Fix permitted_param generation for belongs_to when :param is us…

…ed (#6460)

When playing around with our sample application, I noticed that you
couldn't create post taggings due to this bug.

features/belongs_to.feature

@@ -50,6 +50,27 @@ Feature: Belongs To
     When I press "Update Post"
     Then I should see "Post was successfully updated."
 
+  Scenario: Updating a child resource page with custom configuration
+    Given a configuration of:
+    """
+      ActiveAdmin.register User
+      ActiveAdmin.register Post do
+        belongs_to :author, class_name: "User", param: "user_id", route_name: "user"
+        permit_params :title
+
+        form do |f|
+          f.actions
+        end
+      end
+    """
+    When I go to the last author's last post page
+    And I follow "Edit Post"
+    Then I should see the element "form[action='/admin/users/2/posts/2']"
+    And I should see a link to "Hello World" in the breadcrumb
+
+    When I press "Update Post"
+    Then I should see "Post was successfully updated."
+
   Scenario: Creating a child resource page
     Given a configuration of:
     """

lib/active_admin/resource/belongs_to.rb

@@ -46,7 +46,7 @@ def required?
       end
 
       def to_param
-        :"#{@target_name}_id"
+        (@options[:param] || "#{@target_name}_id").to_sym
       end
     end
   end