Multiply field by value in Mongodb

Question

I\'ve been looking for a way to create an update statement that will take an existing numeric field and modify it using an expression. For example, if I have a field called

Answer 1

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update of a field based on another field:

// { price: 19.99 }
// { price: 2.04  }
db.collection.update(
  {},
  [{ $set: { price: { $multiply: [ 0.5, "$price" ] } } }],
  { multi: true }
)
// { price: 9.995 }
// { price: 1.02  }

• The first part {} is the match query, filtering which documents to update (all documents in this case).

• The second part [{ $set: { price: ... } }] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline). $set is a new aggregation operator and an alias of $addFields. Note how price is modified directly based on the its own value ($price).

• Don't forget { multi: true }, otherwise only the first matching document will be updated.