def build_dependency_graph(self):
        def get_url(resource):
            """Get the URL of the resource."""
            if hasattr(resource, 'href'):
                return resource.href
            else:
                return resource.absolute_url

        # Even with the API cache, we don't want to be making more requests
        # than necessary. The review request resource will be cached by an
        # ETag, so there will still be a round trip if we don't cache them
        # here.
        review_requests_by_url = {}
        review_requests_by_url[self.absolute_url] = self

        def get_review_request_resource(resource):
            url = get_url(resource)

            if url not in review_requests_by_url:
                review_requests_by_url[url] = resource.get(expand='repository')

            return review_requests_by_url[url]

        repository = self.get_repository()

        graph = defaultdict(set)

        visited = set()

        unvisited = deque()
        unvisited.append(self)

        while unvisited:
            head = unvisited.popleft()

            if head in visited:
                continue

            visited.add(get_url(head))

            for tail in head.depends_on:
                tail = get_review_request_resource(tail)

                if path_exists(graph, tail.id, head.id):
                    raise ValueError('Circular dependencies.')

                # We don't want to include review requests for other
                # repositories, so we'll stop if we reach one. We also don't
                # want to re-land submitted review requests.
                if (repository.id == tail.repository.id and
                    tail.status != 'submitted'):
                    graph[head].add(tail)
                    unvisited.append(tail)

        graph.default_factory = None
        return graph
Build the dependency graph for the review request.
dataset/ETHPy150Open reviewboard/rbtools/rbtools/api/resource.py/ReviewRequestResource.build_dependency_graph