db={"first":[{"cardType":"card","xyz":"XYZ","fields":[{"abc":"abc","xyz":"XYZ","inputMethod":"Entry",},{"abc":"abc","xyz":"XYZ","inputMethod":"List","ListId":"1234"}]}],"second":[{"abc":"abc","xyz":"xyz","itemId":"1234"}]}

db.first.aggregate([{"$match":{}},{$project:{firstCollection:"$$ROOT"}},{"$lookup":{"from":"second","localField":"firstCollection.fields.ListId","foreignField":"itemId","as":"secondCollection"}}])