MongoDB MapReduce. $exists on nested field

Question

I want to execute map-reduce operation on specific documents from MongoDB.

Let's say, we have this "document":

{
  a: {
    b: {
      c:{}
    }
  }
}

I want to select only those documents, that have an a.b.c field. From this documentation page I know, that $exists operator can help.

To check, that a exists I should use:

query: {
  a: { $exists : true }
}

But what query should I use for nested field, described before?

query: {
  //?
}

Answer 1

This isn't related to the fact of using map-reduce.

query: {
  'a.b.c' : { $exists : true }
}

So is a standard query on an embedded document: http://docs.mongodb.org/manual/tutorial/query-documents/#embedded-documents