// Override the original behaviour from sewdn:collection-behaviours
CollectionBehaviours.defineBehaviour("trackable", function (getTransform, args) {
  if (_.isArray(args[0]))
    args = args[0];

  // Track all updated fields or only a list of fields
  if (args[0] === true) {
    var track_field = args[1] || "updatedTrack",
        slice = false;

    if (_.isObject(track_field)) {
      var prop_pair = _.pairs(track_field)[0];

      track_field = prop_pair[0];
      slice = prop_pair[1];
    }

    this.before.update(function (user_id, doc, field_names, modifier, options) {
      var entry = {};

      entry["userId"] = user_id;
      entry["fields"] = _.pick(doc, field_names);

      updateModifier(modifier, track_field, entry, slice);
    });
  }
  else {
    var fields = {};

    _.each(args, function (field) {
      if (_.isString(field))
        fields[field] = null;
      else if (_.isObject(field))
        _.extend(fields, field);
    });

    this.before.update(function (user_id, doc, field_names, modifier, options) {
      _.each(field_names, function (field_name) {
        if (_.has(fields, field_name) && _.has(doc, field_name)) {
          var entry = {},
              slice = fields[field_name];

          entry["userId"] = user_id;
          entry[field_name] = doc[field_name];

          updateModifier(modifier, field_name + "Track", entry, slice);
        }
      });
    });
  }

  // Push the new entry onto modifier
  function updateModifier(modifier, track_field_name, entry, slice) {
    entry["trackedAt"] = new Date;

    if (_.isFinite(slice))
      entry = {
        $each: [entry],
        $slice: slice
      };

    if (!modifier.$push)
      modifier.$push = {};

    modifier.$push[track_field_name] = entry;
  }
});