book, namespace: Api::V3).serializable_hash
+
+        expected = {
+          title: 'A Post', author_name: 'hi',
+          pages: nil,
+          publisher: {
+            name: 'Disney'
+          }
+        }
+        assert_equal expected, result
+      end
+    end
+  end
+end