Merge pull request #1618 from bf4/RomanKapitonov-master

Empty collection root key from explicit serializer option

lib/active_model/serializer.rb

@@ -182,7 +182,7 @@ module ActiveModel
 
     # Used by adapter as resource root.
     def json_key
-      root || object.class.model_name.to_s.underscore
+      root || _type || object.class.model_name.to_s.underscore
     end
 
     def read_attribute_for_serialization(attr)

lib/active_model/serializer/collection_serializer.rb

@@ -8,10 +8,11 @@ module ActiveModel
       attr_reader :object, :root
 
       def initialize(resources, options = {})
-        @root = options[:root]
-        @object = resources
+        @object                  = resources
+        @options                 = options
+        @root                    = options[:root]
+        serializer_context_class = options.fetch(:serializer_context_class, ActiveModel::Serializer)
         @serializers = resources.map do |resource|
-          serializer_context_class = options.fetch(:serializer_context_class, ActiveModel::Serializer)
           serializer_class = options.fetch(:serializer) { serializer_context_class.serializer_for(resource) }
 
           if serializer_class.nil? # rubocop:disable Style/GuardClause
@@ -26,9 +27,28 @@ module ActiveModel
         true
       end
 
+      # TODO: unify naming of root, json_key, and _type.  Right now, a serializer's
+      # json_key comes from the root option or the object's model name, by default.
+      # But, if a dev defines a custom `json_key` method with an explicit value,
+      # we have no simple way to know that it is safe to call that instance method.
+      # (which is really a class property at this point, anyhow).
+      # rubocop:disable Metrics/CyclomaticComplexity
+      # Disabling cop since it's good to highlight the complexity of this method by
+      # including all the logic right here.
       def json_key
-        root || derived_root
+        return root if root
+        # 1. get from options[:serializer] for empty resource collection
+        key = object.empty? &&
+          (explicit_serializer_class = options[:serializer]) &&
+          explicit_serializer_class._type
+        # 2. get from first serializer instance in collection
+        key ||= (serializer = serializers.first) && serializer.json_key
+        # 3. get from collection name, if a named collection
+        key ||= object.respond_to?(:name) ? object.name && object.name.underscore : nil
+        # 4. key may be nil for empty collection and no serializer option
+        key && key.pluralize
       end
+      # rubocop:enable Metrics/CyclomaticComplexity
 
       def paginated?
         object.respond_to?(:current_page) &&
@@ -38,14 +58,7 @@ module ActiveModel
 
       protected
 
-      attr_reader :serializers
-
-      private
-
-      def derived_root
-        key = serializers.first.try(:json_key) || object.try(:name).try(:underscore)
-        key.try(:pluralize)
-      end
+      attr_reader :serializers, :options
     end
   end
 end