4 files changed, 34 insertions, 1 deletions
diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index 6585808fa2..500d8dc42f 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   `#to_param` returns `nil` if `#to_key` returns `nil`. Fixes #11399.
+
+    *Yves Senn*
+
 *   Ability to specify multiple contexts when defining a validation.
 
     Example:
diff --git a/activemodel/lib/active_model/conversion.rb b/activemodel/lib/active_model/conversion.rb
index 6b0a752566..0a19ef686d 100644
--- a/activemodel/lib/active_model/conversion.rb
+++ b/activemodel/lib/active_model/conversion.rb
@@ -62,7 +62,7 @@ module ActiveModel
     #   person = Person.create
     #   person.to_param # => "1"
     def to_param
-      persisted? ? to_key.join('-') : nil
+      (persisted? && key = to_key) ? key.join('-') : nil
     end
 
     # Returns a +string+ identifying the path associated with the object.
diff --git a/activemodel/test/cases/conversion_test.rb b/activemodel/test/cases/conversion_test.rb
index 3bb177591d..c5cfbf909d 100644
--- a/activemodel/test/cases/conversion_test.rb
+++ b/activemodel/test/cases/conversion_test.rb
@@ -24,6 +24,16 @@ class ConversionTest < ActiveModel::TestCase
     assert_equal "1", Contact.new(id: 1).to_param
   end
 
+  test "to_param returns nil if to_key is nil" do
+    klass = Class.new(Contact) do
+      def persisted?
+        true
+      end
+    end
+
+    assert_nil klass.new.to_param
+  end
+
   test "to_partial_path default implementation returns a string giving a relative path" do
     assert_equal "contacts/contact", Contact.new.to_partial_path
     assert_equal "helicopters/helicopter", Helicopter.new.to_partial_path,
diff --git a/activemodel/test/cases/dirty_test.rb b/activemodel/test/cases/dirty_test.rb
index 8b55901a65..2853476c91 100644
--- a/activemodel/test/cases/dirty_test.rb
+++ b/activemodel/test/cases/dirty_test.rb
@@ -41,6 +41,10 @@ class DirtyTest < ActiveModel::TestCase
     def save
       changes_applied
     end
+
+    def reload
+      reset_changes
+    end
   end
 
   setup do
@@ -157,4 +161,19 @@ class DirtyTest < ActiveModel::TestCase
     @model.size = 1
     assert @model.size_changed?
   end
+
+  test "reload should reset all changes" do
+    @model.name = 'Dmitry'
+    @model.name_changed?
+    @model.save
+    @model.name = 'Bob'
+
+    assert_equal [nil, 'Dmitry'], @model.previous_changes['name']
+    assert_equal 'Dmitry', @model.changed_attributes['name']
+
+    @model.reload
+
+    assert_equal ActiveSupport::HashWithIndifferentAccess.new, @model.previous_changes
+    assert_equal ActiveSupport::HashWithIndifferentAccess.new, @model.changed_attributes
+  end
 end