elixir-phoenix-phoenix-contexts

Phoenix Contexts Reference

Reference for designing and implementing Phoenix contexts (bounded contexts).

Iron Laws — Never Violate These

1. CONTEXTS OWN THEIR DATA — Never query another context's schema directly via Repo
2. SCOPES ARE MANDATORY (Phoenix 1.8+) — Every context function MUST accept scope as first parameter
3. THIN CONTROLLERS/LIVEVIEWS — Controllers translate HTTP, business logic stays in contexts
4. NO SIDE EFFECTS IN SCHEMAS — Use Ecto.Multi for transactions with side effects

Context Structure

lib/my_app/
├── accounts/           # Context directory
│   ├── user.ex         # Schema
│   ├── scope.ex        # Scope struct (Phoenix 1.8+)
├── accounts.ex         # Context module (public API)

Phoenix 1.8+ Scopes (CRITICAL)

All context functions MUST accept scope as first parameter:

def list_posts(%Scope{} = scope) do
  from(p in Post, where: p.user_id == ^scope.user.id)
  |> Repo.all()
end

def create_post(%Scope{} = scope, attrs) do
  %Post{user_id: scope.user.id}
  |> Post.changeset(attrs)
  |> Repo.insert()
  |> broadcast(scope, :created)
end

Quick Decisions

When to SPLIT contexts?

• Module exceeds ~400 lines
• Functions don't share domain language
• Could theoretically be a separate microservice
• Team member could own it independently

When to KEEP together?

• Resources share vocabulary and domain concepts
• Functions frequently operate on same data together
• Splitting would create excessive cross-context calls

Cross-Context References

# ✅ Reference by ID, convert at boundary
def create_order(%Scope{} = scope, user_id, product_ids) do
  with {:ok, user} <- Accounts.fetch_user(scope, user_id) do
    do_create_order(scope, user.id, product_ids)
  end
end

# ❌ Reaching into other context's internals
alias MyApp.Accounts.User  # Don't do this
Repo.all(from o in Order, join: u in User, ...)  # Don't query other schemas

Anti-patterns

Wrong	Right
Service objects (UserCreationService)	Context functions (Accounts.create_user/2)
Repository pattern wrapping Repo	Repo IS the repository
Direct Repo calls in controllers	Delegate to context
Schema callbacks with side effects	Use Ecto.Multi