Log `UnnestedInFilters#rewrite` calls to make it easier to debug

lib/unnested_in_filters/rewriter.rb

@@ -105,6 +105,8 @@ def initialize(relation)
     #   LIMIT 20
     #
     def rewrite
+      log_rewrite
+
       model.from(from)
            .limit(limit_value)
            .order(order_values)
@@ -125,6 +127,10 @@ def rewrite?
 
     delegate :model, :order_values, :limit_value, :where_values_hash, to: :relation, private: true
 
+    def log_rewrite
+      ::Gitlab::AppLogger.info(message: 'Query is being rewritten by `UnnestedInFilters`', model: model.name)
+    end
+
     def from
       [value_tables.map(&:to_sql) + [lateral]].join(', ')
     end

spec/lib/unnested_in_filters/rewriter_spec.rb

@@ -138,5 +138,20 @@
         end
       end
     end
+
+    describe 'logging' do
+      subject(:load_reload) { rewriter.rewrite }
+
+      before do
+        allow(::Gitlab::AppLogger).to receive(:info)
+      end
+
+      it 'logs the call' do
+        load_reload
+
+        expect(::Gitlab::AppLogger)
+          .to have_received(:info).with(message: 'Query is being rewritten by `UnnestedInFilters`', model: 'User')
+      end
+    end
   end
 end