MongoDB Auto-Increment

MongoDB does not have an auto-increment feature like SQL does, MongoDB's _id is a system-generated 12-byte unique identifier.

However, in some cases, we might need to implement an auto-increment functionality for ObjectId.

Since MongoDB does not implement this feature, we can achieve it through programming. Below, we will implement auto-increment for the _id field in the counters collection.

Using the counters Collection

Consider the following products document. We want the _id field to auto-increment from 1, 2, 3, 4 to n.

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

To achieve this, create a counters collection where the sequence field value can be auto-incremented:

>db.createCollection("counters")

Now, insert the following document into the counters collection, using productid as the key:

{
  "_id": "productid",
  "sequence_value": 0
}

The sequence_value field is the value of the sequence after auto-increment.

Insert the sequence document into the counters collection using the following command:

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

Creating a Javascript Function

Now, create the function getNextSequenceValue to take a sequence name as input, which will auto-increment the specified sequence by 1 and return the latest sequence value. In this example, 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

Next, we will use the getNextSequenceValue function to create a new document and set the document _id to the returned sequence value:

>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 use the getNextSequenceValue function to set the _id field.

To verify the function's effectiveness, we can read the documents using the following command:

>db.products.find()

The above command will return the following result, showing that the _id field is auto-incremented:

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

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