Database

[{"caseNumber":"2020921008981","diagnosis":[{"_id":ObjectId("5b55aabe0550de0021097bf0"),"term":"INFECT.DIS - Chickenpox"},{"_id":ObjectId("5b55aabe0550de0021097bf1"),"term":"INFECT.DIS - Herpes zoster"},{"_id":ObjectId("5b55aabf0550de0021097ed2"),"term":"Gastrointestinal mucormycosis"}],"patient":"Test Twit"}]

Query

db.collection.aggregate([{$addFields:{diagnosis:{$reduce:{input:"$diagnosis.term",initialValue:"",in:{$cond:[{"$eq":["$$value",""]},"$$this",{$concat:["$$value","\n","$$this"]}]}}}}}])

Result