MongoDB Exists

MongoDB $exists Query

• The $exists MongoDB operator is an efficient way to check whether a specified field exists in any document.
• If multiple documents match the criteria, all of them are returned.
• If no document exists in MongoDB which matches the criteria, nothing is returned.

Syntax

• Here "field" is the name of the field that we want to check for existence in the collection. We have to replace "field" with the name of the field that we want to query.

• If MongoDB exists operator takes the boolean value “true” as an argument then it returns documents where the specified field exists, including documents where the field value is "null".

• If Exists MongoDB operator takes the boolean value “false” as an argument then it returns the documents where the specified field does not exist.

• The $exists MongoDB operator can also be used to check if a nested field exists.

• Here "embeddedField" is the name of the nested field within the field "field" that we want to check for existence in the collection. We have to replace "embeddedField" with the name of the nested field that we want to query.

• For example, in the document given below, "crew" is a field, and "director" and "producer" are two nested field inside "crew".

• Therefore, to check if the nested field "producer" exists the above syntax can be written as:

Methods

a. Check if the Field Exists:

Syntax:

Explanation:

This method checks if the field exists in the collection. All the documents that contain the field name are returned.

b. Check if the Embedded Field Exists:

• Embedded fields are fields that are nested inside a field or an object in a document.
• $exists MongoDB operator can also be used to check if an embedded field exists.
• In the query, we will have to specify the entire path to the specified embedded field.

Syntax:

Explanation:

MongoDB exists operator is used for determining whether the embedded field exists inside the field in any document in the collection. If any document exists in MongoDB which satisfies the MongoDB exists query criteria, it is returned. If multiple documents satisfy the criteria, all of them are returned.

Examples

Example - 1:

We have a collection named "movies" with 6 documents. Only 3 out of 6 documents have a field named "crew".

A. We want to find all the documents having the field named "title" using the Exists MongoDB operator.

Syntax:

Therefore, we can see that all 6 documents have been returned by $exists MongoDB operator since all of them contain the field "title".

B. We will find the documents containing the field named "crew".

Syntax:

Therefore, we can see that 3 documents have been returned by the MongoDB exists operator, since they contain the field "crew".

C. Now we will find the documents which do not contain the field named "crew".

Syntax:

After MongoDB exists query has been executed we can see that 3 documents have been returned since they do not contain the field "crew".

Example - 2:

Now we will find the documents which have the embedded field "director" inside the field "crew".

Syntax:

Therefore, we can see that 3 documents have been returned, since they contain the embedded field "director" inside the field "crew".