db={"coll1":[{"_id":ObjectId("5cec2dc69b806c4a00f91f16"),"currentStep":["flowkuec8ta1o"],"previousStep":["conditioncplwf7pw1","condition00yokr6jv","conditionzq9koi6i3"],"userMessage":"Liked It :)","intents":[{"_id":ObjectId("5cec2dc69b806c4a00f91f17"),"intent":"General_Positive_Feedback","score":"0.9774518966674806"}]}],"coll2":[{"_id":ObjectId("5cec2dd29b806c4a00f91f18"),"currentStep":[],"previousStep":["flowkuec8ta1o"],"userMessage":"ILikedIt#1","intents":[{"_id":ObjectId("5cec2dd29b806c4a00f91f19"),"intent":"Feedback","score":"1"}]}]}

db.coll1.aggregate([{$match:{"intents.intent":"General_Positive_Feedback"}},{"$lookup":{"from":"coll2","let":{"currentStep":"$currentStep"},"pipeline":[{"$match":{"$expr":{"$eq":["$previousStep","$$currentStep"]}}},{$project:{userMessage:1}}],"as":"previousStep"}},{$unwind:"$previousStep"},{$replaceRoot:{newRoot:"$previousStep"}}])