[{"user_id":7,"action":1},{"user_id":8,"action":1},{"user_id":9,"action":1},{"user_id":7,"action":2}]

db.collection.aggregate([{$group:{_id:"$user_id",actions:{$addToSet:"$action"}}},{$match:{$and:[{"actions":1},{"actions":{$not:{$eq:2}}}]}},{$group:{_id:"$_id"}}])