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How to select fields in a document in MongoDB

January 05, 2020🍿 2 min readEdit this post on Github

This is part 3 of 3 in my series on "Mongodb Bootcamp: Beginners to Advanced"

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Let’s assume we have a collection with lot of data in a single document. We have a countries collection. We are going to select just few fields we want,

// Query without selection
db.posts.find({});
// Result will look like
[
  {
    _id: ObjectId('5e14ba178e47a9a3b8b7af52'),
    name: 'Estonia',
    shortName: 'EST',
    cities: ['Tartu', 'Tallinn', 'Viljandi'],
  },
  {
    _id: ObjectId('5e14ba3c8e47a9a3b8b7af5a'),
    name: 'India',
    shortName: 'IND',
    cities: ['Chennai', 'Mumbai', 'Delhi'],
  },
  {
    _id: ObjectId('5e14ba558e47a9a3b8b7af62'),
    name: 'United states of America',
    shortName: 'USA',
    cities: [],
  },
  {
    _id: ObjectId('5e14ba6a8e47a9a3b8b7af67'),
    name: 'Sweden',
    shortName: 'SE',
  },
];

Now, I want to select only the name field and display only the result with name field.

Inorder to do that, we need to first understand the mongoDB query syntax

db.countries.find(
  // For basic filtering based on the key values passed
  {},
  // for selecting the fields
  {}
);

You can pass the second parameter to the find method. It will work for findOne too.

Now let’s select the name field. Here you need to learn one more trick, The key will be the field you want to select, what about the value? Value will be 1 for selecting a field. 0 for deselecting a field.

Let’s see it as example

Selecting only country name field

// Query with name selection
db.posts.find({}, { name: 1 });

// Result will look like
[
  {
    _id: ObjectId('5e14ba178e47a9a3b8b7af52'),
    name: 'Estonia',
  },
  {
    _id: ObjectId('5e14ba3c8e47a9a3b8b7af5a'),
    name: 'India',
  },
  {
    _id: ObjectId('5e14ba558e47a9a3b8b7af62'),
    name: 'United states of America',
  },
  {
    _id: ObjectId('5e14ba6a8e47a9a3b8b7af67'),
    name: 'Sweden',
  },
];

Here we have successfully selected only the name field. Wait? there is also the _id field without we pass it explicitly?

by default, mongo query will always include the _id field.

Deselecting any field

Let’s deselect the _id field itself, it’s the same way,

// Query with name selection and _id deselection
db.posts.find({}, { name: 1, _id: 0 });

// Result will look like
[
  {
    name: 'Estonia',
  },
  {
    name: 'India',
  },
  {
    name: 'United states of America',
  },
  {
    name: 'Sweden',
  },
];

Hope you find this tutorial series helpful. Next we will learn more into this series. Share it with your groups and happy learning 🤗

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