Avg in Query Commands in MongoDB

Create Database

Create a database with the name is learnmongodb. This database have 2 collections: Category collection and Product collection. Category collection and Product collection have a One to Many. One category can have many products and One product belongs to one and only one category.

/* Create learnmongodb database */
use learnmongodb

/* Create Category collection */
db.createCollection('category');

/* Dumping data for `category` collection */
db.getCollection('category').insertOne({
    name: 'Mobile'
})

db.getCollection('category').insertOne({
    name: 'Laptop'
})

db.getCollection('category').insertOne({
    name: 'Tivi'
})

/* Create Product collection */
db.createCollection('product');

/* Dumping data for `product` collection */
db.getCollection('product').insertOne({
	name: 'Mobile 1',
	price: 45,
    quantity: 4,
    status: true,
    date: ISODate('2016-10-20'),
    categoryId: ObjectId('5a30de130867edfa45711668'),
    brand: {
    	_id: new ObjectId(),
        name: 'brand 1'
	}
});

db.getCollection('product').insertOne({
	name: 'Mobile 2',
	price: 12,
	quantity: 7,
	status: true,
	date: ISODate('2017-11-14'),
	categoryId: ObjectId('5a30de130867edfa45711668'),
	brand: {
		_id: new ObjectId(),
		name: 'brand 2'
	}
});

db.getCollection('product').insertOne({
	name: 'Mobile 3',
	price: 28,
	quantity: 8,
	status: true,
	date: ISODate('2017-11-20'),
	categoryId: ObjectId('5a30de130867edfa45711668'),
	brand: {
		_id: new ObjectId(),
		name: 'brand 3'
	}
});

db.getCollection('product').insertOne({
	name: 'Laptop 1',
	price: 39,
	quantity: 12,
	status: false,
	date: ISODate('2017-12-26'),
	categoryId: ObjectId('5a30de130867edfa45711669'),
	brand: {
		_id: new ObjectId(),
		name: 'brand 1'
	}
});

db.getCollection('product').insertOne({
	name: 'Laptop 2',
	price: 86,
	quantity: 23,
	status: true,
	date: ISODate('2017-03-11'),
	categoryId: ObjectId('5a30de130867edfa45711669'),
	brand: {
		_id: new ObjectId(),
		name: 'brand 1'
	}
});

db.getCollection('product').insertOne({
	name: 'Tivi 1',
	price: 22,
	quantity: 7,
	status: true,
	date: ISODate('2017-06-26'),
	categoryId: ObjectId('5a30de130867edfa4571166a'),
	brand: {
		_id: new ObjectId(),
		name: 'brand 1'
	}
});

db.getCollection('product').insertOne({
	name: 'Tivi 2',
	price: 86,
	quantity: 23,
	status: false,
	date: ISODate('2017-09-24'),
	categoryId: ObjectId('5a30de130867edfa4571166a'),
	brand: {
		_id: new ObjectId(),
		name: 'brand 3'
	}
});

Average Price of The Products

db.getCollection('product').aggregate( [ { $group: { _id: '', total: { $avg: "$price" } } } ] )

Output

/* 1 */
{
    "_id" : "",
    "total" : 45.4285714285714
}