Sooner or later, you’ll come to the point, that you don’t only want to get objects by Id or the whole set of objects of a specific type in Realm. The easiest, but probably most unsophisticated way to do it, would be just use a function on a Linq-Where statement. That’s easy and has reasonable performance, no need to iterate and implement it yourself. But it actually is a big waste of resources, because depending upon the function, every single element needs to be evaluated.
But there is a better way to do so, eventhough it’s support in Realm is very confined. You can use Linq-Expressions. In the next table you find a plan, on what is supported or not:
| Expression | Example | Realm | Info |
| Int-Query Double-Query String-Query | class.Number==4 class.Number>3.4 class.Number<=2 class.Text==”Hello World” | ✓ | |
| String-Functions: Contains, StartsWith, EndsWith | class.Text.StartsWith(“T”) | ✓ | as StringComparison you can use Ordinal or OrdinalIgnoreCase |
| advanced calculation | class.Number*20>100 | ╳ | no left-side operations are supported, but right-side operations |
| Same-Object Comparison | class.Number>class.Number2 | ╳ | only comparison with constant values are supported |
| List-Functions | class.List.Contains(3) | ╳ | |
| Sub-Object-Access | class.SubClass.Number==1 | ╳ | |
| Take() | Take(20) | ╳ | |
| Skip() | Skip(5) | ╳ | |
That’s really not a lot. Actually it ends up with numeric type comparisons (==,<,>..) and some string-Comparer (==,Contains, StartsWith, EndsWith). I really hope Mongo Inc. will spend some more time on implementing more Linq-Expression features in Realm.
| Expression | Filter | |
| advanced calculation | ╳ | |
| Same-Object Comparison | SubClass.Number>Number | |
| List-Functions | ANY SubClassArray.Number>10 | ANY or SOME works only with RealmObject-Arrays, not with atomic type arrays |
| Sub-Object-Access | SubClass.Number>20 | |
| Take() | ╳ | |
| Skip() | ╳ |
Summarizing my investigation and research on this, I can say: There’s space for improvement. I am very keen on finding out, whether MongoRealm will have a more sufficient way to query data.