[{"time":1422471890,"user_id":"256f5280-fb49-4ad6-b7f5-65c4329d46e0","message":"This is an example of my db"},{"time":1622471890,"user_id":"256f5280-fb49-4ad6-b7f5-65c4329d46e0","message":"This is an example of my db (1)"},{"time":1622471890,"user_id":"256f5280-fb49-4ad6-b7f5-65c4329d46e0","message":"This is an example of my db (2)"},{"time":1622471890,"user_id":"e194d667-d79f-4262-94b1-ecf4561c9418","message":"This is an example of my db (3)"},{"time":1922471890,"user_id":"256f5280-fb49-4ad6-b7f5-65c4329d46e0","message":"This is an example of my db (4)"}]

db.collection.aggregate([{$match:{user_id:{$regex:""}}},{$sort:{time:-1}},{$group:{_id:null,user_id:{$addToSet:"$user_id",},}},{$unwind:"$user_id"},{$project:{_id:0}},{$out:"collection"}])