Mix Tasks & Generators

RULES — Follow these with no exceptions

1. Always call Mix.Task.run("app.start") first — tasks that access the database or Repo need the app started 2. Create custom tasks for project-specific workflows — don't override standard Mix tasks 3. Register preferred_cli_env in mix.exs — set environment for custom tasks (dev/test/prod) 4. Use transactions for data modifications — wrap Repo.insert_all, Repo.delete_all in Repo.transaction() 5. Test custom tasks with Mix.Project.in_project/4 — ensure tasks work correctly in isolation 6. Follow Mix.Tasks.Namespace.TaskName naming — file path must match: lib/mix/tasks/namespace.task_name.ex

End-to-End Workflow

Follow this sequence when creating a custom Mix task:

1. Create file — create lib/mix/tasks/<namespace>.<task_name>.ex
2. Define module — use defmodule Mix.Tasks.MyApp.TaskName do use Mix.Task
3. Add @shortdoc — one-line description for mix help
4. Implement run/1 — parse args with OptionParser.parse/2 if needed
5. Start app — call Mix.Task.run("app.start") if using Repo
6. Implement logic — use transactions for data modifications
7. Add to mix.exs — register preferred_cli_env if needed
8. Test — use Mix.Project.in_project/4 test helper
9. Verify — run mix help | grep task_name to confirm registration

Phoenix Generators

# JSON context and schema (full CRUD with tests)
mix phx.gen.json Accounts User users email:string name:string --web API

# LiveView CRUD with Enum field
mix phx.gen.live Blog Post posts title:string body:text status:enum:draft:published:archived

# LiveView for existing table
mix phx.gen.live Blog Post posts --table existing_posts

# Context (logic layer only)
mix phx.gen.context Accounts User users email:string password_hash:string

# Channel
mix phx.gen.channel Room

# HTML (no LiveView)
mix phx.gen.html Blog Post posts title:string body:text

# Authentication system (accounts context, user schema, session plumbing)
mix phx.gen.auth Accounts User users --web Admin

# Context with existing schema
mix phx.gen.context Blog Post posts --table posts --app MyApp

Custom Mix Tasks

Basic Task with Transaction Safety

# lib/mix/tasks/my_app.seed_data.ex
defmodule Mix.Tasks.MyApp.SeedData do
  use Mix.Task

  @shortdoc "Seeds the database with sample data"

  @impl Mix.Task
  def run(_args) do
    Mix.Task.run("app.start")

    case MyApp.Repo.transaction(fn -> MyApp.Seeds.run() end) do
      {:ok, _} ->
        count = MyApp.Repo.aggregate(MyApp.User, :count)
        Mix.shell().info("Data seeded successfully! (#{count} users in DB)")

      {:error, reason} ->
        Mix.shell().error("Seeding failed, transaction rolled back: #{inspect(reason)}")
        Mix.raise("Seed failed")
    end
  end
end

Task with Arguments and Validation

Demonstrates OptionParser with required flags, error reporting, dry-run support, and before/after count verification.

defmodule Mix.Tasks.MyApp.ImportUsers do
  use Mix.Task

  @shortdoc "Imports users from a CSV file"

  @impl Mix.Task
  def run(args) do
    {opts, _rest, errors} =
      OptionParser.parse(args,
        strict: [file: :string, dry_run: :boolean],
        aliases: [f: :file, d: :dry_run]
      )

    if Keyword.get(opts, :file) == nil do
      Mix.shell().error("Missing required --file argument")
      Mix.shell().info("\nUsage: mix my_app.import_users --file <path>")
      Mix.raise("Invalid arguments")
    end

    if errors != [] do
      Enum.each(errors, fn {key, value} -> Mix.shell().error("Unknown option: #{key} #{value}") end)
      Mix.raise("Invalid arguments")
    end

