API Resource Expansion Designer

You are an API optimization specialist reducing N+1 query problems and network round trips.

## CONTEXT
I need to implement resource expansion for related data.

**Requirements:**
- Primary Resources: {{RESOURCES}}
- Relationships: {{RELATIONSHIPS}}
- Depth Limits: {{DEPTH}}
- Performance: {{PERFORMANCE}}

## TASK
Design resource expansion:

### 1. EXPAND PARAMETER
```
GET /api/orders/123?expand=customer,items
GET /api/orders/123?expand=customer.address,items.product
```

### 2. RESPONSE FORMAT
```json
{
  "id": "order_123",
  "status": "shipped",
  "customer": {
    "id": "cust_456",
    "name": "John Doe",
    "email": "john@example.com"
  },
  "items": [
    {
      "id": "item_1",
      "quantity": 2,
      "product": {
        "id": "prod_789",
        "name": "Widget",
        "price": 29.99
      }
    }
  ]
}
```

### 3. IMPLEMENTATION
```typescript
async function expandResources(data, expansions) {
  for (const expansion of expansions) {
    const [relation, nested] = expansion.split('.');
    const relatedData = await loadRelation(data, relation);
    data[relation] = nested
      ? await expandResources(relatedData, [nested])
      : relatedData;
  }
  return data;
}
```

### 4. DEPTH LIMITING
```typescript
const MAX_EXPANSION_DEPTH = 3;
// Reject: ?expand=a.b.c.d.e
```

### 5. PERFORMANCE OPTIMIZATION
- Batch loading
- Dataloader pattern
- Caching expanded resources

### 6. SECURITY
- Whitelist expandable relations
- Permission checks on relations