Phoenix Context Creator

This skill guides the creation of well-designed Phoenix contexts following bounded context principles and Phoenix best practices.

When to Use

•Creating new business domains
•Organizing related functionality
•Refactoring code into contexts
•Designing API boundaries
•Building feature modules

What is a Context?

A context is a module that groups related functionality and provides a public API for that domain. Contexts enforce boundaries between different parts of your application.

Examples:

•Accounts - User management, authentication
•Catalog - Products, categories, inventory
•Sales - Orders, shopping cart, checkout
•CMS - Blog posts, pages, comments
•Notifications - Emails, SMS, push notifications

Context Design Principles

1. Bounded Context

Each context should have clear responsibilities:

elixir

# Good: Focused contexts
Accounts.create_user()
Catalog.list_products()
Sales.place_order()

# Bad: Mixed responsibilities
Users.create_user()
Users.list_products()  # Products don't belong here
Users.send_email()     # Email sending doesn't belong here

2. Public API Only

Contexts expose intentional APIs, hide implementation:

elixir

# Good: Clear, intention-revealing API
defmodule MyApp.Accounts do
  def list_users, do: Repo.all(User)
  def get_user!(id), do: Repo.get!(User, id)
  def create_user(attrs), do: %User{} |> User.changeset(attrs) |> Repo.insert()
end

# Bad: Exposing internal details
defmodule MyApp.Accounts do
  # Don't expose User schema directly
  def user_schema, do: User

  # Don't expose changesets
  def user_changeset(attrs), do: User.changeset(%User{}, attrs)
end

3. No Cross-Context Dependencies

Contexts should not directly reference other contexts' schemas:

elixir

# Bad: Post directly references User schema
defmodule Blog.Post do
  schema "posts" do
    belongs_to :user, Accounts.User  # Direct schema reference
  end
end

# Good: Use IDs to reference across contexts
defmodule Blog.Post do
  schema "posts" do
    field :user_id, :id  # Just store the ID
  end
end

# Then in Blog context, delegate user lookups to Accounts
defmodule Blog do
  def get_post_with_author!(id) do
    post = get_post!(id)
    author = Accounts.get_user!(post.user_id)
    %{post | author: author}
  end
end

Creating a New Context

Step 1: Plan the Domain

Questions to answer:

•What is the primary responsibility?
•What are the main entities?
•What operations will be needed?
•How does it interact with other contexts?

Example: Building a Blog

•Primary responsibility: Content management
•Entities: Post, Comment, Tag
•Operations: CRUD posts, publish/unpublish, add comments
•Interactions: Needs user data from Accounts context

Step 2: Generate the Context

bash

# Generate context with primary schema
mix phx.gen.context Blog Post posts \
  title:string \
  body:text \
  published:boolean \
  user_id:references:users \
  slug:string:unique

Generates:

•Context: lib/my_app/blog.ex
•Schema: lib/my_app/blog/post.ex
•Migration: priv/repo/migrations/*_create_posts.exs
•Tests: test/my_app/blog_test.exs

Step 3: Design the Public API

Start with CRUD:

elixir

defmodule MyApp.Blog do
  alias MyApp.Blog.Post

  # List operations
  def list_posts
  def list_published_posts

  # Get operations
  def get_post!(id)
  def get_post_by_slug(slug)

  # Create/Update/Delete
  def create_post(attrs)
  def update_post(post, attrs)
  def delete_post(post)

  # Domain-specific operations
  def publish_post(post)
  def unpublish_post(post)
  def increment_view_count(post)
end

Add business logic:

elixir

def publish_post(%Post{} = post) do
  post
  |> Post.publish_changeset()
  |> Repo.update()
end

def list_posts_by_user(user_id) do
  Post
  |> where(user_id: ^user_id)
  |> order_by([desc: :inserted_at])
  |> Repo.all()
end

