db={"first_collection":[{"UID":3398,"name":"test"}],"second_collection":[{"ques_id":"q1","ri_score":5.3,"uc_id":3398},{"ques_id":"q2","ri_score":5.3,"uc_id":3398}]}

db.first_collection.aggregate([{"$lookup":{"from":"second_collection","localField":"UID","foreignField":"uc_id","as":"values"}},{"$project":{"UID":1,"name":1,"values.ri_score":1}},{"$addFields":{"values":{"$reduce":{"input":"$values","initialValue":[],"in":{"$cond":[{"$in":["$$this.ri_score","$$value.ri_score"]},"$$value",{$concatArrays:["$$value",["$$this"]]}]}}}}}])