Be sure to pass a Decorator class and not a string to 'with:'

lib/draper.rb

@@ -50,9 +50,8 @@ def self._decorate(object_or_enumerable, with: nil, namespace: nil)
 
   def self._guess_decorator(object_or_enumerable, with: nil, namespace: nil)
     object_or_enumerable = object_or_enumerable.first if object_or_enumerable.is_a? Enumerable
-    klass     = with || object_or_enumerable.class.name
-    namespace = "#{namespace}::" if namespace.present?
-    decorator = "#{namespace}#{klass}Decorator"
+    klass     = with || "#{object_or_enumerable.class.name}Decorator"
+    decorator = [namespace, klass].compact.join('::')
     decorator.safe_constantize || raise(Draper::UninferrableDecoratorError.new(klass, decorator))
   end
 

lib/draper/decorator.rb

@@ -31,7 +31,7 @@ def delegate(*methods)
       #   end
       def decorates_association(relation_name, with: nil, namespace: nil, scope: nil)
         relation_sym = ":#{relation_name}"
-        with         = with.nil? ? "nil" : "'#{with}'"
+        with         = with.nil? ? "nil" : "#{with}"
         namespace    = namespace.nil? ? "nil" : "'#{namespace}'"
         scope        = scope.nil? ? "nil" : ":#{scope}"
 

spec/dummy/app/decorators/email/comment_decorator.rb

@@ -2,6 +2,7 @@
 
 module Email
   class CommentDecorator < CommentDecorator
-    decorates_association :author, with: 'Foo', namespace: '::Bar'
+    # reset namespace to '' to not inherit 'Email' module namespace
+    decorates_association :author, with: ::Bar::FooDecorator, namespace: ''
   end
 end

spec/dummy/app/decorators/post_decorator.rb

@@ -9,7 +9,7 @@ class PostDecorator < Draper::Decorator
   delegate :id, :created_at, :new_record?
 
   decorates_association :comments
-  decorates_association :author, with: 'User'
+  decorates_association :author, with: UserDecorator
 
   def posted_date
     if created_at.to_date == DateTime.now.utc.to_date