MongoDB: unconditional updates?

This seems like a silly question but I haven't yet found the answer. If I simply wanted to add the same field->value to EVERY record in a MongoDB collection, what would be the appropriate shell command to do so? I tried doing a multi update with a blank query ({}) but that resulted in this error:

multi update only works with $ operators

I'm a bit puzzled about how to get around this. Any suggestions?

The error says it all: You can only modify multiple documents using the $ modifier operators. You probably had something like this:

> db.coll.update({ }, { a: 'b' }, false, true);

Which would normally replace the first object in the collection with { a: 'b' } if multi was false. You wouldn't want to replace all the objects in your collection with the same document!

Use the $set operator instead:

> db.coll.update({ }, { '$set': { a: 'b' } }, false, true);

This will set the a property of every document (creating it as necessary) to 'b'.