diff --git a/spec/lib/acts_as_watchable/lib/open_project/acts/registry_methods_spec.rb b/spec/lib/acts_as_watchable/lib/open_project/acts/registry_methods_spec.rb
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+++ b/spec/lib/acts_as_watchable/lib/open_project/acts/registry_methods_spec.rb
@@ -0,0 +1,75 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) 2012-2024 the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe OpenProject::Acts::RegistryMethods do
+  def described_module = described_class
+
+  let(:registry) { Class.new.tap { |c| c.extend described_module } }
+  let(:acts_as_dummy) { Module.new }
+  let(:instance_methods_module) { Module.new }
+  let(:model) { Class.new }
+
+  before do
+    allow(registry).to receive_messages(module_parent: acts_as_dummy, module_parent_name: "OpenProject::Acts::Dummy")
+    allow(acts_as_dummy).to receive(:const_get).with(:InstanceMethods).and_return(instance_methods_module)
+
+    allow(model).to receive(:name).and_return("Model")
+  end
+
+  describe ".add" do
+    it "allows adding class including instance methods module" do
+      model.include instance_methods_module
+
+      expect { registry.add(model) }.not_to raise_error
+    end
+
+    it "forbids adding class not including instance methods module" do
+      expect { registry.add(model) }.to raise_error(/does not include acts_as_dummy/)
+    end
+  end
+
+  describe ".instance" do
+    before do
+      model.include instance_methods_module
+
+      allow(ActiveSupport::Inflector).to receive(:constantize).with("Model").and_return(model)
+    end
+
+    it "returns nil for non registered model" do
+      expect(registry.instance("models")).to be_nil
+    end
+
+    it "returns model class for registered model" do
+      registry.add(model)
+
+      expect(registry.instance("models")).to eq(model)
+    end
+  end
+end