I'll add the Integration test for mongodb to make sure that this case works

One issue is that MongoDB uses an abstraction that isn't fully applicable to how MongoDB represents paths. Accepting property paths that start with a whitespace is indeed odd, and we should look forward to reject these unless there's a good reason to keep them. Java properties cannot start with a whitespace, however, other JVM languages can create method names containing whitespace.

One issue is that MongoDB uses an abstraction that isn't fully applicable to how MongoDB represents paths

I agree. I think the separate issue can be created for this in Spring Data MongoDB to think about the design around it. Since PropertyPath is indeed not exactly the great fit for expressing paths within a mongodb document, at least it seems.

Java properties cannot start with a whitespace, however, other JVM languages can create method names containing whitespace.

Well, yeah, that is true and it is a valid concern.

Just my 2 cents 😄 : If Spring Data decide to consider the whitespace starting paths in general, that means we need to differentiate between hypothetical foo, foo and \tfoo as paths. And then it becomes unclear how to reference those fields using for instance derived queries like:

public MyEntity `findByFoo(SomeType foo)` // It is unclear how to express ` foo` vs `\tfoo` vs `foo`...

Apart from that, it seems that the Kotlin Language, the one that is most often used apart from Java with Spring applications, does not permit the identifier to start with whitespace, see specification:

identifier : (letter | '_') (letter | digit | '_')*

So, my humble opinion is that it is not worth it to consider the paths that start with a whitespace. But I might be wrong.

CC: @odrotbohm @mp911de