MongoDB: How to query for records where field is null or not set?

Question

I have an Email document which has a sent_at date field:

{
  \'sent_at\': Date( 1336776254000 )
}

If this E

Answer 1

Use:

db.emails.count({sent_at: null})

Which counts all emails whose sent_at property is null or is not set. The above query is same as below.

db.emails.count($or: [
  {sent_at: {$exists: false}},
  {sent_at: null}
])