Step 4: Enhance the Schema

elixir

defmodule MyApp.Blog.Post do
  use Ecto.Schema
  import Ecto.Changeset

  schema "posts" do
    field :title, :string
    field :body, :text
    field :published, :boolean, default: false
    field :slug, :string
    field :view_count, :integer, default: 0
    field :user_id, :id

    timestamps()
  end

  # Creation changeset
  def changeset(post, attrs) do
    post
    |> cast(attrs, [:title, :body, :user_id])
    |> validate_required([:title, :body, :user_id])
    |> validate_length(:title, min: 3, max: 100)
    |> generate_slug()
    |> unique_constraint(:slug)
  end

  # Publishing changeset
  def publish_changeset(post) do
    change(post, published: true, published_at: DateTime.utc_now())
  end

  defp generate_slug(changeset) do
    case get_change(changeset, :title) do
      nil -> changeset
      title -> put_change(changeset, :slug, Slug.slugify(title))
    end
  end
end

Step 5: Add Additional Schemas

bash

# Add comments to blog context
mix phx.gen.context Blog Comment comments \
  body:text \
  post_id:references:posts \
  user_id:references:users \
  --merge-with-existing-context

Step 6: Write Comprehensive Tests

elixir

defmodule MyApp.BlogTest do
  use MyApp.DataCase

  alias MyApp.Blog

  describe "posts" do
    test "list_posts/0 returns all posts" do
      post = fixture(:post)
      assert Blog.list_posts() == [post]
    end

    test "get_post!/1 returns the post with given id" do
      post = fixture(:post)
      assert Blog.get_post!(post.id) == post
    end

    test "create_post/1 with valid data creates a post" do
      attrs = %{title: "Title", body: "Body", user_id: 1}
      assert {:ok, %Post{} = post} = Blog.create_post(attrs)
      assert post.title == "Title"
    end

    test "publish_post/1 marks post as published" do
      post = fixture(:post)
      assert {:ok, %Post{} = published} = Blog.publish_post(post)
      assert published.published == true
    end
  end
end

Context Interaction Patterns

Pattern 1: ID References (Recommended)

elixir

# Blog context references users by ID only
defmodule MyApp.Blog do
  def create_post(user_id, attrs) do
    attrs
    |> Map.put(:user_id, user_id)
    |> create_post()
  end

  # When you need user data, delegate to Accounts
  def get_post_with_author(post_id) do
    post = get_post!(post_id)
    author = MyApp.Accounts.get_user!(post.user_id)
    Map.put(post, :author, author)
  end
end

Pattern 2: Data Transfer Objects

elixir

# Blog context accepts struct from Accounts
defmodule MyApp.Blog do
  def create_post_for_user(%Accounts.User{id: user_id}, attrs) do
    create_post(Map.put(attrs, :user_id, user_id))
  end
end

Pattern 3: Event-Based Communication

elixir

# Publish events when something important happens
defmodule MyApp.Blog do
  def publish_post(post) do
    with {:ok, post} <- do_publish(post) do
      Phoenix.PubSub.broadcast(
        MyApp.PubSub,
        "posts",
        {:post_published, post}
      )
      {:ok, post}
    end
  end
end

# Other contexts subscribe to events
defmodule MyApp.Notifications do
  def handle_info({:post_published, post}, state) do
    send_notifications(post)
    {:noreply, state}
  end
end

Common Context Patterns

Accounts Context

elixir

defmodule MyApp.Accounts do
  # User management
  def list_users
  def get_user!(id)
  def create_user(attrs)
  def update_user(user, attrs)
  def delete_user(user)

  # Authentication
  def authenticate(email, password)
  def change_password(user, password)

  # Authorization
  def assign_role(user, role)
  def has_permission?(user, permission)
end

Catalog Context (E-commerce)

elixir

defmodule MyApp.Catalog do
  # Products
  def list_products
  def get_product!(id)
  def create_product(attrs)

