View Source Mix tasks

There are currently a number of built-in Phoenix-specific and Ecto-specific Mix tasks available to us within a newly-generated application. We can also create our own application specific tasks.

Note to learn more about mix, you can read Elixir's official Introduction to Mix.

Phoenix tasks

$ mix help --search "phx"
mix local.phx          # Updates the Phoenix project generator locally
mix phx                # Prints Phoenix help information
mix phx.digest         # Digests and compresses static files
mix phx.digest.clean   # Removes old versions of static assets.
mix phx.gen.auth       # Generates authentication logic for a resource
mix phx.gen.cert       # Generates a self-signed certificate for HTTPS testing
mix phx.gen.channel    # Generates a Phoenix channel
mix phx.gen.context    # Generates a context with functions around an Ecto schema
mix phx.gen.embedded   # Generates an embedded Ecto schema file
mix phx.gen.html       # Generates controller, views, and context for an HTML resource
mix phx.gen.json       # Generates controller, views, and context for a JSON resource
mix phx.gen.live       # Generates LiveView, templates, and context for a resource
mix phx.gen.notifier   # Generates a notifier that delivers emails by default
mix phx.gen.presence   # Generates a Presence tracker
mix phx.gen.schema     # Generates an Ecto schema and migration file
mix phx.gen.secret     # Generates a secret
mix phx.gen.socket     # Generates a Phoenix socket handler
mix phx.new            # Creates a new Phoenix application
mix phx.new.ecto       # Creates a new Ecto project within an umbrella project
mix phx.new.web        # Creates a new Phoenix web project within an umbrella project
mix phx.routes         # Prints all routes
mix phx.server         # Starts applications and their servers

We have seen all of these at one point or another in the guides, but having all the information about them in one place seems like a good idea.

We will cover all Phoenix Mix tasks, except phx.new, phx.new.ecto, and phx.new.web, which are part of the Phoenix installer. You can learn more about them or any other task by calling mix help TASK.

`mix phx.gen.html`

Phoenix offers the ability to generate all the code to stand up a complete HTML resource — Ecto migration, Ecto context, controller with all the necessary actions, view, and templates. This can be a tremendous time saver. Let's take a look at how to make this happen.

The mix phx.gen.html task takes the following arguments: the module name of the context, the module name of the schema, the resource name, and a list of column_name:type attributes. The module name we pass in must conform to the Elixir rules of module naming, following proper capitalization.

$ mix phx.gen.html Blog Post posts body:string word_count:integer
* creating lib/hello_web/controllers/post_controller.ex
* creating lib/hello_web/controllers/post_html/edit.html.heex
* creating lib/hello_web/controllers/post_html/post_form.html.heex
* creating lib/hello_web/controllers/post_html/index.html.heex
* creating lib/hello_web/controllers/post_html/new.html.heex
* creating lib/hello_web/controllers/post_html/show.html.heex
* creating lib/hello_web/controllers/post_html.ex
* creating test/hello_web/controllers/post_controller_test.exs
* creating lib/hello/blog/post.ex
* creating priv/repo/migrations/20211001233016_create_posts.exs
* creating lib/hello/blog.ex
* injecting lib/hello/blog.ex
* creating test/hello/blog_test.exs
* injecting test/hello/blog_test.exs
* creating test/support/fixtures/blog_fixtures.ex
* injecting test/support/fixtures/blog_fixtures.ex

When mix phx.gen.html is done creating files, it helpfully tells us that we need to add a line to our router file as well as run our Ecto migrations.

Add the resource to your browser scope in lib/hello_web/router.ex:

    resources "/posts", PostController

Remember to update your repository by running migrations:

    $ mix ecto.migrate

Important: If we don't do this, we will see the following warnings in our logs, and our application will error when compiling.

