Design a Task Management Model (OOD)

Problem Design the object model for a task/project management tool.

Requirements

• Hierarchy: Spaces -> Lists -> Tasks -> Subtasks.
• Status, assignees, due dates, custom fields.
• Dependencies between tasks.
• Comments and activity feed.

Areas to design

• The containment hierarchy and composition.
• The task-dependency graph and cycle detection.
• Extensible custom fields/statuses.