[{"_id":1,/** no activities*/},{"_id":2,/** no issue*/"activities":[{"message":"Activity 2.1","data":{}},{"message":"Activity 2.2","data":{}}]},{"_id":3,/** no dismissed*/"activities":[{"message":"Activity 3.1","data":{}},{"message":"Activity 3.2",/** no dismissed*/"data":{},"issue":{"errors":[]}}]},{"_id":4,/** dismiss exists*/"activities":[{"message":"Activity 4.1","data":{},"issue":{"errors":[],"dismissed":{"timestamp":"","message":"any"}}},{"message":"Activity 4.2","data":{},"issue":{"errors":[],"dismissed":{"timestamp":"","message":""}}}]},{"_id":5,/** dismiss exists only in one activity*/"activities":[{"message":"Activity 5.1","data":{},"issue":{"errors":[],"dismissed":{"timestamp":"","message":"any"}}},{"message":"Activity 5.2","data":{}}]}]

db.collection.find({"activities":{$not:{$elemMatch:{"issue.dismissed":{$exists:false}}}}})