$ mix phx.server
Compiling 17 files (.ex)

warning: no route path for HelloWeb.Router matches \"/posts\"
  lib/hello_web/controllers/post_controller.ex:22: HelloWeb.PostController.index/2

If we don't want to create a context or schema for our resource we can use the --no-context flag. Note that this still requires a context module name as a parameter.

$ mix phx.gen.html Blog Post posts body:string word_count:integer --no-context
* creating lib/hello_web/controllers/post_controller.ex
* creating lib/hello_web/controllers/post_html/edit.html.heex
* creating lib/hello_web/controllers/post_html/post_form.html.heex
* creating lib/hello_web/controllers/post_html/index.html.heex
* creating lib/hello_web/controllers/post_html/new.html.heex
* creating lib/hello_web/controllers/post_html/show.html.heex
* creating lib/hello_web/controllers/post_html.ex
* creating test/hello_web/controllers/post_controller_test.exs

It will tell us we need to add a line to our router file, but since we skipped the context, it won't mention anything about ecto.migrate.

Add the resource to your browser scope in lib/hello_web/router.ex:

    resources "/posts", PostController

Similarly, if we want a context created without a schema for our resource we can use the --no-schema flag.

$ mix phx.gen.html Blog Post posts body:string word_count:integer --no-schema
* creating lib/hello_web/controllers/post_controller.ex
* creating lib/hello_web/controllers/post_html/edit.html.heex
* creating lib/hello_web/controllers/post_html/post_form.html.heex
* creating lib/hello_web/controllers/post_html/index.html.heex
* creating lib/hello_web/controllers/post_html/new.html.heex
* creating lib/hello_web/controllers/post_html/show.html.heex
* creating lib/hello_web/controllers/post_html.ex
* creating test/hello_web/controllers/post_controller_test.exs
* creating lib/hello/blog.ex
* injecting lib/hello/blog.ex
* creating test/hello/blog_test.exs
* injecting test/hello/blog_test.exs
* creating test/support/fixtures/blog_fixtures.ex
* injecting test/support/fixtures/blog_fixtures.ex

It will tell us we need to add a line to our router file, but since we skipped the schema, it won't mention anything about ecto.migrate.

`mix phx.gen.json`

Phoenix also offers the ability to generate all the code to stand up a complete JSON resource — Ecto migration, Ecto schema, controller with all the necessary actions and view. This command will not create any template for the app.

The mix phx.gen.json task takes the following arguments: the module name of the context, the module name of the schema, the resource name, and a list of column_name:type attributes. The module name we pass in must conform to the Elixir rules of module naming, following proper capitalization.

$ mix phx.gen.json Blog Post posts title:string content:string
* creating lib/hello_web/controllers/post_controller.ex
* creating lib/hello_web/controllers/post_json.ex
* creating test/hello_web/controllers/post_controller_test.exs
* creating lib/hello_web/controllers/changeset_json.ex
* creating lib/hello_web/controllers/fallback_controller.ex
* creating lib/hello/blog/post.ex
* creating priv/repo/migrations/20170906153323_create_posts.exs
* creating lib/hello/blog.ex
* injecting lib/hello/blog.ex
* creating test/hello/blog/blog_test.exs
* injecting test/hello/blog/blog_test.exs
* creating test/support/fixtures/blog_fixtures.ex
* injecting test/support/fixtures/blog_fixtures.ex

When mix phx.gen.json is done creating files, it helpfully tells us that we need to add a line to our router file as well as run our Ecto migrations.

Add the resource to the "/api" scope in lib/hello_web/router.ex:

    resources "/posts", PostController, except: [:new, :edit]

Remember to update your repository by running migrations:

    $ mix ecto.migrate

Important: If we don't do this, we'll get the following warning in our logs and the application will error when attempting to compile:

$ mix phx.server
Compiling 19 files (.ex)

warning: no route path for HelloWeb.Router matches \"/posts\"
  lib/hello_web/controllers/post_controller.ex:22: HelloWeb.PostController.index/2

mix phx.gen.json also supports --no-context, --no-schema, and others, as in mix phx.gen.html.

