How to set a max length on collection index in mongodb

Question

Using Pymongo to set/query/insert in my mongodb database.

It is possible to indicate that a value should be a unique index in mongodb.

db.mycoll.create_index("sid", unique=True) # This is what I have so far

Now, I want to make sure that the index has a minimum and maximum length. (3-20 chars).

I didn't find any information about adding length restrictions, is it possible with mongodb ?

I would expect something like this:

db.mycoll.create_index("sid", unique=True, max_len=20, min_len=3) # Is there a way to do this ? 

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If you need a bit more context, I have a collection in mongodb and for every document I need a SID (string ID). I would like to put a technical size limit on this value, so I can be sure on every side of my application that any string which doesn't match this size is not a proper id.

Thanks a lot