Fixed Link validation

[iterlace]

In cases where you tried to save a Document with a field containing a Link/DBRef instead of the wrapped object itself, the validate_on_save hook would stop working, and return an error.

This was because the Link object was first serialized into a dict, and any further validation with Link.validate would always fail trying to deserialize a dict like {"id": ..., "collection": ...} as a real Document rather than a Link.

I've committed a simple solution, however, you can refine it to make it more elegant.

As my reserch has shown, there is no sense tweaking the way Document.validate_self validates the document. Instead, the easiest way would be to teach the Link to recognize its own schema.
This approach has the only obvious corner case: if the user creates a custom model with the same structure as Link, and then tries to wrap it with Link, he would get an unexpected behavior.

[iterlace]

Ping?

[roman-right]

Hi! Sorry for the delay. Thank you for the PR!
I'll check it by the end of this week

[roman-right]

Merged! Thank you! It will be published by the end of this week with others.