Mongoose/MongoDB 结果字段在 Javascript 中出现未定义

Question

Is there something that I'm missing that would allow item to log as an object with a parameter, but when I try to access that parameter, it's undefined?

What I've tried so far:

• console.log(item) => { title: "foo", content: "bar" } , that's fine
• console.log(typeof item) => object
• console.log(item.title) => "undefined"

I'll include some of the context just in case it's relevant to the problem.

var TextController = function(myCollection) {
  this.myCollection = myCollection
}

TextController.prototype.list = function(req, res, next) {
  this.myCollection.find({}).exec(function(err, doc) {
    var set = new Set([])
    doc.forEach(function(item) {
      console.log(item)         // Here item shows the parameter
      console.log(item.title)   // "undefined"
      set.add(item.title)       
    })
    res.json(set.get());
  })
}

Based on suggestion I dropped debugger before this line to check what item actually is via the node repl debugger. This is what I found: http://hastebin.com/qatireweni.sm

From this I tried console.log(item._doc.title) and it works just fine.. So, this seems more like a mongoose question now than anything.

There are questions similar to this, but they seem to be related to 'this' accessing of objects or they're trying to get the object outside the scope of the function. In this case, I don't think I'm doing either of those, but inform me if I'm wrong. Thanks

Answer 1

Solution

You can call the toObject method in order to access the fields. For example:

var itemObject = item.toObject();
console.log(itemObject.title); // "foo"

Why

As you point out that the real fields are stored in the _doc field of the document .

But why console.log(item) => { title: "foo", content: "bar" } ?

From the source code of mongoose(document.js) , we can find that the toString method of Document call the toObject method. So console.log will show fields 'correctly'. The source code is shown below:

var inspect = require('util').inspect;

...

/**
 * Helper for console.log
 *
 * @api public
 */
Document.prototype.inspect = function(options) {
  var isPOJO = options &&
    utils.getFunctionName(options.constructor) === 'Object';
  var opts;
  if (isPOJO) {
    opts = options;
  } else if (this.schema.options.toObject) {
    opts = clone(this.schema.options.toObject);
  } else {
    opts = {};
  }
  opts.minimize = false;
  opts.retainKeyOrder = true;
  return this.toObject(opts);
};

/**
 * Helper for console.log
 *
 * @api public
 * @method toString
 */

Document.prototype.toString = function() {
  return inspect(this.inspect());
};

Answer 2

Make sure that you have defined title in your schema:

var MyCollectionSchema = new mongoose.Schema({
    _id: String,
    title: String
});

Answer 3

Try performing a for in loop over item and see if you can access values.

for (var k in item) {
    console.log(item[k]);
}

If it works, it would mean your keys have some non-printable characters or something like this.

From what you said in the comments, it looks like somehow item is an instance of a String primitive wrapper.

Eg

var s = new String('test');
typeof s; //object
s instanceof String; //true

To verify this theory, try this:

eval('(' + item + ')').title;

It could also be that item is an object that has a toString method that displays what you see.

EDIT: To identify these issues quickly, you can use console.dir instead of console.log , since it display an interactive list of the object properties. You can also but a breakpoint and add a watch.

Answer 4

Old question, but since I had a problem with this too, I'll answer it.
This probably happened because you're using find() instead of findOne() . So in the end, you're calling a method for an array of documents instead of a document, resulting in finding an array and not a single document. Using findOne() will let you get access the object normally.

Answer 5

Use findOne() instead of find() .

The find() method returns an array of values, even if you have only one possible result, you'll need to use item[0] to get it.

The findOne method returns one object or none, then you'll be able to access its properties with no issues.

Answer 6

A better way to tackle an issue like this is using doc.toObject() like this

doc.toObject({ getters: true })

other options include:

• getters: apply all getters (path and virtual getters)
• virtuals: apply virtual getters (can override getters option)
• minimize: remove empty objects (defaults to true)
• transform: a transform function to apply to the resulting document before returning
• depopulate: depopulate any populated paths, replacing them with their original refs (defaults to false)
• versionKey: whether to include the version key (defaults to true)

so for example you can say

Model.findOne().exec((err, doc) => {
   if (!err) {
      doc.toObject({ getters: true })
      console.log('doc _id:', doc._id) // or title
   }
})

and now it will work

Answer 7

You don't have whitespace or funny characters in ' title' , do you? They can be defined if you've quoted identifiers into the object/map definition. For example:

var problem = {
    ' title': 'Foo',
    'content': 'Bar'
};

That might cause console.log(item) to display similar to what you're expecting, but cause your undefined problem when you access the title property without it's preceding space.

Answer 8

I think using 'find' method returns an array of Documents.I tried this and I was able to print the title

for (var i = 0; i < doc.length; i++) {
   console.log("iteration " + i);
   console.log('ID:' + docs[i]._id);
   console.log(docs[i].title);
}

Answer 9

If you only want to get the info without all mongoose benefits, save ie, you can use .lean() in your query. It will get your info quicker and you'll can use it as an object directly.

https://mongoosejs.com/docs/api.html#query_Query-lean

As says in docs, this is the best to read-only scenarios.

Answer 10

Are you initializing your object?

function MyObject()
{
    this.Title = "";
    this.Content = "";
}

var myo1 = new MyObject();

If you do not initialize or have not set a title. You will get undefined.

Answer 11

当您进行周二查询时，请使用.lean()例如

const order = await Order.findId("84578437").lean()

Answer 12

find returns an array of object, so to access element use indexing, like doc[0].title