This feature would allow users to set default values for fields within in a collection. If the field is not present in the document inserted, the field would be set to the default value.

A possible implementation could look like this:

> db.createCollection( "coll", 
     { "validator": { "a": { "$type": "string" }, "b": { "$type": "string" }, "c": { "$type": "string" }}}
     {"fieldValueDefaults": { "c": "defaultString"}}
 )
{ "ok" : 1 }
> db.coll.insert({ "a": "A", "b": "B" })
WriteResult({ "nInserted" : 1 })
> db.coll.findOne()
{
	"_id" : ObjectId("575721a2dc744047e1566f0f"),
	"a" : "A",
	"b" : "B",
	"c" : "defaultString"
}

Original summary

Allow $or and $and in validation rule expression

Original Description

From MongoDB 3.2, we can validate our documents during update or insertion, but we can't use the $or and the $and operators for complex validation rules.

db.createCollection( "collection", 
     { "validator": { 
         "$and": [         
              { "a": { "$type": "string" } },         
              { "b": { "$type": "string" } }, 
              { "c": { "$or": [ { "$type": "string" }, "default" ] }
          ]
     }}
 )

Failed with the following error:

{ "ok" : 0, "errmsg" : "unknown operator: $or", "code" : 2 }

source: https://stackoverflow.com/questions/37674538/how-can-i-ensure-that-only-certain-keys-are-present-in-a-document-using-mongodb