3.45. Automatic growth of MongoDB

3.45.1. Use counters collections ¶

Consider the following products documents. We want the_ id field to grow automatically from 1, 2, 3, 4 to n.

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

To do this, create a counters collection, and the sequence field values can be automatically long:

>db.createCollection("counters")

Now let’s insert the following documents into the counters collection, using productid as the key:

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

sequence_value A field is a value after the sequence is automatically incremented.

Use the following command to insert counters In the sequence document of the collection:

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

3.45.2. Create a Javascript function ¶

Now, we create the function getNextSequenceValue as the input to the sequence name, and the specified sequence automatically grows by 1 and returns the latest sequence value. In the example in this article, the sequence is named productid.

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

3.45.3. Use the Javascript function ¶

Next we will use the getNextSequenceValue Function to create a new document and set the document_ 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 getNextSequenceValue Function to set the_ id field.

To verify that the function is valid, we can read the document using the following command:

>db.products.find()

The above command returns the following result, and we find that the_ id field is self-growing:

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

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