Multiply field by value in Mongodb

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.

You can run server-side code with db.eval().

db.eval(function() { 
    db.collection.find({tag : "refurb"}).forEach(function(e) {
        e.Price = e.Price * 0.5;
        db.collection.save(e);
    });
});

Note this will block the DB, so it's better to do find-update operation pair.

See https://docs.mongodb.com/manual/core/server-side-javascript/

In the new Mongo 2.6.x there is a $mul operator. It would multiply the value of the field by the number with the following syntax.

{
  $mul: { field: <number> }
}

So in your case you will need to do the following:

db.collection.update(
  { tag : "refurb"},
  { $mul: { Price : 0.5 } }
);