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Sometimes, we need to find a specific document using special properties like string length. In this article, we will see how to find a specific document using string length in MongoDB, and we will also see an example on this topic for easy understanding.

Find specific documents in a collection based on string length

To find a specific document from a collection, we use a very common MongoDB built-in method called find(). But when we want to find documents based on string length, we need to provide some exceptions.

In the next example, we will see two ways to find specific documents based on the length of a string.

In the following example, we will see how to find documents based on string length.

But first, let's check what documents are available in our collection. To do this, we will use the following command:

db.mydata.find()

The above command will display all the documents in the mydata collection.

{ _id: ObjectId("63713371117701ff3d627b56"),
  Name: 'Alen',
  Email: 'abc@gmail.com',
  Year: 2018 }
{ _id: ObjectId("63713371117701ff3d627b57"),
  Name: 'Max',
  Email: 'max@gmail.com',
  Year: 2017 }
{ _id: ObjectId("63713371117701ff3d627b58"),
  Name: 'Ethen',
  Email: 'ethen@gmail.com',
  Year: 2019 }
{ _id: ObjectId("63713371117701ff3d627b59"),
  Name: 'Alex',
  Email: 'alex@gmail.com',
  Year: 2020 }

Above are the files available in our collection. Let’s move to the next step.

Now let's see a method we can use to find documents based on the length of a string. To do this, we will take the length of the name field.

Using the $where keyword

To do this, we can use the keyword $where. The command will look like this:

db.mydata.find({ $where: 'this.Name.length > 4' })

Using Aggregation

If you want to use aggregation, then you can follow the command below:

db.mydata.find({
Name: { $exists: true },
$expr: { $gt: [{ $strLenCP: '$Name' }, 4] } })

In the above command, the keyword $gtmeans greater than. Now, after executing both the commands in MongoDB console, you will get the same output as shown below:

{ _id: ObjectId("63713371117701ff3d627b58"),
  Name: 'Ethen',
  Email: 'ethen@gmail.com',
  Year: 2019 }

请注意The commands shown in this article are for MongoDB databases and need to be run on the MongoDB console.