  # Categories
  def list_categories
  def get_category_products(category_id)

  # Search
  def search_products(query)
  def filter_products(filters)

  # Inventory
  def check_availability(product_id, quantity)
  def reserve_stock(product_id, quantity)
end

Sales Context (E-commerce)

elixir

defmodule MyApp.Sales do
  # Cart
  def get_cart(user_id)
  def add_to_cart(user_id, product_id, quantity)
  def update_cart_item(cart_item, quantity)

  # Orders
  def create_order(user_id, cart_id)
  def get_order!(id)
  def cancel_order(order)

  # Checkout
  def calculate_total(cart)
  def apply_discount(cart, code)
  def process_payment(order, payment_details)
end

Anti-Patterns to Avoid

1. God Contexts

elixir

# Bad: Kitchen sink context
defmodule MyApp.Core do
  def create_user(attrs)
  def create_product(attrs)
  def send_email(attrs)
  def process_payment(attrs)
end

# Good: Focused contexts
MyApp.Accounts.create_user(attrs)
MyApp.Catalog.create_product(attrs)
MyApp.Notifications.send_email(attrs)
MyApp.Billing.process_payment(attrs)

2. Direct Schema Access

elixir

# Bad: Controllers accessing schemas directly
def index(conn, _params) do
  users = Repo.all(User)  # Don't do this!
  render(conn, "index.html", users: users)
end

# Good: Use context API
def index(conn, _params) do
  users = Accounts.list_users()
  render(conn, "index.html", users: users)
end

3. Context Coupling

elixir

# Bad: Blog directly importing Accounts
defmodule MyApp.Blog do
  alias MyApp.Accounts.User

  def create_post_with_user(attrs) do
    user = Repo.get!(User, attrs.user_id)  # Direct coupling
    # ...
  end
end

# Good: Use IDs and delegate
defmodule MyApp.Blog do
  def create_post(attrs) do
    # Just verify user exists via ID
    unless Accounts.user_exists?(attrs.user_id) do
      {:error, :user_not_found}
    else
      # Create post
    end
  end
end

Context Organization

code

lib/my_app/
├── accounts/
│   ├── user.ex
│   ├── session.ex
│   └── role.ex
├── accounts.ex          # Public API
├── blog/
│   ├── post.ex
│   ├── comment.ex
│   └── tag.ex
├── blog.ex              # Public API
└── catalog/
    ├── product.ex
    ├── category.ex
    └── variant.ex

Testing Contexts

elixir

defmodule MyApp.BlogTest do
  use MyApp.DataCase

  alias MyApp.Blog

  # Test context API, not internal functions
  describe "list_posts/0" do
    test "returns all posts" do
      # Setup
      post1 = fixture(:post)
      post2 = fixture(:post)

      # Execute
      posts = Blog.list_posts()

      # Assert
      assert length(posts) == 2
      assert post1 in posts
      assert post2 in posts
    end
  end

  describe "create_post/1" do
    test "with valid data" do
      attrs = %{title: "Title", body: "Body", user_id: 1}
      assert {:ok, post} = Blog.create_post(attrs)
      assert post.title == "Title"
    end

    test "with invalid data" do
      assert {:error, changeset} = Blog.create_post(%{})
      assert %{title: ["can't be blank"]} = errors_on(changeset)
    end
  end
end

Migration Strategy

Adding to Existing Codebase

•Identify Boundaries - Group related functionality
•Create Context - Start with one clear boundary
•Move Schemas - Relocate related schemas
•Extract Functions - Pull functions into context
•Update References - Update controllers/views
•Write Tests - Ensure nothing broke
•Repeat - Continue with other boundaries

Refactoring Example

Before:

elixir

# Everything in one place
defmodule MyAppWeb.UserController do
  def index(conn, _params) do
    users = Repo.all(User)
    render(conn, "index.html", users: users)
  end
end

After:

elixir

# Context layer
defmodule MyApp.Accounts do
  def list_users, do: Repo.all(User)
end

# Controller uses context
defmodule MyAppWeb.UserController do
  def index(conn, _params) do
    users = Accounts.list_users()
    render(conn, "index.html", users: users)
  end
end