Auto-increment Sequence in Mongodb

MongoDB does not have the auto-increment out-of-the-box feature like SQL Database. By default, it uses a 12-byte ObjectId for the _id field as the Primary key to uniquely identify the Documents. However, there are situations when we want the _id field to have some values that automatically increase beyond ObjectId.

Since this is not the default feature in MongoDB, we will program this feature using a counters collection as suggested by MongoDB Documentation.

Use counter Collection

You follow the following product document. We want the _id field to be an ascending sequence of integers starting from 1, 2, 3, 4 to n.

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

To do this, create a counters collection to track the final range value for all sequence fields.

 > db . createCollection ( "counters" ) 

We will now insert the following Document in counters Collection with its productid key.

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

Field sequence_value tracks the last value of that sequence.

Use the following code to insert this sequence document into counters collection:

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

Create JavaScript function

Now, we will create a getNextSequenceValue function, which accepts the array name as its input, increments the array value by 1 and returns the updated number. In this example, the array name is the productid .

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

Use JavaScript function

Now we use the getNextSequenceValue function while creating a new Document and assigning the returned range value as the Document's _id field.

Insert two Document templates by 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 used the getNextSequenceValue function to set the value for the _id field.

To test this feature, we retrieve the Documents using the find command:

 > db . prodcuts . find ()