In mongoDb, how do you remove an array element by its index?

There is no straight way of pulling/removing by array index. In fact, this is an open issue http://jira.mongodb.org/browse/SERVER-1014 , you may vote for it.

The workaround is using $unset and then $pull:

db.lists.update({}, {$unset : {"interests.3" : 1 }}) 
db.lists.update({}, {$pull : {"interests" : null}})

Update: as mentioned in some of the comments this approach is not atomic and can cause some race conditions if other clients read and/or write between the two operations. If we need the operation to be atomic, we could:

• Read the document from the database
• Update the document and remove the item in the array
• Replace the document in the database. To ensure the document has not changed since we read it, we can use the update if current pattern described in the mongo docs

You can use $pull modifier of update operation for removing a particular element in an array. In case you provided a query will look like this:

db.people.update({"name":"dannie"}, {'$pull': {"interests": "guitar"}})

Also, you may consider using $pullAll for removing all occurrences. More about this on the official documentation page - http://www.mongodb.org/display/DOCS/Updating#Updating-%24pull

This doesn't use index as a criteria for removing an element, but still might help in cases similar to yours. IMO, using indexes for addressing elements inside an array is not very reliable since mongodb isn't consistent on an elements order as fas as I know.