When attempting to query on one field of an array element, the docs make it very clear that $elemMatch is not necessary.

For example, find({"list.field":"value"}) should match the following document: { "list":[ {"field":"value"}, {"otherField":"otherValue"} ] }.

However, doing an upsert using findAndModify on an empty collection:

 findAndModify(
  {"list.field":"value"},
  {"$push":{"list":{"field":"value"}}},
  {"upsert":1}
)

causes the following error:
"list" must be an array but is of type Object in document {INVALID-MUTABLE-ELEMENT}