Database

[{"class":"apple"},{"class":"appl"},{"class":"orange","nested":[{"classification":"app"},{"classification":"A"},{"classification":"orang"}]},{"nested":[{"classification":"O"},{"classification":"unknown"}]}]

Query

db.collection.aggregate([{"$addFields":{standard:{$reduce:{input:[{k:"apple",v:"a"},{k:"appl",v:"a"},{k:"orange",v:"o"}],initialValue:"",in:{$cond:[{$eq:["$$this.k","$class"]},"$$this.v","$$value"]}}}}},{"$addFields":{standard:{$reduce:{input:{"$ifNull":["$nested",[]]},initialValue:[{v:"$standard"}],in:{$concatArrays:["$$value",{$filter:{input:[{k:"app",v:"a"},{k:"A",v:"a"},{k:"orang",v:"o"},{k:"O",v:"o"},{k:"unknown",v:"u"}],as:"nested",cond:{$eq:["$$this.classification","$$nested.k"]}}}]}}}}},{"$addFields":{"standard":{$reduce:{input:"$standard.v",initialValue:"",in:{"$concat":["$$value","$$this"]}}}}}])

Result