Phoenix Contexts Reference

Phoenix Contexts Reference

Ash projects: Ash.Domain replaces Phoenix contexts for data access — use the ash-framework skill. Context boundary and PubSub patterns still apply.

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