mongodb-function.js

db.getCollection('patient').find().snapshot().forEach(
  function (p) {
    var lower_middle_name;
    if(p.middle_name) lower_middle_name = p.middle_name.toLowerCase(); else lower_middle_name = null;
     
    var lower_mrn;
    if(p.mrn) lower_mrn = p.mrn.toLowerCase(); else lower_mrn = null;
     
    var lower_email_address;
    if(p.email_address) lower_email_address = p.email_address.toLowerCase(); else lower_email_address = null;
      
    // update document, using its own properties
    p.lower_first_name = p.first_name.toLowerCase();
    p.lower_middle_name = lower_middle_name;
    p.lower_last_name = p.last_name.toLowerCase();
    p.lower_mrn = lower_mrn;
    p.lower_email_address = lower_email_address;

    // save the updated document
    db.getCollection('patient').save(p);
  }
)