[{_id:123,uId:111,msg:"test 1",attachments:[{name:"attach1",url:"https://example.com",isDeleted:false},{name:"attach2",url:"https://example.com",isDeleted:true}]},{_id:456,uId:222,msg:"test 2",attachments:[{name:"attach1",url:"https://example.com",isDeleted:true}]},{_id:789,uId:333,msg:"test 3",attachments:[{name:"attach1",url:"https://example.com",isDeleted:false}]}]

db.collection.aggregate([{"$project":{_id:1,uId:1,msg:1,attachments:{"$filter":{"input":"$attachments","as":"attachment","cond":{"$eq":["$$attachment.isDeleted",false]}}}}}])