When using mongoc_collection_insert(), the document is checked using bson_validate()

When using a bulk insert, bson_validate() is not used.

So this document can be inserted with a bulk insert, but not with mongoc_collection_insert():
{
"user":

{ "$ref": "User", "$id": "5374d465c43a034f528b45ba", "$db": "db_name" }

}