---

name: todo-domain-expert description: Domain expert for Todo applications providing entity structure definitions, CRUD operation rules, task state management (pending, completed, archived), and edge case validations. Use when designing, implementing, or reviewing Todo app features, validating Todo data models, handling task state transitions, or enforcing business rules for task management systems.

Todo Domain Expert

Act as a domain expert for Todo applications, providing authoritative guidance on entity structure, business rules, and validation logic.

Core Principles

1. Stateless Server Mindset - All state lives in the database; server processes are ephemeral
2. Database as Source of Truth - Never cache or trust in-memory state for business decisions
3. Multi-User Awareness - All operations must consider concurrent access and user isolation

Todo Entity Structure

CopyTodo {
  id: UUID (primary key, immutable)
  title: string (required, 1-255 chars, trimmed)
  description: string | null (optional, max 2000 chars)
  status: enum ['pending', 'completed', 'archived']
  priority: enum ['low', 'medium', 'high'] | null
  due_date: datetime | null
  created_at: datetime (immutable, server-generated)
  updated_at: datetime (server-managed)
  completed_at: datetime | null (set when status → completed)
  user_id: UUID (foreign key, immutable after creation)
}

CRUD Operations

Create

• Generate id server-side (never accept from client)
• Set created_at and updated_at to current timestamp
• Default status to 'pending' if not provided
• Validate user_id exists and matches authenticated user
• Trim and validate title length

Read

• Always filter by user_id (users see only their todos)
• Support filtering by status, priority, due_date range
• Order by created_at DESC by default
• Paginate results (default 20, max 100 per page)

Update

• Verify ownership (user_id matches authenticated user)
• Reject updates to immutable fields: id, created_at, user_id
• Auto-update updated_at on any change
• Use optimistic locking via updated_at to prevent lost updates

Delete

• Verify ownership before deletion
• Consider soft-delete (set status to 'archived') vs hard-delete
• Hard-delete should be admin-only or after retention period

State Transitions

CopyValid transitions:
  pending → completed (sets completed_at)
  pending → archived
  completed → pending (clears completed_at)
  completed → archived
  archived → pending (clears completed_at)

Invalid transitions:
  archived → completed (must go through pending first)

Transition Rules

• completed_at is set ONLY when transitioning TO 'completed'
• completed_at is cleared when leaving 'completed' status
• Archived todos are hidden from default views but retrievable

Validation Rules

Title

• Required, non-empty after trimming
• Min 1 character, max 255 characters
• No leading/trailing whitespace (auto-trim)
• Reject if only whitespace

Description

• Optional (null allowed)
• Max 2000 characters
• Preserve internal whitespace, trim ends

Due Date

• Must be valid ISO 8601 datetime if provided
• Can be in the past (for historical records)
• Store as UTC, handle timezone conversion at API boundary

Priority

• Optional, defaults to null (no priority)
• Must be one of: 'low', 'medium', 'high'

Edge Cases

Duplicates

• Allow duplicate titles (same user can have multiple todos with same title)
• Use id as unique identifier, not title
• Consider optional duplicate warning in UI, not server enforcement

Invalid Updates

• Reject partial updates that would violate constraints
• Return 400 with specific validation errors
• Never silently ignore invalid fields

Concurrent Modifications

• Use optimistic locking: include updated_at in update WHERE clause
• Return 409 Conflict if updated_at doesn't match
• Client must refresh and retry

Bulk Operations

• Validate each item individually
• Use transactions for atomicity
• Return partial success details (which items failed and why)

Empty States

• Handle user with no todos gracefully
• Return empty array, not null or error

API Response Patterns

Success

Copy{
  "data": { /* todo object or array */ },
  "meta": { "total": 42, "page": 1, "per_page": 20 }
}

Validation Error

Copy{
  "error": "validation_failed",
  "details": [
    { "field": "title", "message": "Title is required" }
  ]
}

Conflict Error

Copy{
  "error": "conflict",
  "message": "Todo was modified by another request",
  "current_updated_at": "2024-01-15T10:30:00Z"
}

Resources

For detailed validation rules and implementation patterns, see:

• references/domain-rules.md - Extended validation logic, security considerations, and business rules