MongoDB $exists Operator – Get documents if field exists

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  • Post category:MongoDB
  • Post last modified:May 9, 2024
  • Reading time:9 mins read

In MongoDB, the $exists operator is used to query documents that contain a specific field or property. It takes boolean value true/false. When <boolean> is true, it returns the documents that contain the field, including documents where the field value is null. If <boolean> is false, it returns only the documents that do not contain the field.

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1. Syntax of $exists operator

Following is the syntax or usage of the $exists operator.


# Syntax
{ field_name: { $exists: boolean_value } }

Where field_name defines the field’s name to check, and boolean_value is either true or false.

Let’s create the sample collection with a few documents to explain the $exists operator.


# Create Collection
db.student.insertMany([
      {  
         _id: 1,
         name: "James",
         age: 25,
         email: "[email protected]",
         marks: 75,
         course: ["SQL", "MongoDB"]
 
      },
      {
         _id: 2,
         name: "Elena",
         age: 21,
         marks: 85,
         course: ["Java", "SQL"]
      },
      {
         _id: 3,
         name: "Emily",
         age: 20,
         marks: 70,
         personal: {"phone#": 3627791, "city":"New York"}
      },
      {
         _id: 4,
         name: "Alex",
         age: 23,
         email: "[email protected]",
         marks: 90,
         course: ["Python", "PHP"]
      }
   ]
)

2. MongoDB $exists – Get documents if the field exists or not exists

The $exists operator in MongoDB is used to retrieve the documents only if a field exists or not. Use value true, to get all documents that have the specified field. Use the value false to get the documents that have no specified field.

Let’s look at the example with the value true.


# $exists operator inside the find() method
db.student.find({email:{$exists:true}})

Here, the email field is set with the $exits operator with the value true. This specifies that only documents with a email field should be returned. The $exists operator here checks if a field is present in the document, and the value true specifies that it should exist.

The output shows the document with the email field exists.

$exists operator in MongoDB

If you want to read about find() method using in ($in operator), use the following link.

3. MongoDB $exists operator with an embedded or nested field

The $exists operator can also be used to retrieve the document by checking the existence of the given field in the embedded or nested document.


# $exists operator for embedded field
db.student.find({"personal.city":{$exists:true}})

The query filter provided above specifies that only documents with a non-null city field inside a personal subdocument should be returned by the $exits operator. The value true set to the $exist operator indicates that a field should exist when a field’s existence is being checked using the $exists operator.

Moreover, the $exists operator can be used with various other operators to satisfy the criteria. Here, the $exists operator is called along with the $nin operator. Here is an example.


# exists operator with $nin operator
db.student.find( { marks: { $exists: true, $nin: [ 70, 75 ] } } )

We retrieve documents with a marks field exists and does not have a value of 70 or 75. Here, a field’s existence in the document is checked by the $exists operator, and the inclusion of the value in a given array is determined by the $nin operator.

Yields below output.

$exists operator in MongoDB

5. $exists operator with the $gt operator

Similarly, the MongoDB $exists operator also be utilized with the conditional operator to check the existence of the field according to the specified value.


# $exists operator with the $gt operator
db.student.find({
    age: {
        $exists: true,
        $gt: 21
    }
});

Above, the query filter defines that only documents that have a age field, not null, and have a value greater than 21 are to be returned. The purpose of the $exists operator is to determine whether a field is present in the document, whereas the $gt operator is used to determine if a value exceeds a predefined value.

Yields below output.

$exists operator in MongoDB

6. $exists operator with the false value

As we know, the $exists operator sets with the boolean value. Here, we provide the false value to the $exists operator to get the documents that provide the null field or field not exists.


# $exists operator with the false value
db.student.find( { course: { $exists: false } } )

Here, we fetched only those documents where the course field does not exist or is null.

Yields below output.

$exists operator in MongoDB

7. Conclusion

In conclusion, the $exists operator is explored in this article with the syntax explanation and the implementation of the examples. Therefore, the $exists operator is useful in cases where we need to filter documents based on the existence or non-existence of a specific field.

When <boolean> is true, it returns the documents that contain the field, including documents where the field value is null. If <boolean> is false, it returns only the documents that do not contain the field.

More details about this topic can be found here.