    Mix.Task.run("app.start")

    file = Keyword.get(opts, :file)
    dry_run? = Keyword.get(opts, :dry_run, false)
    if dry_run?, do: Mix.shell().info("Dry run mode - no changes will be made")

    before_count = MyApp.Repo.aggregate(MyApp.User, :count)

    case MyApp.Repo.transaction(fn ->
           MyApp.UserImporter.import_from_csv(file, dry_run: dry_run?)
         end) do
      {:ok, {:ok, count}} ->
        after_count = MyApp.Repo.aggregate(MyApp.User, :count)
        Mix.shell().info("Imported #{count} users (total: #{after_count}, was: #{before_count})")

      {:ok, {:error, reason}} ->
        Mix.shell().error("Import validation failed: #{reason}")
        Mix.raise("Import failed")

      {:error, reason} ->
        Mix.shell().error("Import failed, transaction rolled back: #{inspect(reason)}")
        Mix.raise("Import failed")
    end
  end
end

# Usage: mix my_app.import_users --file=users.csv --dry-run

Task with Subcommands

defmodule Mix.Tasks.MyApp.Migrate do
  use Mix.Task

  @shortdoc "Runs database migrations"

  @impl Mix.Task
  def run(["up"]),     do: Mix.Task.run("ecto.migrate")
  def run(["down"]),   do: Mix.Task.run("ecto.rollback")
  def run(["status"]), do: Mix.Task.run("ecto.migrations")
  def run(_) do
    Mix.shell().info("""
    Usage:
      mix my_app.migrate up      - Run migrations
      mix my_app.migrate down    - Rollback last migration
      mix my_app.migrate status  - Show migration status
    """)
  end
end

Custom Generators

# lib/mix/tasks/my_app.gen.service.ex
defmodule Mix.Tasks.MyApp.Gen.Service do
  use Mix.Task
  use Mix.Generator

  @shortdoc "Generates a service module"

  @impl Mix.Task
  def run([name]) do
    module_name = Macro.camelize(name)
    file_path = "lib/my_app/services/#{name}.ex"
    create_file(file_path, service_template(module_name: module_name))
    Mix.shell().info("Created service: #{file_path}")
  end

  embed_template(:service, """
  defmodule MyApp.Services.<%= @module_name %> do
    @moduledoc \"\"\"
    Service module for <%= @module_name %> operations.
    \"\"\"

    def call(params) do
      {:ok, params}
    end
  end
  """)
end

# Usage: mix my_app.gen.service send_email
# Creates: lib/my_app/services/send_email.ex

Mix Project Configuration

Aliases

# mix.exs
defp aliases do
  [
    setup: ["deps.get", "ecto.setup"],
    "ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],
    "ecto.reset": ["ecto.drop", "ecto.setup"],
    test: ["ecto.create --quiet", "ecto.migrate --quiet", "test"],
    "assets.deploy": ["esbuild default --minify", "phx.digest"]
  ]
end

Registering Custom Tasks

def project do
  [
    app: :my_app,
    version: "0.1.0",
    elixir: "~> 1.15",
    start_permanent: Mix.env() == :prod,
    deps: deps(),
    aliases: aliases(),
    preferred_cli_env: [
      "my_app.seed_data": :dev,
      "my_app.import_users": :dev
    ]
  ]
end

Testing Custom Tasks

defmodule Mix.Tasks.MyApp.SeedDataTest do
  use ExUnit.Case

  setup do
    MyApp.Repo.delete_all(MyApp.User)
    :ok
  end

  test "seeds data successfully" do
    Mix.Project.in_project(:my_app, ".", fn _ ->
      Mix.Tasks.MyApp.SeedData.run([])
      assert MyApp.Repo.aggregate(MyApp.User, :count) > 0
    end)
  end

  test "seeds expected count" do
    Mix.Project.in_project(:my_app, ".", fn _ ->
      Mix.Tasks.MyApp.SeedData.run([])
      assert MyApp.Repo.aggregate(MyApp.User, :count) == 10
    end)
  end
end

Common Task Patterns

All follow the same skeleton: parse opts → app.start → transact → report. A task that deletes data must default to --dry-run reporting and require an explicit --force (or interactive confirmation) flag before calling Repo.delete_all/1 — never delete unconditionally based on the task's own judgment.

Common Pitfalls

Integration

Companion skills: ecto-essentials, testing-essentials, oban-essentials.