`mix phx.gen.context`

If we don't need a complete HTML/JSON resource and only need a context, we can use the mix phx.gen.context task. It will generate a context, a schema, a migration and a test case.

The mix phx.gen.context task takes the following arguments: the module name of the context, the module name of the schema, the resource name, and a list of column_name:type attributes.

$ mix phx.gen.context Accounts User users name:string age:integer
* creating lib/hello/accounts/user.ex
* creating priv/repo/migrations/20170906161158_create_users.exs
* creating lib/hello/accounts.ex
* injecting lib/hello/accounts.ex
* creating test/hello/accounts/accounts_test.exs
* injecting test/hello/accounts/accounts_test.exs
* creating test/support/fixtures/accounts_fixtures.ex
* injecting test/support/fixtures/accounts_fixtures.ex

Note: If we need to namespace our resource we can simply namespace the first argument of the generator.

$ mix phx.gen.context Admin.Accounts User users name:string age:integer
* creating lib/hello/admin/accounts/user.ex
* creating priv/repo/migrations/20170906161246_create_users.exs
* creating lib/hello/admin/accounts.ex
* injecting lib/hello/admin/accounts.ex
* creating test/hello/admin/accounts_test.exs
* injecting test/hello/admin/accounts_test.exs
* creating test/support/fixtures/admin/accounts_fixtures.ex
* injecting test/support/fixtures/admin/accounts_fixtures.ex

`mix phx.gen.schema`

If we don't need a complete HTML/JSON resource and are not interested in generating or altering a context we can use the mix phx.gen.schema task. It will generate a schema, and a migration.

The mix phx.gen.schema task takes the following arguments: the module name of the schema (which may be namespaced), the resource name, and a list of column_name:type attributes.

$ mix phx.gen.schema Accounts.Credential credentials email:string:unique user_id:references:users
* creating lib/hello/accounts/credential.ex
* creating priv/repo/migrations/20170906162013_create_credentials.exs

`mix phx.gen.auth`

Phoenix also offers the ability to generate all of the code to stand up a complete authentication system — Ecto migration, phoenix context, controllers, templates, etc. This can be a huge time saver, allowing you to quickly add authentication to your system and shift your focus back to the primary problems your application is trying to solve.

The mix phx.gen.auth task takes the following arguments: the module name of the context, the module name of the schema, and a plural version of the schema name used to generate database tables and route paths.

Here is an example version of the command:

$ mix phx.gen.auth Accounts User users
* creating priv/repo/migrations/20201205184926_create_users_auth_tables.exs
* creating lib/hello/accounts/user_notifier.ex
* creating lib/hello/accounts/user.ex
* creating lib/hello/accounts/user_token.ex
* creating lib/hello_web/controllers/user_auth.ex
* creating test/hello_web/controllers/user_auth_test.exs
* creating lib/hello_web/controllers/user_confirmation_html.ex
* creating lib/hello_web/templates/user_confirmation/new.html.heex
* creating lib/hello_web/templates/user_confirmation/edit.html.heex
* creating lib/hello_web/controllers/user_confirmation_controller.ex
* creating test/hello_web/controllers/user_confirmation_controller_test.exs
* creating lib/hello_web/templates/user_registration/new.html.heex
* creating lib/hello_web/controllers/user_registration_controller.ex
* creating test/hello_web/controllers/user_registration_controller_test.exs
* creating lib/hello_web/controllers/user_registration_html.ex
* creating lib/hello_web/controllers/user_reset_password_html.ex
* creating lib/hello_web/controllers/user_reset_password_controller.ex
* creating test/hello_web/controllers/user_reset_password_controller_test.exs
* creating lib/hello_web/templates/user_reset_password/edit.html.heex
* creating lib/hello_web/templates/user_reset_password/new.html.heex
* creating lib/hello_web/controllers/user_session_html.ex
* creating lib/hello_web/controllers/user_session_controller.ex
* creating test/hello_web/controllers/user_session_controller_test.exs
* creating lib/hello_web/templates/user_session/new.html.heex
* creating lib/hello_web/controllers/user_settings_html.ex
* creating lib/hello_web/templates/user_settings/edit.html.heex
* creating lib/hello_web/controllers/user_settings_controller.ex
* creating test/hello_web/controllers/user_settings_controller_test.exs
* creating lib/hello/accounts.ex
* injecting lib/hello/accounts.ex
* creating test/hello/accounts_test.exs
* injecting test/hello/accounts_test.exs
* creating test/support/fixtures/accounts_fixtures.ex
* injecting test/support/fixtures/accounts_fixtures.ex
* injecting test/support/conn_case.ex
* injecting config/test.exs
* injecting mix.exs
* injecting lib/hello_web/router.ex
* injecting lib/hello_web/router.ex - imports
* injecting lib/hello_web/router.ex - plug
* injecting lib/hello_web/templates/layout/root.html.heex

