public function assignRelated($source, $relationNames=null, $meta=null)
    {
        if (!$source) { return; }

        $stack = [];
        $sourceIsArray = is_array($source) || $source instanceof \Traversable;
        if (!$sourceIsArray) {
            $source = array($source);
        }

        if ($meta !== null) {
            $meta = !$meta instanceof Meta ? $this->mapper->getMeta($meta) : $meta;
        } else {
            $meta = $this->mapper->getMeta(get_class($source[0]));
        }

        if (!$relationNames) {
            if ($meta->autoRelations) {
                $relationNames = $meta->autoRelations;
            } else {
                throw new Exception("relationNames not passed, class {$meta->class} does not define autoRelations");
            }
        }

        $relationMap = [];
        foreach ((array)$relationNames as $relationName) {
            if (!isset($meta->relations[$relationName])) {
                throw new Exception("Unknown relation $relationName on {$meta->class}");
            }
            $relationMap[$relationName] = $rel = $meta->relations[$relationName];
            if ($rel['mode'] == 'class') {
                throw new Exception("Relation $relationName is not assignable for class {$meta->class}");
            }
        }

        $done = [];
        foreach ($relationMap as $relationName=>$relation) {
            if (isset($done[$relationName])) {
                continue;
            }
            $done[$relationName] = true;

            $missing = [];
            foreach ($source as $idx=>$item) {
                // we have to assume it's missing if we see a getter as there
                // may be serious unintended side effects from calling a getter
                // that may be unpopulated. it might lazy load if it's an active
                // record, or it might throw an exception because the 'set' hasn't
                // been called.

                // check for isset as well as falsey for the situation where we're using
                // stdClasses as the response object
                if (isset($relation['getter']) || !isset($item->{$relationName}) || !$item->{$relationName}) {
                    $missing[$idx] = $item;
                }
            }

            if ($missing) {
                $result = $this->getRelated($missing, $relationName, ['stack'=>$stack], $meta);
                $relator = $this->getRelator($meta, $relationName);
                $this->populateObjectsWithRelated($missing, $result, $relation);
            }
        }
    }