Agent skill

m09-domain

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

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SKILL.md

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

Domain Concept Rust Pattern Ownership Implication
Entity struct + Id Owned, unique identity
Value Object struct + Clone/Copy Shareable, immutable
Aggregate Root struct owns children Clear ownership tree
Repository trait Abstracts persistence
Domain Event enum Captures state changes
Service impl block / free fn Stateless operations

Thinking Prompt

Before creating a domain type:

  1. What's the concept's identity?

    • Needs unique identity → Entity (Id field)
    • Interchangeable by value → Value Object (Clone/Copy)

  2. What invariants must hold?

    • Always valid → private fields + validated constructor
    • Transition rules → type state pattern

  3. Who owns this data?

    • Single owner (parent) → owned field
    • Shared reference → Arc/Rc
    • 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?)
Design Question Trace To Ask
Entity vs Value Object domain-* What makes two instances "the same"?
Aggregate boundaries domain-* What must be consistent together?
Validation rules domain-* What business rules apply?

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

DDD Concept Rust Pattern Example
Value Object Newtype struct Email(String);
Entity Struct + ID struct User { id: UserId, ... }
Aggregate Module boundary mod order { ... }
Repository Trait trait UserRepo { fn find(...) }
Domain Event Enum enum OrderEvent { Created, ... }

Pattern Templates

Value Object

rust
struct Email(String);

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

Entity

rust
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

rust
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

Mistake Why Wrong Better
Primitive obsession No type safety Newtype wrappers
Public fields with invariants Invariants violated Private + accessor
Leaked aggregate internals Broken encapsulation Methods on root
String for semantic types No validation Validated newtype

Related Skills

When See
Type-driven implementation m05-type-driven
Ownership for aggregates m01-ownership
Domain error handling m13-domain-error
Specific domain rules domain-*

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