Database
db={"colectionA":[{"_id":1,"item":"almonds"},{"_id":2,"item":"pecans"}],"colectionB":[{"_fid":1,"date":"2021-01-10"},{"_fid":1,"date":"2021-01-11"},{"_fid":1,"date":"2021-01-12"},{"_fid":2,"date":"2021-01-03"}]}
Query
db.colectionA.aggregate([{"$lookup":{"from":"colectionB","localField":"_id","foreignField":"_fid","as":"matches"}}])