Update all documents in a collection with random numbers - mongodb

Question

I am trying to update all documents in a collection with random numbers.
Each document should have a different number.
My current code
db.myDoc.update(
{ rand : { $exists : false } },
{ $set : { rand: Math.random() } },
{ multi : true }
)
populates ALL documents with the SAME random value.
How to fix?

Answer 1

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()}})
})

Answer 2

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.

Answer 3

Starting in Mongo 4.4.2, the $rand aggregation operator provides a random float between 0 and 1 each time it is called:
// { "x" : 1 }
// { "x" : 2 }
db.collection.updateMany(
{ rand: { $exists: false } },
[{ $set: { rand: { $rand: {} } } }]
)
// { "x" : 1, "rand" : 0.22252333583221115 }
// { "x" : 2, "rand" : 0.9811303782541574 }