title: pig - how to reference columns in a FOREACH after a JOIN?
body: <pre><code>A = load 'a.txt' as (id, a1);
  B = load 'b.txt as (id, b1);
  C = join A by id, B by id;
  D = foreach C generate id,a1,b1;
  dump D;
  </code></pre>
  
  <p>4th line fails on:
  <code>Invalid field projection. Projected field [id] does not exist in schema</code></p>
  
  <p>I tried to change to A.id but then the last line fails on: <code>ERROR 0: Scalar has more than one row in the output.</code></p>