MongoDB does not have out-of-the-box auto-increment functionality, like SQL databases. By default, it uses the 12-byte ObjectId for the _id field as the primary key to uniquely identify the documents. However, there may be scenarios where we may want the _id field to have some auto-incremented value other than the ObjectId.

Since this is not a default feature in MongoDB, we will programmatically achieve this functionality by using a counters collection as suggested by the MongoDB documentation.

Using Counter Collection

Consider the following products document. We want the _id field to be an auto-incremented integer sequence starting from 1,2,3,4 upto n.

{
  "_id":1,
  "product_name": "Apple iPhone",
  "category": "mobiles"
}

For this, create a counters collection, which will keep track of the last sequence value for all the sequence fields.

>db.createCollection("counters")

Now, we will insert the following document in the counters collection with productid as its key −

> db.counters.insert({
	"_id":"productid",
	"sequence_value": 0
})
WriteResult({ "nInserted" : 1 })
>

The field sequence_value keeps track of the last value of the sequence.

Use the following code to insert this sequence document in the counters collection −

>db.counters.insert({_id:"productid",sequence_value:0})

Creating Javascript Function

Now, we will create a function getNextSequenceValue which will take the sequence name as its input, increment the sequence number by 1 and return the updated sequence number. In our case, the sequence name is productid.

>function getNextSequenceValue(sequenceName){
   var sequenceDocument = db.counters.findAndModify({
      query:{_id: sequenceName },
      update: {$inc:{sequence_value:1}},
      new:true
   });
   return sequenceDocument.sequence_value;
}

Using the Javascript Function

We will now use the function getNextSequenceValue while creating a new document and assigning the returned sequence value as document”s _id field.

Insert two sample documents using the following code −

>db.products.insert({
   "_id":getNextSequenceValue("productid"),
   "product_name":"Apple iPhone",
   "category":"mobiles"
})
>db.products.insert({
   "_id":getNextSequenceValue("productid"),
   "product_name":"Samsung S3",
   "category":"mobiles"
})

As you can see, we have used the getNextSequenceValue function to set value for the _id field.

To verify the functionality, let us fetch the documents using find command −

>db.products.find()

The above query returned the following documents having the auto-incremented _id field −

{ "_id" : 1, "product_name" : "Apple iPhone", "category" : "mobiles"}
{ "_id" : 2, "product_name" : "Samsung S3", "category" : "mobiles" }