DDD Rails Modeling

Use this skill when the domain concepts are clear enough that the next question is how to model them in Rails.

Core principle: Model real domain pressure, not textbook DDD vocabulary.

HARD-GATE

DO NOT introduce repositories, aggregates, or domain events just to sound "DDD".
DO NOT fight Rails defaults when a normal model or service expresses the domain clearly.
ALWAYS start from domain invariants, ownership, and lifecycle before choosing a pattern.

Modeling Order

1. List domain concepts: Entities, values, policies, workflows, and events from the ubiquitous language.
2. Identify invariants: Decide which object or boundary must keep each rule true.
3. Choose the aggregate entry point: Name the object that guards state transitions and consistency.
4. Place behavior: Keep behavior on the entity/aggregate when cohesive; extract a domain service only when behavior spans multiple concepts cleanly.
5. Pick Rails homes: Choose the simplest location that matches the boundary and repo conventions.
6. Verify with tests: Hand off to rails-tdd-slices and rspec-best-practices before implementation.

Rails-First Mapping

Output Style

When using this skill, return for each domain concept:

1. Domain concept — name from the ubiquitous language
2. Recommended modeling choice — entity, value object, service, etc.
3. Suggested Rails home — file path
4. Invariant or ownership reason — why this boundary
5. Patterns to avoid — what not to reach for
6. Next skill to chain — generate-tasks, rails-tdd-slices, etc.

Examples

Value Object — Money

# app/models/money.rb
class Money
  attr_reader :amount_cents, :currency

  def initialize(amount_cents, currency = "USD")
    @amount_cents = Integer(amount_cents)
    @currency = currency.upcase.freeze
    freeze
  end

  def ==(other)
    other.is_a?(Money) && amount_cents == other.amount_cents && currency == other.currency
  end

  alias eql? ==

  def hash
    [amount_cents, currency].hash
  end
end

• Modeling choice: Value object — equality by value, immutable, no database identity needed.
• Suggested home: app/models/money.rb
• Avoid: Adding belongs_to or a database table — this is a calculation value, not an entity.

Application Service — CreateOrder

# app/services/orders/create_order.rb
module Orders
  class CreateOrder
    Result = Struct.new(:success?, :order, :errors, keyword_init: true)

    def self.call(**args) = new(**args).call

    def initialize(user:, product_id:, quantity:)
      @user, @product_id, @quantity = user, product_id, quantity
    end

    def call
      order = @user.orders.build(product: Product.find(@product_id), quantity: @quantity)
      order.save ? Result.new(success?: true, order: order, errors: [])
                 : Result.new(success?: false, order: nil, errors: order.errors.full_messages)
    rescue ActiveRecord::RecordNotFound
      Result.new(success?: false, order: nil, errors: ["Product not found"])
    end
  end
end

• Modeling choice: Application service — coordinates persistence and follows up side effects for one use case.
• Suggested home: app/services/orders/create_order.rb
• Avoid: Fat controller method or a callback chain on Order.

Common Mistakes

Red Flags

• The same invariant enforced from several unrelated entry points
• New abstractions increase indirection without clarifying ownership
• Primitive obsession persists while modeling is focused only on folder names

Integration