Support constraints on resource custom params when nesting

The Mapper looks for a :id constraint in the scope to see whether it
should apply a constraint for nested resources. Since #5581 added support
for resource params other than :id, we need to check for a constraint on
the parent resource's param name and not assume it's :id.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -917,7 +917,7 @@ def initialize(entities, options = {})
             @path       = (options[:path] || @name).to_s
             @controller = (options[:controller] || @name).to_s
             @as         = options[:as]
-            @param      = options[:param] || :id
+            @param      = (options[:param] || :id).to_sym
             @options    = options
           end
 
@@ -969,8 +969,12 @@ def new_scope(new_path)
             "#{path}/#{new_path}"
           end
 
+          def nested_param
+            :"#{singular}_#{param}"
+          end
+
           def nested_scope
-            "#{path}/:#{singular}_#{param}"
+            "#{path}/:#{nested_param}"
           end
 
         end
@@ -1481,18 +1485,18 @@ def resource_scope(kind, resource) #:nodoc:
           def nested_options #:nodoc:
             options = { :as => parent_resource.member_name }
             options[:constraints] = {
-              :"#{parent_resource.singular}_id" => id_constraint
-            } if id_constraint?
+              parent_resource.nested_param => param_constraint
+            } if param_constraint?
 
             options
           end
 
-          def id_constraint? #:nodoc:
-            @scope[:constraints] && @scope[:constraints][:id].is_a?(Regexp)
+          def param_constraint? #:nodoc:
+            @scope[:constraints] && @scope[:constraints][parent_resource.param].is_a?(Regexp)
           end
 
-          def id_constraint #:nodoc:
-            @scope[:constraints][:id]
+          def param_constraint #:nodoc:
+            @scope[:constraints][parent_resource.param]
           end
 
           def canonical_action?(action, flag) #:nodoc:

actionpack/test/dispatch/routing_test.rb

@@ -482,7 +482,7 @@ def self.call(params, request)
         get :preview, :on => :member
       end
 
-      resources :profiles, :param => :username do
+      resources :profiles, :param => :username, :username => /[a-z]+/ do
         get :details, :on => :member
         resources :messages
       end
@@ -2243,6 +2243,17 @@ def test_custom_param
     assert_equal '34', @request.params[:id]
   end
 
+  def test_custom_param_constraint
+    get '/profiles/bob1'
+    assert_equal 404, @response.status
+
+    get '/profiles/bob1/details'
+    assert_equal 404, @response.status
+
+    get '/profiles/bob1/messages/34'
+    assert_equal 404, @response.status
+  end
+
 private
   def with_https
     old_https = https?