code: function(association, rel, expanded, fn, thisArg) {
      var assoc = association.split('.');
      var index = assoc.length - 1;
      var sourceRelation = _.last(expanded);
      var expandedAssoc = assoc.slice(1);
  
      expanded.forEach(function(rel) {
        expandedAssoc.push(rel.expansionName);
        fn.call(thisArg, expandedAssoc.join('.'), rel, {
          through: rel,
          expanded: true,
          source: (rel === sourceRelation),
          index: -1,
        });
      });
  
      fn.call(thisArg, assoc.join('.'), sourceRelation, {
        through: rel,
        expanded: false,
        source: true,
        index: index,
      });
    }
docs: Enumerate an expanded (a.k.a. _through_) relation. This is a helper for
  {@link BoundHelpers#_relationForEach} and should not be used directly.
  
  Calls the callback with arguments compatible with
  {@link BoundHelpers#_relationForEach}.
  
  Each relation in the through relation will cause the callback to be
  called. Each call will have the following details set: `through` will be
  the association given to this method, `expanded` will be true, `index`
  will be -1, and `source` will be true for the last through relation, the
  _source_ relation. Each of these calls will use the expanded association
  name, that is the name from each of the _non-through_ relations.
  
  Finally, the callback will be called again with the _source_ relation, but
  will use the _through_ association name instead of the expanded
  association name and the actual index in the association string. The
  `expanded` option will be false and the `source` option true. The
  `through` option will be set to the same value as the previous invocation.
  
  @method
  @private
  
  @param {String} association Relation key path.
  @param {Relation} rel The through relation that's been expanded.
  @param {Array.<Relation>} expanded The expanded relations to iterate.
  @param {BoundHelpers~RelationIterator} fn The callback function.
  @param {Object} thisArg The value of `this` for the callback function.