When mix phx.gen.auth is done creating files, it helpfully tells us that we need to re-fetch our dependencies as well as run our Ecto migrations.

Please re-fetch your dependencies with the following command:

    mix deps.get

Remember to update your repository by running migrations:

  $ mix ecto.migrate

Once you are ready, visit "/users/register"
to create your account and then access to "/dev/mailbox" to
see the account confirmation email.

A more complete walk-through of how to get started with this generator is available in the mix phx.gen.auth authentication guide.

`mix phx.gen.channel` and `mix phx.gen.socket`

This task will generate a basic Phoenix channel, the socket to power the channel (if you haven't created one yet), as well a test case for it. It takes the module name for the channel as the only argument:

$ mix phx.gen.channel Room
* creating lib/hello_web/channels/room_channel.ex
* creating test/hello_web/channels/room_channel_test.exs

If your application does not have a UserSocket yet, it will ask if you want to create one:

The default socket handler - HelloWeb.UserSocket - was not found
in its default location.

Do you want to create it? [Y/n]

By confirming, a channel will be created, then you need to connect the socket in your endpoint:

Add the socket handler to your `lib/hello_web/endpoint.ex`, for example:

    socket "/socket", HelloWeb.UserSocket,
      websocket: true,
      longpoll: false

For the front-end integration, you need to import the `user_socket.js`
in your `assets/js/app.js` file:

    import "./user_socket.js"

In case a UserSocket already exists or you decide to not create one, the channel generator will tell you to add it to the Socket manually:

Add the channel to your `lib/hello_web/channels/user_socket.ex` handler, for example:

    channel "rooms:lobby", HelloWeb.RoomChannel

You can also create a socket any time by invoking mix phx.gen.socket.

`mix phx.gen.presence`

This task will generate a presence tracker. The module name can be passed as an argument, Presence is used if no module name is passed.

$ mix phx.gen.presence Presence
* lib/hello_web/channels/presence.ex

Add your new module to your supervision tree,
in lib/hello/application.ex:

    children = [
      ...
      HelloWeb.Presence
    ]

`mix phx.routes`

This task has a single purpose, to show us all the routes defined for a given router. We saw it used extensively in the routing guide.

If we don't specify a router for this task, it will default to the router Phoenix generated for us.

$ mix phx.routes
GET  /  TaskTester.PageController.index/2

We can also specify an individual router if we have more than one for our application.

$ mix phx.routes TaskTesterWeb.Router
GET  /  TaskTesterWeb.PageController.index/2

`mix phx.server`

This is the task we use to get our application running. It takes no arguments at all. If we pass any in, they will be silently ignored.

$ mix phx.server
[info] Running TaskTesterWeb.Endpoint with Cowboy on port 4000 (http)

It will silently ignore our DoesNotExist argument:

$ mix phx.server DoesNotExist
[info] Running TaskTesterWeb.Endpoint with Cowboy on port 4000 (http)

If we would like to start our application and also have an IEx session open to it, we can run the Mix task within iex like this, iex -S mix phx.server.

