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

Version Notes

• •Phoenix 1.8+: Uses built-in %Scope{} struct for authorization context
• •Phoenix 1.7: Requires manual authorization context (see references/scopes-auth.md "Pre-Scopes Patterns")

References

For detailed patterns, see:

• •references/context-patterns.md - Full context module, PubSub, Multi, cross-boundary
• •references/scopes-auth.md - Scope struct, multi-tenant, authorization, plugs
• •references/routing-patterns.md - Verified routes, pipelines, API auth
• •references/plug-patterns.md - Function/module plugs, placement, guards
• •references/json-api-patterns.md - JSON controllers, FallbackController, API auth