Can I generate nested bags using nested FOREACH statements in Pig Latin?

Question

Let's say I have a data set of restaurant reviews:

User,City,Restaurant,Rating
Jim,New York,Mecurials,3
Jim,New York,Whapme,4.5
Jim,London,Pint Size,2
Lisa,London,Pint Size,4
Lisa,London,Rabbit Whole,3.5

And I want to produce a list by user and city of average review. I.e. output:

User,City,AverageRating
Jim,New York,3.75
Jim,London,2
Lisa,London,3.75

I could write a Pig script as follows:

Data = LOAD 'data.txt' USING PigStorage(',') AS (
    user:chararray, city:chararray, restaurant:charray, rating:float
);

PerUserCity = GROUP Data BY (user, city);

ResultSet = FOREACH PerUserCity {
    GENERATE group.user, group.city, AVG(Data.rating);
}

However I'm curious whether I can first group the higher level group (the users) and then sub group the next level (the cities) later: i.e.

PerUser = GROUP Data BY user;

Intermediate = FOREACH PerUser {
    B = GROUP Data BY city;
    GENERATE group AS user, B;
}

I get:

Error during parsing.
Invalid alias: GROUP in {
  group: chararray,
  Data: {
    user: chararray,
    city: chararray,
    restaurant: chararray,
    rating: float
  }
}

Has anyone tried this with success? Is it simply not possible to GROUP within a FOREACH?

My goal is to do something like:

ResultSet = FOREACH PerUser {
    FOREACH City {
        GENERATE user, city, AVG(City.rating)
    }
}

Answer 1

Currently the allowed operations are DISTINCT, FILTER, LIMIT, and ORDER BY inside a FOREACH.

For now grouping directly by (user, city) is the good way to do as you said.