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]

lib/active_model/serializer.rb

@@ -44,7 +44,7 @@ module ActiveModel
       elsif resource.respond_to?(:to_ary)
         config.collection_serializer
       else
-        options.fetch(:serializer) { get_serializer_for(resource.class) }
+        options.fetch(:serializer) { get_serializer_for(resource.class, options[:namespace]) }
       end
     end
 
@@ -59,13 +59,14 @@ module ActiveModel
     end
 
     # @api private
-    def self.serializer_lookup_chain_for(klass)
+    def self.serializer_lookup_chain_for(klass, namespace = nil)
       chain = []
 
       resource_class_name = klass.name.demodulize
       resource_namespace = klass.name.deconstantize
       serializer_class_name = "#{resource_class_name}Serializer"
 
+      chain.push("#{namespace}::#{serializer_class_name}") if namespace
       chain.push("#{name}::#{serializer_class_name}") if self != ActiveModel::Serializer
       chain.push("#{resource_namespace}::#{serializer_class_name}")
 
@@ -84,11 +85,14 @@ module ActiveModel
     #   1. class name appended with "Serializer"
     #   2. try again with superclass, if present
     #   3. nil
-    def self.get_serializer_for(klass)
+    def self.get_serializer_for(klass, namespace = nil)
       return nil unless config.serializer_lookup_enabled
-      serializers_cache.fetch_or_store(klass) do
+
+      cache_key = ActiveSupport::Cache.expand_cache_key(klass, namespace)
+      serializers_cache.fetch_or_store(cache_key) do
         # NOTE(beauby): When we drop 1.9.3 support we can lazify the map for perfs.
-        serializer_class = serializer_lookup_chain_for(klass).map(&:safe_constantize).find { |x| x && x < ActiveModel::Serializer }
+        lookup_chain = serializer_lookup_chain_for(klass, namespace)
+        serializer_class = lookup_chain.map(&:safe_constantize).find { |x| x && x < ActiveModel::Serializer }
 
         if serializer_class
           serializer_class