title: How to compare two fields from different MongoDB Collections
body: <p>I am trying to write a query to compare fields in two different MongoDB collections (same DB).  Specifically, I have two collections that each contain a field that will match a portion of the time.  I would, therefore, like to iterate through each row of each collection performing a comparison.  When a match is found, I want to be able to perform a follow-on action (insert value of third field into one of the collections).  A suggestion of syntax for this comparison query would be very helpful.</p>
  
  <p>Thank you.</p>