[{_id:1,field_value:[{_id:2,name:"Batman"},{_id:3,name:"Robin"}]}]

db.collection.aggregate([{$project:{field_value:{$reduce:{input:"$field_value",initialValue:"",in:{$concat:["$$value",{$cond:[{$eq:["$$value",""]},"",","]},{$toString:"$$this.name"}]}}}}}])