name: m09-domain
description: "CRITICAL: Use for domain modeling. Triggers: domain model, DDD, domain-driven design, entity, value object, aggregate, repository pattern, business rules, validation, invariant, 领域模型, 领域驱动设计, 业务规则"
user-invocable: false

Domain Modeling

Layer 2: Design Choices

Core Question

What is this concept's role in the domain?

Before modeling in code, understand:
- Is it an Entity (identity matters) or Value Object (interchangeable)?
- What invariants must be maintained?
- Where are the aggregate boundaries?

Domain Concept → Rust Pattern

Thinking Prompt

Before creating a domain type:

1. What's the concept's identity?
2. Needs unique identity → Entity (Id field)

3. Interchangeable by value → Value Object (Clone/Copy)

4. What invariants must hold?

5. Always valid → private fields + validated constructor

6. Transition rules → type state pattern

7. Who owns this data?

8. Single owner (parent) → owned field
9. Shared reference → Arc/Rc
10. Weak reference → Weak

Trace Up ↑

To domain constraints (Layer 3):

"How should I model a Transaction?"
    ↑ Ask: What domain rules govern transactions?
    ↑ Check: domain-fintech (audit, precision requirements)
    ↑ Check: Business stakeholders (what invariants?)

Trace Down ↓

To implementation (Layer 1):

"Model as Entity"
    ↓ m01-ownership: Owned, unique
    ↓ m05-type-driven: Newtype for Id

"Model as Value Object"
    ↓ m01-ownership: Clone/Copy OK
    ↓ m05-type-driven: Validate at construction

"Model as Aggregate"
    ↓ m01-ownership: Parent owns children
    ↓ m02-resource: Consider Rc for shared within aggregate

Quick Reference

Pattern Templates

Value Object

struct Email(String);

impl Email {
    pub fn new(s: &str) -> Result<Self, ValidationError> {
        validate_email(s)?;
        Ok(Self(s.to_string()))
    }
}

Entity

struct UserId(Uuid);

struct User {
    id: UserId,
    email: Email,
    // ... other fields
}

impl PartialEq for User {
    fn eq(&self, other: &Self) -> bool {
        self.id == other.id  // Identity equality
    }
}

Aggregate

mod order {
    pub struct Order {
        id: OrderId,
        items: Vec<OrderItem>,  // Owned children
        // ...
    }

    impl Order {
        pub fn add_item(&mut self, item: OrderItem) {
            // Enforce aggregate invariants
        }
    }
}

Common Mistakes

Related Skills