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# MongoDB Auto-Increment MongoDB does not have an auto-increment feature like SQL. MongoDB's _id is a system-generated 12-byte unique identifier. However, in some cases, we may need to implement an auto-incrementing ObjectId feature. Since MongoDB does not implement this feature, we can achieve it through programming. Below, we will implement auto-increment for the _id field in a 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 auto-increment: >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 after the sequence has auto-incremented. Use the following command to insert the sequence document into the counters collection: >db.counters.insert({_id:"productid",sequence_value:0}) * * * ## Creating a Javascript Function Now, we create a function getNextSequenceValue that takes a sequence name as input. The specified sequence will auto-increment 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's _id automatically 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 if the function works, we can use the following command to read the documents: >db.products.find() The above command will return the following results, showing that the _id field is auto-incrementing: { "_id" : 1, "product_name" : "Apple iPhone", "category" : "mobiles"}{ "_id" : 2, "product_name" : "Samsung S3", "category" : "mobiles" }