Queries the collection for existing documents.
auto findOne(R, T, U) (
QueryFlags flags = QueryFlags
auto findOne(R, T) (
By default, a Bson value of the matching document is returned, or
when no document matched. For types R that are not Bson, the returned value is either
R, or of type , if
R is not a reference/pointer type.
Exception if a DB communication error or a query error occurred.