$ iex -S mix phx.server
Erlang/OTP 17 [erts-6.4] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

[info] Running TaskTesterWeb.Endpoint with Cowboy on port 4000 (http)
Interactive Elixir (1.0.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)>

`mix phx.digest`

This task does two things, it creates a digest for our static assets and then compresses them.

"Digest" here refers to an MD5 digest of the contents of an asset which gets added to the filename of that asset. This creates a sort of fingerprint for it. If the digest doesn't change, browsers and CDNs will use a cached version. If it does change, they will re-fetch the new version.

Before we run this task let's inspect the contents of two directories in our hello application.

First priv/static/ which should look similar to this:

├── assets
│   ├── app.css
│   └── app.js
├── favicon.ico
└── robots.txt

And then assets/ which should look similar to this:

├── css
│   └── app.css
├── js
│   └── app.js
├── tailwind.config.js
└── vendor
    └── topbar.js

All of these files are our static assets. Now let's run the mix phx.digest task.

$ mix phx.digest
Check your digested files at 'priv/static'.

We can now do as the task suggests and inspect the contents of priv/static/ directory. We'll see that all files from assets/ have been copied over to priv/static/ and also each file now has a couple of versions. Those versions are:

the original file
a compressed file with gzip
a file containing the original file name and its digest
a compressed file containing the file name and its digest

We can optionally determine which files should be gzipped by using the :gzippable_exts option in the config file:

config :phoenix, :gzippable_exts, ~w(.js .css)

Note: We can specify a different output folder where mix phx.digest will put processed files. The first argument is the path where the static files are located.

$ mix phx.digest priv/static/ -o www/public/
Check your digested files at 'www/public/'

Note: You can use mix phx.digest.clean to prune stale versions of the assets. If you want to remove all produced files, run mix phx.digest.clean --all.

Ecto tasks

Newly generated Phoenix applications now include Ecto and Postgrex as dependencies by default (which is to say, unless we use mix phx.new with the --no-ecto flag). With those dependencies come Mix tasks to take care of common Ecto operations. Let's see which tasks we get out of the box.

$ mix help --search "ecto"
mix ecto               # Prints Ecto help information
mix ecto.create        # Creates the repository storage
mix ecto.drop          # Drops the repository storage
mix ecto.dump          # Dumps the repository database structure
mix ecto.gen.migration # Generates a new migration for the repo
mix ecto.gen.repo      # Generates a new repository
mix ecto.load          # Loads previously dumped database structure
mix ecto.migrate       # Runs the repository migrations
mix ecto.migrations    # Displays the repository migration status
mix ecto.reset         # Alias defined in mix.exs
mix ecto.rollback      # Rolls back the repository migrations
mix ecto.setup         # Alias defined in mix.exs

Note: We can run any of the tasks above with the --no-start flag to execute the task without starting the application.

`mix ecto.create`

This task will create the database specified in our repo. By default it will look for the repo named after our application (the one generated with our app unless we opted out of Ecto), but we can pass in another repo if we want.

Here's what it looks like in action.

$ mix ecto.create
The database for Hello.Repo has been created.

There are a few things that can go wrong with ecto.create. If our Postgres database doesn't have a "postgres" role (user), we'll get an error like this one.

$ mix ecto.create
** (Mix) The database for Hello.Repo couldn't be created, reason given: psql: FATAL:  role "postgres" does not exist

We can fix this by creating the "postgres" role in the psql console with the permissions needed to log in and create a database.

=# CREATE ROLE postgres LOGIN CREATEDB;
CREATE ROLE

If the "postgres" role does not have permission to log in to the application, we'll get this error.

$ mix ecto.create
** (Mix) The database for Hello.Repo couldn't be created, reason given: psql: FATAL:  role "postgres" is not permitted to log in

To fix this, we need to change the permissions on our "postgres" user to allow login.

