Avoid allocating extra hash and arrays when as_json is called without options

Co-authored-by: NEugene Kenny <elkenny@gmail.com>

activemodel/lib/active_model/serialization.rb

@@ -121,17 +121,17 @@ module Serialization
     #   user.serializable_hash(include: { notes: { only: 'title' }})
     #   # => {"name" => "Napoleon", "notes" => [{"title"=>"Battle of Austerlitz"}]}
     def serializable_hash(options = nil)
-      options ||= {}
-
       attribute_names = attributes.keys
+
+      return serializable_attributes(attribute_names) unless options
+
       if only = options[:only]
         attribute_names &= Array(only).map(&:to_s)
       elsif except = options[:except]
         attribute_names -= Array(except).map(&:to_s)
       end
 
-      hash = {}
-      attribute_names.each { |n| hash[n] = read_attribute_for_serialization(n) }
+      hash = serializable_attributes(attribute_names)
 
       Array(options[:methods]).each { |m| hash[m.to_s] = send(m) }
 
@@ -165,6 +165,10 @@ def serializable_hash(options = nil)
       #   end
       alias :read_attribute_for_serialization :send
 
+      def serializable_attributes(attribute_names)
+        attribute_names.index_with { |n| read_attribute_for_serialization(n) }
+      end
+
       # Add associations specified via the <tt>:include</tt> option.
       #
       # Expects a block that takes as arguments:

activerecord/lib/active_record/serialization.rb

@@ -11,10 +11,12 @@ module Serialization
     end
 
     def serializable_hash(options = nil)
-      options = options ? options.dup : {}
+      if self.class.has_attribute?(self.class.inheritance_column)
+        options = options ? options.dup : {}
 
-      options[:except] = Array(options[:except]).map(&:to_s)
-      options[:except] |= Array(self.class.inheritance_column)
+        options[:except] = Array(options[:except]).map(&:to_s)
+        options[:except] |= Array(self.class.inheritance_column)
+      end
 
       super(options)
     end