1 files changed, 17 insertions, 6 deletions
diff --git a/actionpack/lib/action_dispatch/journey/formatter.rb b/actionpack/lib/action_dispatch/journey/formatter.rb
index 0323360faa..b5a8ca7262 100644
--- a/actionpack/lib/action_dispatch/journey/formatter.rb
+++ b/actionpack/lib/action_dispatch/journey/formatter.rb
@@ -1,10 +1,11 @@
-require 'action_controller/metal/exceptions'
+require "action_controller/metal/exceptions"
 
 module ActionDispatch
+  # :stopdoc:
   module Journey
     # The Formatter class is used for formatting URLs. For example, parameters
     # passed to +url_for+ in Rails will eventually call Formatter#generate.
-    class Formatter # :nodoc:
+    class Formatter
       attr_reader :routes
 
       def initialize(routes)
@@ -32,15 +33,24 @@ module ActionDispatch
 
           defaults       = route.defaults
           required_parts = route.required_parts
-          parameterized_parts.keep_if do |key, value|
-            (defaults[key].nil? && value.present?) || value.to_s != defaults[key].to_s || required_parts.include?(key)
+
+          route.parts.reverse_each do |key|
+            break if defaults[key].nil? && parameterized_parts[key].present?
+            break if parameterized_parts[key].to_s != defaults[key].to_s
+            break if required_parts.include?(key)
+
+            parameterized_parts.delete(key)
           end
 
           return [route.format(parameterized_parts), params]
         end
 
-        message = "No route matches #{Hash[constraints.sort_by{|k,v| k.to_s}].inspect}"
-        message << " missing required keys: #{missing_keys.sort.inspect}" if missing_keys && !missing_keys.empty?
+        unmatched_keys = (missing_keys || []) & constraints.keys
+        missing_keys = (missing_keys || []) - unmatched_keys
+
+        message = "No route matches #{Hash[constraints.sort_by { |k,v| k.to_s }].inspect}"
+        message << ", missing required keys: #{missing_keys.sort.inspect}" if missing_keys && !missing_keys.empty?
+        message << ", possible unmatched constraints: #{unmatched_keys.sort.inspect}" if unmatched_keys && !unmatched_keys.empty?
 
         raise ActionController::UrlGenerationError, message
       end
@@ -169,4 +179,5 @@ module ActionDispatch
         end
     end
   end
+  # :startdoc:
 end