Learn how to add a field in MongoDB Aggregation Pipeline. Our MongoDB Support team is here to help you with your questions and concerns.

How to add a field in MongoDB Aggregation Pipeline

Did you know that the aggregation pipeline in MongoDB is a framework?

It allows us to process and transform documents as they pass through a series of stages. Furthermore, each stage carries out an operation on the input documents and produces an output that becomes the input for the next stage.

In fact, a common operation is adding new fields to the documents with the $addFields stage.

The $addFields stage in the aggregation pipeline adds new fields to the documents in the output stream without changing the original documents in the collection.

It helps include computed values in the results of our aggregation query.

We can use the $addFields stage to add a new field to our documents as seen here:

db.collection.aggregate([
{
$addFields: {
newField: "value"
}
}
])

Here:

• db.collection.aggregate:

  This command starts the aggregation pipeline on a collection.
• $addFields:

  This is the stage that adds new fields to the documents.
• newField:

  We have to replace this with the name of the field we want to add.
• “value”:

  We have to replace this with the value we want to assign to the new field.

An Example

Here is an example. Suppose we have a collection of books, and we want to add a new field called “discountedPrice” that is 10% less than the “price” field:

db.books.aggregate([
{
$addFields: {
discountedPrice: { $multiply: ["$price", 0.9] }
}
}
])

Here, the $multiply expression multiplies the value of the “price” field by 0.9 to calculate the new price.

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Conclusion

In summary, our Support Techs demonstrated how to add a field in MongoDB Aggregation Pipeline.