Function to Pre-Heat data using ObjectID timestamp component in MongoDB

preheat.js

// function will take a number of days, a collection name, an index Field name, and a boolean as args
// it assumes the index is ObjectID based and creates an ObjectID with a timestamp X days in the past
// Finally, it queries that index/data (controlled by boolean), loading it into memory
// 
// Example - 2 days data, foo collection, _id index, pre-heat index only
// preHeatData(2, "foo", "_id", true)
// Example - 7 days data, foo collection, _id index, pre-heat data also
// preHeatData(7, "foo", "_id", false)
// Example - 2 days data, bar collection, blah index, pre-heat index only
// preHeatData(2, "bar", "blah", false)

preHeatData = function(numDays, collName, indexField, indexOnly) {
  // rounding is needed here to chop off the fractional seconds
  // ObjectID is to the second
  lastXDays = Math.round(((new Date().getTime() - (numDays * 24 * 60 * 60))/1000));
  // convert to hexadecimal 
  hexDays = lastXDays.toString(16);
  // create and pad a string and pass it to the ObjectId constructor
  historicId = ObjectId(hexDays+"0000000000000000");
  // Now query for it:
  if (indexOnly && indexField != "_id") {
    // if indexOnly is true, and not the _id index include a projection to make this a covered query
    db.collName.find({indexField : {"$gt" : historicId}}, {"_id" : 0, indexField : 1}).hint({indexField : 1}).explain();
    print("Loaded index Only, pre-heated the " + indexField + " index from the " + collName + " collection");
    }
    else if(indexOnly) {
      // if it is the _id field, simpler projection
      db.collName.find({indexField : {"$gt" : historicId}}, {indexField : 1}).hint({indexField : 1}).explain();
      print("Loaded index Only, pre-heated the " + indexField + " index from the " + collName + " collection");
    }
    else {
      // finally, the general case, where we return the data also
      db.collName.find({indexField : {"$gt" : historicId}}).hint({indexField : 1}).explain();
      print("Pre-heated the " + indexField + " index and data from the " + collName + " collection");

    }
}