Make serializer lookup configurable (#1757)

lib/active_model/serializer.rb

@@ -60,17 +60,10 @@ module ActiveModel
 
     # @api private
     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}")
-
-      chain
+      lookups = ActiveModelSerializers.config.serializer_lookup_chain
+      Array[*lookups].flat_map do |lookup|
+        lookup.call(klass, self, namespace)
+      end.compact
     end
 
     # Used to cache serializer name => serializer class

lib/active_model/serializer/concerns/configuration.rb

@@ -32,6 +32,26 @@ module ActiveModel
         config.jsonapi_include_toplevel_object = false
         config.include_data_default = true
 
+        # For configuring how serializers are found.
+        # This should be an array of procs.
+        #
+        # The priority of the output is that the first item
+        # in the evaluated result array will take precedence
+        # over other possible serializer paths.
+        #
+        # i.e.: First match wins.
+        #
+        # @example output
+        # => [
+        #   "CustomNamespace::ResourceSerializer",
+        #   "ParentSerializer::ResourceSerializer",
+        #   "ResourceNamespace::ResourceSerializer" ,
+        #   "ResourceSerializer"]
+        #
+        # If CustomNamespace::ResourceSerializer exists, it will be used
+        # for serialization
+        config.serializer_lookup_chain = ActiveModelSerializers::LookupChain::DEFAULT.dup
+
         config.schema_path = 'test/support/schemas'
       end
     end