存储类型： BSON BSON documents (objects) consist of a well ordered list of elements. Each element consists of a field name, a type, and a value. Field names are strings. Types include: * string * integer * double * date * byte array (binary data) * boolean (true and false) * null * BSON object This is nominally a superset of JSON types (JSON does not have a byte array type, for example), but because of length limitations, some valid JSON values (such as very long strings) are not valid BSON values.
Cursor Stages A MongoCursor has two "life stages": pre- and post- query. When a cursor is created, it has not yet contacted the database, so it is in its pre-query state. In this state, the client can further specify what they want the query to do, including adding limits, skips, sorts, and more advanced options. When the client attempts to get a result (by calling MongoCursor::next(), directly or indirectly), the cursor moves into the post-query stage. At this point, the query has been executed by the database and cannot be modified anymore.