为什么对象在输出属性时返回 undefined？

Question

At the server side with Node js, I am using Mongoose to query by the ID a document. I queried a document from mongodb to use the ID of that document to add some data to it. But when I want to use de ID of that document it is undefined. Here is the code:

user.find({user: req.body.user}, function(err, doc) {

        if(err) {
            console.log(err);
        } else {
            console.log(doc + ' the document');  // Here returns the object with all properties
            console.log(typeof doc + ' type of variable'); // Outputs Object
            console.log(doc._id + ' the ID');              // Undefined
            console.log(doc.user+ ' the user');     //Also undefined

            user.findById(doc._id, function(err, user){
                if(err) {
                    console.log(err);
                } else {
                    console.log(user + ' <---');
                    // for (const iterator of req.body.list) {
                    //     console.log(iterator);
                    //     user.cases = iterator;
                    //     user.save().then(person => {
                    //         console.log('Added')
                    //     })
                    //     .catch(err => {
                    //         console.log('Failed')
                    //     });
                    // }
                }
            })
            // console.log(doc + ' <--- Document');

        }

    });
})

Answer 1

Mongoose's Model.find() always provides documents inside of a collection, even if there's only 1. This method is a shorthand for calling Query.find() , which notes this more directly:

The result will be an array of documents.

With it, you'll have to access the desired document from the collection before accessing the _id .

user.find({user: req.body.user}, function(err, docs /* note: plural */) {
    if(err) {
        console.log(err);
    } else if (docs.length === 0) {
        console.log('Not found');
    } else {
        console.log(docs[0]._id, 'the user');

        let userDoc = docs[0];
        console.log(userDoc._id, 'the user');

        // ...
    }
});

You can alternately use Model.findOne() to get back only the first/single document that matches the query:

user.findOne({user: req.body.user}, function(err, userDoc) {
    if(err) {
        console.log(err);
    } else if (userDoc == null) {
        console.log('Not found');
    } else {
        console.log(userDoc._id, 'the user');

        // ...
    }
});