=# ALTER ROLE postgres LOGIN;
ALTER ROLE

If the "postgres" role does not have permission to create a database, we'll get this error.

$ mix ecto.create
** (Mix) The database for Hello.Repo couldn't be created, reason given: ERROR:  permission denied to create database

To fix this, we need to change the permissions on our "postgres" user in the psql console to allow database creation.

=# ALTER ROLE postgres CREATEDB;
ALTER ROLE

If the "postgres" role is using a password different from the default "postgres", we'll get this error.

$ mix ecto.create
** (Mix) The database for Hello.Repo couldn't be created, reason given: psql: FATAL:  password authentication failed for user "postgres"

To fix this, we can change the password in the environment specific configuration file. For the development environment the password used can be found at the bottom of the config/dev.exs file.

Finally, if we happen to have another repo called OurCustom.Repo that we want to create the database for, we can run this.

$ mix ecto.create -r OurCustom.Repo
The database for OurCustom.Repo has been created.

`mix ecto.drop`

This task will drop the database specified in our repo. By default it will look for the repo named after our application (the one generated with our app unless we opted out of Ecto). It will not prompt us to check if we're sure we want to drop the database, so do exercise caution.

$ mix ecto.drop
The database for Hello.Repo has been dropped.

If we happen to have another repo that we want to drop the database for, we can specify it with the -r flag.

$ mix ecto.drop -r OurCustom.Repo
The database for OurCustom.Repo has been dropped.

`mix ecto.gen.repo`

Many applications require more than one data store. For each data store, we'll need a new repo, and we can generate them automatically with ecto.gen.repo.

If we name our repo OurCustom.Repo, this task will create it here lib/our_custom/repo.ex.

$ mix ecto.gen.repo -r OurCustom.Repo
* creating lib/our_custom
* creating lib/our_custom/repo.ex
* updating config/config.exs
Don't forget to add your new repo to your supervision tree
(typically in lib/hello/application.ex):

    {OurCustom.Repo, []}

Notice that this task has updated config/config.exs. If we take a look, we'll see this extra configuration block for our new repo.

. . .
config :hello, OurCustom.Repo,
  username: "user",
  password: "pass",
  hostname: "localhost",
  database: "hello_repo",
. . .

Of course, we'll need to change the login credentials to match what our database expects. We'll also need to change the config for other environments.

We certainly should follow the instructions and add our new repo to our supervision tree. In our Hello application, we would open up lib/hello/application.ex, and add our repo as a worker to the children list.

. . .
children = [
  Hello.Repo,
  # Our custom repo
  OurCustom.Repo,
  # Start the endpoint when the application starts
  HelloWeb.Endpoint,
]
. . .

`mix ecto.gen.migration`

Migrations are a programmatic, repeatable way to affect changes to a database schema. Migrations are also just modules, and we can create them with the ecto.gen.migration task. Let's walk through the steps to create a migration for a new comments table.

We simply need to invoke the task with a snake_case version of the module name that we want. Preferably, the name will describe what we want the migration to do.

$ mix ecto.gen.migration add_comments_table
* creating priv/repo/migrations
* creating priv/repo/migrations/20150318001628_add_comments_table.exs

Notice that the migration's filename begins with a string representation of the date and time the file was created.

Let's take a look at the file ecto.gen.migration has generated for us at priv/repo/migrations/20150318001628_add_comments_table.exs.

defmodule Hello.Repo.Migrations.AddCommentsTable do
  use Ecto.Migration

  def change do
  end
end

Notice that there is a single function change/0 which will handle both forward migrations and rollbacks. We'll define the schema changes that we want using Ecto's handy DSL, and Ecto will figure out what to do depending on whether we are rolling forward or rolling back. Very nice indeed.

What we want to do is create a comments table with a body column, a word_count column, and timestamp columns for inserted_at and updated_at.

