Database
[{"_id":"5f2b2c45559ecc3e2cceadb5","name":"P40688","flag":"BUY","item_id":168652,"connected_realm_id":[{"_id":1602,"name":"Gordunni"}],"type":"DERIVATIVE",},{"_id":"5f1ec59ed983794988debcdc","name":"AUCTION BUY","flag":"BUY","item_id":168652,"connected_realm_id":[{"_id":1922,"name":"Azuregos"},{"_id":1602,"name":"Gordunni"}],"type":"MARKET"}]
Query
db.collection.aggregate([{$project:{field_value:{$reduce:{input:"$connected_realm_id",initialValue:"",in:{$concat:["$$value",{$cond:[{$eq:["$$value",""]},"",","]},{$toString:"$$this.name"}]}}}}}])