def _toposort(self, rev_by_id):
    children = collections.defaultdict(list)
    in_degree = collections.defaultdict(int)
    for rev_id, rev in rev_by_id.items():
        for parent in rev['parents']:
            in_degree[rev_id] += 1
            children[parent].append(rev_id)

    queue = collections.deque()
    for rev_id in rev_by_id.keys():
        if in_degree[rev_id] == 0:
            queue.append(rev_id)

    while queue:
        rev_id = queue.popleft()
        yield rev_by_id[rev_id]
        for child in children[rev_id]:
            in_degree[child] -= 1
            if in_degree[child] == 0:
                queue.append(child)