Mongoose: From an array of objects in model, extract value of a property as array

Question

I have a schema of users, contains books, and I want to get the book list of one user

var userSchema = new Schema({
    id: {
        type: String,
        unique: true,
        required: true
    },
    gender: String,
    firstName: String,
    lastName: String,
    books: [{
        id: String
        title: String
        author: String
    }]

});

For example,

[{id: "1",
  gender: "Male",
  firstName: "Alan",
  lastName: "Brown",
  books: [
    { id: "ISN001",
      title: "Programming From Beginer to Give Up",
      author: "Someone"
    },
    { id: "ISN002",
      title: "Introduction to Databases",
      author: "BetaCat"
    }
  ]
},
{ id: "2",
  gender: "Female",
  firstName: "Cynthia",
  lastName: "Durand",
  books: [
    { id: "ISN003",
      title: "How to Play with Your Cat",
      author: "UrCat"
    },
    { id: "ISN004",
      title: "Greek Mythology",
      author: "Heracles"
    }
  ]
}]

I can get the user id by req.params.user_id, and return the book list array of this user. For example, I want ["Programming From Beginer to Give Up","Introduction to Databases"] if I get user_id 1. I try to do this by

var getBookList = (req, res) => {
    User.find(req.params.user_id, (err, user) => {
        var books = user.map((item) => {return item.title;});
    });
}

But doesn't work, because map is not a function for user.

And I try to use map from the answer of this question. https://stackoverflow.com/questions/19590865/from-an-array-of-objects-extract-value-of-a-property-as-array — Xun Zhang, Sep 26 '19 at 13:49

Dominic · Accepted Answer · 2019-09-27T08:26:05.937

1

User is an object, you want to map the books array inside it.

const books = user.books.map(book => book.title)

Also change find to findById as you only want 1 record.

When in doubt put a console.log in there and see if the value is what you expect.

edited Sep 27 '19 at 08:26

answered Sep 26 '19 at 14:00

Dominic

62,658
20
139
163

TypeError: Cannot read property 'books' of undefined. It seems that the user I find can not do that – Xun Zhang Sep 27 '19 at 08:06
I just noticed you're using `.find` which is for multiple records so it will return an array of users with one inside it, change to `findById` since you are finding by ID (or `findOne` if you weren't). Never use queries for multiple documents when looking for a single one as this is inefficient too. – Dominic Sep 27 '19 at 08:08

score 0 · Answer 2 · answered Sep 26 '19 at 13:58

0

User.find({id:req.params.user_id}, (err, user) => {
    var books = user.map((item) => {return item.title;});
});

Try the above one

answered Sep 26 '19 at 13:58

Sudhakar

533
1
3
17

no error for this one :D, but empty array output ;-( – Xun Zhang Sep 27 '19 at 08:12

Mongoose: From an array of objects in model, extract value of a property as array

2 Answers2