How to apply constraints in MongoDB?

MongoDB 3.2 Update

Document validation is now supported natively by MongoDB.

Example from the documentation:

db.createCollection( "contacts",
   { validator: { $or:
      [
         { phone: { $type: "string" } },
         { email: { $regex: /@mongodb\.com$/ } },
         { status: { $in: [ "Unknown", "Incomplete" ] } }
      ]
   }
} )

Original answer

To go beyond the uniqueness constraint available natively in indexes, you need to use something like Mongoose and its ability to support field-based validation. That will give you support for things like minimum value, but only when updates go through your Mongoose schemas/models.