Database

[{a:1,"userTasks":[{"done":true,"_id":"6059c57339d5590b04cb0fec","task":"abc","date":"2021-03-23T10:39:47.881Z"},{"done":false,"_id":"6059c57739d5590b04cb0fed","task":"abc","date":"2021-03-23T10:39:51.558Z"},{"done":false,"_id":"6059c57839d5590b04cb0fee","task":"abc","date":"2021-03-23T10:39:52.528Z"},{"done":false,"_id":"6059c57939d5590b04cb0fef","task":"abc","date":"2021-03-23T10:39:53.497Z"},{"done":true,"_id":"6059c57a39d5590b04cb0ff0","task":"abc","date":"2021-03-23T10:39:54.323Z"}]},]

Query

db.collection.aggregate({"$project":{a:1,"userTasks":{$filter:{input:"$userTasks",as:"task",cond:{"$eq":["$$task.done",false]}}}}})

Result