1 files changed, 20 insertions, 0 deletions
diff --git a/spec/models/concerns/from_set_operator_spec.rb b/spec/models/concerns/from_set_operator_spec.rb
new file mode 100644
index 00000000000..8ebbb5550c9
--- /dev/null
+++ b/spec/models/concerns/from_set_operator_spec.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe FromSetOperator do
+  describe 'when set operator method already exists' do
+    let(:redefine_method) do
+      Class.new do
+        def self.from_union
+          # This method intentionally left blank.
+        end
+
+        extend FromSetOperator
+        define_set_operator Gitlab::SQL::Union
+      end
+    end
+
+    it { expect { redefine_method }.to raise_exception(RuntimeError) }
+  end
+end