3. Basic concepts

Let's work with the fundamental concepts of MongoDB.

Databases and collections

In MongoDB, a database contains multiple collections, and each collection contains multiple documents.

To create or switch to a database:

use learn

It doesn't matter that the database doesn't exist yet. It will be created automatically when you insert the first document.

To see all the databases:

show dbs

To see the collections in the current database:

show collections

To see information about the current database:

db.stats()

Documents

Documents are JSON objects (technically BSON) that store your data. Each document must have a unique _id field.

Example document:

{
    _id: ObjectId("507f1f77bcf86cd799439011"),
    name: "Aurora",
    age: 25,
    email: "aurora@example.com",
    hobbies: ["reading", "programming"],
    address: {
        city: "Madrid",
        country: "Spain"
    }
}

Notice that:

• Fields can be of different types (strings, numbers, arrays, objects)
• You can have arrays directly
• You can have embedded documents (objects within objects)
• The _id is generated automatically if you don't provide it

The _id field

Each document must have a unique _id field. If you don't provide it, MongoDB automatically generates an ObjectId.

An ObjectId is a 12-byte identifier that guarantees uniqueness:

ObjectId("507f1f77bcf86cd799439011")

You can provide your own _id:

db.users.insertOne({
    _id: 1,
    name: "María"
})

But you will usually let MongoDB generate it automatically.

Basic queries

To work with the data you need to know these fundamental commands:

countDocuments: counts the number of documents in a collection

// Count all documents
db.listings.countDocuments()

// Count with a filter (we'll see this later)
db.listings.countDocuments({ room_type: "Entire home/apt" })

stats: gets detailed statistics for a collection

db.listings.stats()

You will see information such as:

• Number of documents (count)
• Total size of the data (size)
• Average size of each document (avgObjSize)
• Number of indexes

Solutions