db={"firstCollection":[{"JoinField":"this is my data"}],"secondCollection":[{"FirstField":"food","SecondField":"bar","Properties":{"JoinField":"this is my data"}},{"FirstField":"food","SecondField":"bar","Properties":{"JoinField":"this is my data"}}]}

db.firstCollection.aggregate([{"$lookup":{from:"secondCollection",localField:"JoinField",foreignField:"Properties.JoinField",as:"secondCollection"}},{$set:{secondCollection:{$arrayElemAt:["$secondCollection",0]}}},{"$unwind":"$secondCollection"},{"$project":{"JoinField":1,firstField:"$secondCollection.FirstField",secondField:"$secondCollection.SecondField",}}])