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 up to 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 the 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 the value for the _id field.
To verify the functionality, let us fetch the documents using the 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" }
