Pig: apply a FOREACH operator to each element within a bag

Question

Example: I have a relation "class", with a nested bag of students:

class: {teacher_name: chararray,students: {(firstname: chararray, lastname: chararray)}

I want to perform an operation on each student, while leaving the global structure untouched, ie, obtain:

class: {teacher_name: chararray,students: {(fullname: chararray)}

where for each student, fullname = CONCAT(firstname, lastname)

My understanding is that a nested FOREACH would not be my solution here, as it still only generates 1 record per input tuple, whereas I want something that would apply within each bag item.

Pretty easy to do with an UDF but wondered if it's possible to do it in pure Piglatin

Answer 1

In PIG 0.10 it is possible without the UDF, as FOREACH can be nested in FOREACH. Here is an example:

inpt = load '~/pig/data/bag_concat.dat' as (k : chararray, c1 : chararray, c2 : chararray);
dump inpt;
1   q   w
1   s   d
2   q   a
2   t   y
2   u   i
2   o   p

bags = group inpt by k;
describe bags;

bags: {group: chararray,inpt: {(k: chararray,c1: chararray,c2: chararray)}}

result = foreach bags {
    concat = foreach inpt generate CONCAT(c1, c2); --it will iterate only over the records of the inpt bag
    generate group, concat;
};
dump result;

(1,{(qw),(sd)})
(2,{(qa),(ty),(ui),(op)})