How to change the type of a field?

Question

I am trying to change the type of a field from within the mongo shell.

I am doing this...

db.meta.update(
  {\'fields.properties.default\': { $type :         


        

Answer 1

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

// { a: "45", b: "x" }
// { a:  53,  b: "y" }
db.collection.update(
  { a : { $type: 1 } },
  [{ $set: { a: { $toString: "$a" } } }],
  { multi: true }
)
// { a: "45", b: "x" }
// { a: "53", b: "y" }

• The first part { a : { $type: 1 } } is the match query:

  • It filters which documents to update.
  • In this case, since we want to convert "a" to string when its value is a double, this matches elements for which "a" is of type 1 (double)).
  • This table provides the codes representing the different possible types.

• The second part [{ $set: { a: { $toString: "$a" } } }] is the update aggregation pipeline:

  • Note the squared brackets signifying that this update query uses an aggregation pipeline.
  • $set is a new aggregation operator (Mongo 4.2) which in this case modifies a field.
  • This can be simply read as "$set" the value of "a" to "$a" converted "$toString".
  • What's really new here, is being able in Mongo 4.2 to reference the document itself when updating it: the new value for "a" is based on the existing value of "$a".
  • Also note "$toString" which is a new aggregation operator introduced in Mongo 4.0.

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

---

In case your cast isn't from double to string, you have the choice between different conversion operators introduced in Mongo 4.0 such as $toBool, $toInt, ...

And if there isn't a dedicated converter for your targeted type, you can replace { $toString: "$a" } with a $convert operation: { $convert: { input: "$a", to: 2 } } where the value for to can be found in this table:

db.collection.update(
  { a : { $type: 1 } },
  [{ $set: { a: { $convert: { input: "$a", to: 2 } } } }],
  { multi: true }
)