title: How to specify foreign keys to a table in Parse.com
body: <p>I take Trains as one table and passenger as other table.
  train table has train_no, train_name.
  Passenger table has pnr and train_no.
  i would like to create train_no as foreign key in passengers table.
  please provide sufficient code for it..
  i am rookie in it.
  Thanks in advance</p>