How can I rename a field for all documents in MongoDB?

If ever you need to do the same thing with mongoid:

Model.all.rename(:old_field, :new_field)

UPDATE

There is change in the syntax in monogoid 4.0.0:

Model.all.rename(old_field: :new_field)

You can use the $rename field update operator:

db.collection.update(
  {},
  { $rename: { 'name.additional': 'name.last' } },
  { multi: true }
)

You can use:

db.foo.update({}, {$rename:{"name.additional":"name.last"}}, false, true);

Or to just update the docs which contain the property:

db.foo.update({"name.additional": {$exists: true}}, {$rename:{"name.additional":"name.last"}}, false, true);

The false, true in the method above are: { upsert:false, multi:true }. You need the multi:true to update all your records.

Or you can use the former way:

remap = function (x) {
  if (x.additional){
    db.foo.update({_id:x._id}, {$set:{"name.last":x.name.additional}, $unset:{"name.additional":1}});
  }
}

db.foo.find().forEach(remap);

In MongoDB 3.2 you can also use

db.students.updateMany( {}, { $rename: { "oldname": "newname" } } )

The general syntax of this is

db.collection.updateMany(filter, update, options)

https://docs.mongodb.com/manual/reference/method/db.collection.updateMany/