db={"colA":[{"customerId":"111","name":"Adil","satisfactionLevels":[{"A":"Score 1","B":"R","C":2,"D":1},{"A":"Score 2","B":"S","C":2,"D":2}]}],"colB":[{"customerId":"111","sScore":"20.54","intId":"1527"},{"customerId":"111","sScore":"50.54","intId":"1528"}]}

db.colA.aggregate([{$match:{customerId:"111"}},{$lookup:{from:"colB",localField:"customerId",foreignField:"customerId",as:"colB"}},{$project:{customerId:1,name:1,satisfactionLevels:{$concatArrays:["$satisfactionLevels","$colB"]}}}])