def build_dependency_graph(objects)
      DependencyGraph.new.tap do |graph|
        objects.each do |id,object|
          graph[id] = [] # no duplicate IDs
          object.foreign_keys.each do |property|
            value = object[property]
            if value
              graph[id] << value
            end
          end
        end
      end
    end
def build_dependency_graph(objects)
      DependencyGraph.new.tap do |graph|
        objects.each do |id,object|
          graph[id] = [] # no duplicate IDs
          object.foreign_keys.each do |property|
            value = object[property]
            if value
              graph[id] << value
            end
          end
        end
      end
    end
Builds a dependency graph.

 @param [Hash] objects a hash of scraped objects keyed by ID
 @return [DependencyGraph] the dependency graph