The $inc Operator: What is it?

The $inc operator in MongoDB is an update operator used to increment (or decrement) the value of a specific field in a document. It's particularly useful for updating counters, scores, or any numeric field that needs to be adjusted by a certain amount. It is an atomic operation, ensuring that updates happen without conflicts, even in concurrent environments.

How to Use $inc

The $inc operator is used within the update command. Here's the basic syntax:

 db.collection.updateOne(
  { <query> },
  {
    $inc: {
      <field1>: <amount1>,
      <field2>: <amount2>,
      ...
    }
  }
) 

Explanation:

• db.collection.updateOne(): This specifies that you want to update a single document matching the given criteria. You could also use updateMany() to update multiple documents.
• {<query>}: This is the query document. It's used to identify the document(s) you want to update. This can be a simple match on an _id field, or a more complex query with multiple conditions.
• {$inc: { ... }}: This is the update document, and it uses the $inc operator. Inside the $inc object, you specify the fields you want to increment or decrement.
• <field>: <amount>: For each field, you provide the field name and the amount by which you want to increment it. A positive value increments the field, while a negative value decrements it.

Examples

Incrementing a Field

Suppose you have a collection named products with documents like this:

 {
  "_id": ObjectId("64f0a2b3c8e17d0009b7e0a1"),
  "name": "Awesome Gadget",
  "views": 10
} 

To increment the views field by 1, you would use the following command:

 db.products.updateOne(
  { "_id": ObjectId("64f0a2b3c8e17d0009b7e0a1") },
  { $inc: { "views": 1 } }
) 

After running this command, the document would look like this:

 {
  "_id": ObjectId("64f0a2b3c8e17d0009b7e0a1"),
  "name": "Awesome Gadget",
  "views": 11
} 

Decrementing a Field

To decrement the views field by 1, you would use a negative value:

 db.products.updateOne(
  { "_id": ObjectId("64f0a2b3c8e17d0009b7e0a1") },
  { $inc: { "views": -1 } }
) 

After running this command, the document would look like this (assuming it was 11 before):

 {
  "_id": ObjectId("64f0a2b3c8e17d0009b7e0a1"),
  "name": "Awesome Gadget",
  "views": 10
} 

Incrementing Multiple Fields

You can increment multiple fields in a single updateOne or updateMany call:

 db.products.updateOne(
  { "_id": ObjectId("64f0a2b3c8e17d0009b7e0a1") },
  { $inc: { "views": 1, "likes": 5 } }
) 

This will increment the views field by 1 and the likes field by 5.

Creating a Field if it Doesn't Exist

If the field you are trying to increment does not already exist in the document, $inc will create the field and initialize it with the specified increment value.

 db.products.updateOne(
  { "_id": ObjectId("64f0a2b3c8e17d0009b7e0a1") },
  { $inc: { "shares": 3 } }
) 

If the shares field didn't exist before, it will now be created and set to 3.

Important Considerations

• The field being incremented must be a numeric type (integer, double, etc.). If the field contains a non-numeric value, the $inc operation will fail.
• The increment value can be a positive or negative number, allowing you to both increase and decrease the value of the field.
• $inc is atomic within a single document. This means that even if multiple clients are trying to update the same document concurrently, the updates will be applied correctly without data loss.