counter-cache

var debug = function(str) {
  console.log('counter-cache: ' + str);
};

Meteor.Collection.prototype.maintainCountOf = function(collection, field, counterField) {
  var self = this;

  // what is Meteor.users an instanceof ?
  // if (! (collection instanceof Meteor.Collection))
  //   throw new Error("Expected first parameter to be a Meteor Collection");
  if (! collection)
    throw new Error("Missing parameter: collection");
  if (! field)
    throw new Error("Missing parameter: field name");

  if (typeof counterField === 'undefined')
    counterField = self._name + 'Count';

  debug('setup counts of `' + collection._name + '` onto `' + this._name + '` with counter field `' + counterField + '`');

  var modifier = { $inc: {}};
  var decrement = function(_id) {
    debug('decrement ' + _id);
    if (! _id) return;

    modifier[counterField] = 1;
    self.update(_id, modifier);
  };
  var increment = function(_id) {
    debug('increment ' + _id);
    if (! _id) return;
    
    modifier[counterField] = -1;
    self.update(_id, { $inc: { teachersCount: 1 }});
  };

  collection.after.insert(function(userId, doc) {
    fieldValue = _.dottedProperty(doc, field);
    if (fieldValue)
      increment(fieldValue);
  });

  collection.after.update(function(userId, doc, fieldNames, modifier, options) {
    var self = this;

    // console.log(modifier);
    // console.log(fieldNames);
    // console.log(this.previous);
    // console.log(doc);

    if (modifier.$set) {
      var oldDoc = self.previous;

      // LocalCollection._modify(doc, modifier);

      var oldDocFieldValue = _.dottedProperty(oldDoc, field);
      var newDocFieldValue = _.dottedProperty(doc, field);

      if (oldDocFieldValue && newDocFieldValue !== oldDocFieldValue)
        decrement(oldDocFieldValue);

      increment(newDocFieldValue);
    }
  });

  collection.after.remove(function(userId, doc) {
    decrement(_.dottedProperty(doc, field));
  });
};