Update all documents in a collection with random numbers

You can make use of the cursor.forEach() cursor method in the mongo shell to achieve this:

db.myDoc.find({rand: {$exists : false }}).forEach(function(mydoc) {
  db.myDoc.update({_id: mydoc._id}, {$set: {rand: Math.random()}})
})

Starting in Mongo 4.4, the $function aggregation operator allows applying a custom javascript function to implement behaviour not supported by the MongoDB Query Language.

For instance, in order to update documents with a random value:

// { "x" : 1 }
// { "x" : 2 }
db.collection.updateMany(
  { rand: { $exists: false } },
  [{ $set:
    { rand:
      { $function: {
          body: function() { return Math.random(); },
          args: [],
          lang: "js"
      }}
    }
  }]
)
// { "x" : 1, "rand" : 0.7012578283384967  }
// { "x" : 2, "rand" : 0.21041874709692365 }

---

$function takes 3 parameters:

• body, which is the function to apply.
• args, which contains the fields from the record that the function can take as parameter. In our case we don't need any reference to the document itself in order to compute a random value, thus the empty array.
• lang, which is the language in which the body function is written. Only js is currently available.

Note that this is now way more efficient than a find/foreach option since everything is done server side in one pass.