Read operation

The read operation retrieves documents in a collection. You can either retrieve all the documents in a collection, or only the documents that match a given query parameter.

Retrieve a single document

The findOne() command retrieves a single document from a collection.

First, populate the database with a new collection containing a list of documents.

db.scientists.insertMany([
  {
    name: {
      firstname: 'Alan',
      lastname: 'Turing'
    },
    born: 1912,
    invention: 'Turing Machine'
  },
  {
    name: {
      firstname: 'Graham',
      lastname: 'Bell'
    },
    born: 1847,
    invention: 'telephone'
  },
  {
    name: {
      firstname: 'Ada',
      lastname: 'Lovelace'
    },
    born: 1815,
    invention: 'computer programming'
  }
])

Run the following findOne() operation to retrieve a single document from the collection:

db.scientists.findOne({ invention: 'Turing Machine' })

Retrieve all documents in a collection

The find() command is used for retrieving all the documents in a collection.

db.collection.find()

Run db.scientists.find() to see the complete list of documents in the collection.

Retrieve documents based on a specific query

Using the find() command, you can also filter a collection for only the documents that match the provided query. For example, find the document with the field born set as 1857.

db.scientists.find({ born: 1857 })

Retrieve documents using operator queries

The operator syntax allows users to query and retrieve a document. There are several operator methods that you can use, such as $gt or $lt. For example, to find the list of scientists born after the 1900s, we'll need the $gt operator:

db.scientists.find({ born: { $gt: 1900 } })

Here is a list of the most commonly used operators.

$gt: selects records that are greater than a specific value

$lt: selects records that are less than a specific value

$gte: selects records greater or equal to a specific value

$lte: selects records less than or equal to a specific value

$in: selects any record that contains any of the items present in a defined array

$nin: selects any record that does not contain any of the items in a defined array

$ne: selects records that are not equal to a specific value

$eq: select records that are equal to a specific value

Retrieve documents containing a specific value in an array

Insert the following documents into an employees collection using this command:

db.employees.insertMany([
  {
    name: {
      first: 'Earl',
      last: 'Thomas'
    },
    employeeID: 1234,
    age: 23,
    role: 'salesperson',
    catalog: ['printer', 'cardboard', 'crayons', 'books']
  },
  {
    name: {
      first: 'Sam',
      last: 'Johnson'
    },
    employeeID: 2234,
    age: 35,
    role: 'salesperson',
    catalog: ['cabinet', 'fridge', 'blender', 'utensils']
  },
  {
    name: {
      first: 'Clarke',
      last: 'Dane'
    },
    employeeID: 3234,
    age: 21,
    role: 'salesperson',
    catalog: ['printer', 'pencils', 'crayons', 'toys']
  }
])

To retrieve all documents with a specific array field and value (catalog: "printer"), run the following command:

db.employees.find({ catalog: 'printer' })

The response displays all the retrieved documents:

[
  {
    _id: ObjectId("636b39f80466c61a229bbf9b"),
    name: { first: 'Earl', last: 'Thomas' },
    employeeID: 1234,
    age: 23,
    role: 'salesperson',
    catalog: ['printer', 'cardboard', 'crayons', 'books']
  },
  {
    _id: ObjectId("636b3b0e0466c61a229bbf9d"),
    name: { first: 'Clarke', last: 'Dane' },
    employeeID: 3234,
    age: 21,
    role: 'salesperson',
    catalog: ['printer', 'pencils', 'crayons', 'toys']
  }
]

Retrieve documents in an array using dot notation

To retrieve all documents containing a specific value in an array, use dot notation to reference its position in the employees collection. The following command retrieves all documents containing "blender" in the third field of an array:

db.employees.find({ 'catalog.2': 'blender' })

The document that matches the array query is displayed in the response:

[
  {
    _id: ObjectId("636b3b0e0466c61a229bbf9c"),
    name: { first: 'Sam', last: 'Johnson' },
    employeeID: 2234,
    age: 35,
    role: 'salesperson',
    catalog: ['cabinet', 'fridge', 'blender', 'utensils']
  }
]

Query on an embedded or nested document

To query on an embedded document, use dot notation to specify the fields. The following command queries on the embedded document in theemployees collection:

db.employees.find({ 'name.first': 'Clarke' })