How does RavenDb "In" operator work?

I probably don't understand how the "In" extension method in RavenDb works. I thought that it is an equivalent of the IN command in SQL world, e.g. WHERE Number IN (1, 2, 3). But it behaves quite weird. ``` // i have 3 records - two with Normal severity, one with Low severity using (var session = store.OpenSession()) { session.Store(new TestObject { Name = "o1", Severity = Severity.Normal }); session.Store(new TestObject { Name = "o2", Severity = Severity.Low }); session.Store(new TestObject { Name = "o3", Severity = Severity.Normal }); session.SaveChanges(); } // this writes the Low severity record, it seems correct using (var session = store.OpenSession()) { var data = session.Query<TestObject>() .Where(o => o.Severity.In(new[] { Severity.Low })) .OrderBy(o => o.Name).ToList(); data.ForEach(r => Console.WriteLine(r)); } // this writes also the Low severity record, it still seems correct using (var session = store.OpenSession()) { var data = session.Query<TestObject>() .Where(o => o.Severity.In(new[] { Severity.Low, Severity.High })) .OrderBy(o => o.Name).ToList(); data.ForEach(r => Console.WriteLine(r)); } // this writes all records, still seems correct using (var session = store.OpenSession()) { var data = session.Query<TestObject>() .Where(o => o.Severity.In( new[] { Severity.High, Severity.Low, Severity.Normal })) .OrderBy(o => o.Name).ToList(); data.ForEach(r => Console.WriteLine(r)); } // but this does not write anything // despite there are 2 records with Normal severity using (var session = store.OpenSession()) { var data = session.Query<TestObject>() .Where(o => o.Severity.In(new[] { Severity.High, Severity.Normal })) .OrderBy(o => o.Name).ToList(); data.ForEach(r => Console.WriteLine(r)); } // and this does not write anything either, // I just tried whether the order of values in the array matters using (var session = store.OpenSession()) { var data = session.Query<TestObject>() .Where(o => o.Severity.In(new[] { Severity.Normal, Severity.High })) .OrderBy(o => o.Name).ToList(); data.ForEach(r => Console.WriteLine(r)); } ``` Or did I find a bug in the RavenDb/Lucene engine? Edit I have found another strange thing. It has to do something with the alphabetical order of enum members. When I rename the Severity.Normal to Severity.A, the last two queries behave correctly and return the results. When I rename the Severity.Normal to Severity.La, it still works (because La < Low). But when I rename the Severity.Normal to Severity.Lu (Lu > Low), it breaks and the last two queries are not returning results any more. The original sample does not work because Normal > Low. But I still wonder why it happens because it makes no sense to me.