[{"_id":ObjectId("5a934e000102030405000000"),"acronym":"MC","tickers":[{"name":"Sensor1-front","symbol":"s1f"},{"name":"Sensor1-back","symbol":"s1b"}]}]

db.collection.aggregate([{$addFields:{newField:{"$reduce":{"input":"$tickers","initialValue":[],"in":{"$concatArrays":[[{"$concat":["$$this.symbol",".","$acronym"]}],"$$value"]}}}}}])