如果_id在MongoDB中有什么特殊用途吗？

Question

I have one nodejs app, which define answer like this:

var answerSchema = mongoose.Schema({
    session  : String, // the ID of the session
    question : String, // the ID of the question
    answer   : Number  // the selected answer (1-5)
});

stored as answer

var sessionSchema = mongoose.Schema({
    name   : String, // the name of the session, like 'SAM' or 'SA'
    currQ  : Number, // the current question being answered
    state  : String, // the state of the current question: new, done, asking, summarize
    active : Number  // 1 if active, 0 otherwise
});

stored as session.

In the ejs file, one function get one answer-tpye variable and use

var reply = JSON.parse(res);

then

session = reply.session._id;

I am really confused what's this line means. There are still many case when _id is used but they didn't define it.

Answer 1

_id is added to all MongoDB documents by default.

Mongoose also adds an id getter for a string representation of _id so you can also call

reply.session.id

These properties will be available without being explicitly added to the schema