4. Querying documents

Now that you have imported Airbnb data, you will learn to read and explore it.

The find command

To get all the documents in a collection:

db.listings.find()

This returns the first 20 documents. To see more, type it (iterate).

Searching for specific documents

You can use a filter to search for documents that meet certain conditions:

// Listings in the CIUTAT VELLA neighborhood
db.listings.find({ neighbourhood_group_cleansed: "CIUTAT VELLA" })

// Listings of type "Entire home/apt"
db.listings.find({ room_type: "Entire home/apt" })

The filter is an object with field-value pairs. MongoDB returns only the documents where those fields have those exact values.

The findOne command

To get just one document (useful for seeing the structure):

// Gets the first listing
db.listings.findOne()

// Gets a specific listing
db.listings.findOne({ room_type: "Private room" })

findOne returns a single document instead of a cursor.

Projections

You can select which fields to return using projections:

// Only returns name and price (and _id by default)
db.listings.find(
  { neighbourhood_group_cleansed: "EXTRAMURS" },
  { name: 1, price: 1 }
)

// Excludes the _id field
db.listings.find(
  { neighbourhood_group_cleansed: "EXTRAMURS" },
  { name: 1, price: 1, _id: 0 }
)

In projections: - 1 means "include this field" - 0 means "exclude this field"

You cannot mix inclusions and exclusions (except with _id).

Counting documents

You already know countDocuments from the previous lesson. Now you can use it with filters:

// Count listings in a specific neighborhood
db.listings.countDocuments({ neighbourhood_group_cleansed: "BENIMACLET" })

// Count superhosts
db.listings.countDocuments({ host_is_superhost: "t" })

Solutions