Element query operators

Element query operators return data based on the existence of a specified field or the data type of a particular value.

Operator	Description
$exists	returns documents where a field exists or does not exist
$type	matches document containing elements with the specified type

For the examples in this section, insert the following documents into the electronics collection:

db.electronics.insertMany([
  {
    product: 'laptop',
    price: 1500,
    stock: 5,
    discount: true,
    specifications: [
      {
        processor: 'Intel Core i7'
      },
      {
        memory: 16
      }
    ]
  },
  {
    product: 'phone',
    price: 800,
    stock: 10,
    discount: true,
    specifications: [
      {
        brand: 'Apple'
      },
      {
        model: 'iPhone 12'
      }
    ]
  },
  {
    product: 'tablet',
    price: 500,
    stock: 15,
    discount: true,
    specifications: [
      {
        brand: 'Samsung'
      },
      {
        model: 'Galaxy Tab S7'
      }
    ]
  },
  {
    product: 'keyboard',
    price: 100,
    stock: 20
  },
  {
    product: 'mouse',
    price: 50,
    stock: 25,
    discount: null,
    specifications: []
  },
  {
    product: 'monitor',
    price: 250,
    stock: 30,
    discount: true,
    specifications: [
      {
        size: 27
      },
      {
        resolution: '4K'
      }
    ]
  },
  {
    product: 'printer',
    price: 150,
    stock: 35,
    discount: false
  },
  {
    product: 'scanner',
    price: 100,
    stock: 40,
    discount: true,
    specifications: [
      {
        type: 'flatbed'
      }
    ]
  }
])

$exists

Syntax: { <field>: { $exists: <boolean> } }

To find out if a particular field exists in a document, use the $exists operator.

tip

If the <boolean> value is true, the query returns documents where the specified field exists, even if the value is null or an empty array. If the <boolean> value is false, the query returns documents where the specified field does not exist.

Example: Find documents in the electronics collection where the specifications field exists using the $exists operator:

db.electronics.find({
  specifications: {
    $exists: true
  }
})

The output:

[
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b8e"),
    product: 'laptop',
    price: 1500,
    stock: 5,
    discount: true,
    specifications: [{ processor: 'Intel Core i7' }, { memory: 16 }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b8f"),
    product: 'phone',
    price: 800,
    stock: 10,
    discount: true,
    specifications: [{ brand: 'Apple' }, { model: 'iPhone 12' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b90"),
    product: 'tablet',
    price: 500,
    stock: 15,
    discount: true,
    specifications: [{ brand: 'Samsung' }, { model: 'Galaxy Tab S7' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b92"),
    product: 'mouse',
    price: 50,
    stock: 25,
    discount: null,
    specifications: []
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b93"),
    product: 'monitor',
    price: 250,
    stock: 30,
    discount: true,
    specifications: [{ size: 27 }, { resolution: '4K' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b95"),
    product: 'scanner',
    price: 100,
    stock: 40,
    discount: true,
    specifications: [{ type: 'flatbed' }]
  }
]

In the above output, the query returns all documents where the specifications field exists, even when the field has an empty value.

If you want to find documents where the specifications field exists and has a specific value, use the $exists operator in conjunction with other operators.

Example: The following query returns all documents where the specifications field exists and its value is an array:

db.electronics.find({
  specifications: {
    $exists: true,
    $type: 'array'
  }
})

The output:

[
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b8e"),
    product: 'laptop',
    price: 1500,
    stock: 5,
    discount: true,
    specifications: [{ processor: 'Intel Core i7' }, { memory: 16 }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b8f"),
    product: 'phone',
    price: 800,
    stock: 10,
    discount: true,
    specifications: [{ brand: 'Apple' }, { model: 'iPhone 12' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b90"),
    product: 'tablet',
    price: 500,
    stock: 15,
    discount: true,
    specifications: [{ brand: 'Samsung' }, { model: 'Galaxy Tab S7' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b92"),
    product: 'mouse',
    price: 50,
    stock: 25,
    discount: null,
    specifications: []
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b93"),
    product: 'monitor',
    price: 250,
    stock: 30,
    discount: true,
    specifications: [{ size: 27 }, { resolution: '4K' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b95"),
    product: 'scanner',
    price: 100,
    stock: 40,
    discount: true,
    specifications: [{ type: 'flatbed' }]
  }
]

$type

Syntax: { <field>: { $type: <datatype> } }

Use the $type operator to select documents where the data type of a field matches the specified BSON type

The <datatype> parameter can be the type code or alias of the particular data type.

The following table lists the available BSON type codes and their corresponding aliases:

Type code	Type	Alias
1	Double	double
2	String	string
3	Object	object
4	Array	array
5	Binary data	binData
7	ObjectId	objectId
8	Boolean	bool
9	Date	date
10	Null	null
11	Regular expression	regex
16	32-bit integer	int
17	Timestamp	timestamp
18	64-bit integer	long
19	Decimal128	decimal
-1	Min key	minKey
127	Max key	maxKey
-128	Number	number

caution

Decimal128, Min Key, and Max Key are not currently implemented. FerretDB supports the alias number which matches the following BSON types: Double, 32-bit integer, and 64-bit integer type values.

info

FerretDB supports the alias number which matches the following BSON types: Double, 32-bit integer, and 64-bit integer type values.

Example: The following operation query returns all documents in the electronics collection where the discount field has a boolean data type, which can be represented with the data code 8:

db.electronics.find({
  discount: {
    $type: 8
  }
})

This query can also be written using the alias of the specified data type.

db.electronics.find({
  discount: {
    $type: 'bool'
  }
})

The output:

[
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b8e"),
    product: 'laptop',
    price: 1500,
    stock: 5,
    discount: true,
    specifications: [{ processor: 'Intel Core i7' }, { memory: 16 }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b8f"),
    product: 'phone',
    price: 800,
    stock: 10,
    discount: true,
    specifications: [{ brand: 'Apple' }, { model: 'iPhone 12' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b90"),
    product: 'tablet',
    price: 500,
    stock: 15,
    discount: true,
    specifications: [{ brand: 'Samsung' }, { model: 'Galaxy Tab S7' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b93"),
    product: 'monitor',
    price: 250,
    stock: 30,
    discount: true,
    specifications: [{ size: 27 }, { resolution: '4K' }]
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b94"),
    product: 'printer',
    price: 150,
    stock: 35,
    discount: false
  },
  {
    _id: ObjectId("63a32fc7cf72d6203bb45b95"),
    product: 'scanner',
    price: 100,
    stock: 40,
    discount: true,
    specifications: [{ type: 'flatbed' }]
  }
]