. . .
def change do
  create table(:comments) do
    add :body, :string
    add :word_count, :integer
    timestamps()
  end
end
. . .

Again, we can run this task with the -r flag and another repo if we need to.

$ mix ecto.gen.migration -r OurCustom.Repo add_users
* creating priv/repo/migrations
* creating priv/repo/migrations/20150318172927_add_users.exs

For more information on how to modify your database schema please refer to the Ecto's migration DSL docs. For example, to alter an existing schema see the documentation on Ecto’s alter/2 function.

That's it! We're ready to run our migration.

`mix ecto.migrate`

Once we have our migration module ready, we can simply run mix ecto.migrate to have our changes applied to the database.

$ mix ecto.migrate
[info] == Running Hello.Repo.Migrations.AddCommentsTable.change/0 forward
[info] create table comments
[info] == Migrated in 0.1s

When we first run ecto.migrate, it will create a table for us called schema_migrations. This will keep track of all the migrations which we run by storing the timestamp portion of the migration's filename.

Here's what the schema_migrations table looks like.

hello_dev=# select * from schema_migrations;
version        |     inserted_at
---------------+---------------------
20150317170448 | 2015-03-17 21:07:26
20150318001628 | 2015-03-18 01:45:00
(2 rows)

When we roll back a migration, ecto.rollback will remove the record representing this migration from schema_migrations.

By default, ecto.migrate will execute all pending migrations. We can exercise more control over which migrations we run by specifying some options when we run the task.

We can specify the number of pending migrations we would like to run with the -n or --step options.

$ mix ecto.migrate -n 2
[info] == Running Hello.Repo.Migrations.CreatePost.change/0 forward
[info] create table posts
[info] == Migrated in 0.0s
[info] == Running Hello.Repo.Migrations.AddCommentsTable.change/0 forward
[info] create table comments
[info] == Migrated in 0.0s

The --step option will behave the same way.

mix ecto.migrate --step 2

The --to option will run all migrations up to and including given version.

mix ecto.migrate --to 20150317170448

`mix ecto.rollback`

The ecto.rollback task will reverse the last migration we have run, undoing the schema changes. ecto.migrate and ecto.rollback are mirror images of each other.

$ mix ecto.rollback
[info] == Running Hello.Repo.Migrations.AddCommentsTable.change/0 backward
[info] drop table comments
[info] == Migrated in 0.0s

ecto.rollback will handle the same options as ecto.migrate, so -n, --step, -v, and --to will behave as they do for ecto.migrate.

Creating our own Mix task

As we've seen throughout this guide, both Mix itself and the dependencies we bring in to our application provide a number of really useful tasks for free. Since neither of these could possibly anticipate all our individual application's needs, Mix allows us to create our own custom tasks. That's exactly what we are going to do now.

The first thing we need to do is create a mix/tasks/ directory inside of lib/. This is where any of our application specific Mix tasks will go.

$ mkdir -p lib/mix/tasks/

Inside that directory, let's create a new file, hello.greeting.ex, that looks like this.

defmodule Mix.Tasks.Hello.Greeting do
  use Mix.Task

  @shortdoc "Sends a greeting to us from Hello Phoenix"

  @moduledoc """
  This is where we would put any long form documentation and doctests.
  """

  @impl Mix.Task
  def run(_args) do
    Mix.shell().info("Greetings from the Hello Phoenix Application!")
  end

  # We can define other functions as needed here.
end

Let's take a quick look at the moving parts involved in a working Mix task.

The first thing we need to do is name our module. All tasks must be defined in the Mix.Tasks namespace. We'd like to invoke this as mix hello.greeting, so we complete the module name with Hello.Greeting.

The use Mix.Task line brings in functionality from Mix that makes this module behave as a Mix task.

The @shortdoc module attribute holds a string which will describe our task when users invoke mix help.

@moduledoc serves the same function that it does in any module. It's where we can put long-form documentation and doctests, if we have any.

The run/1 function is the critical heart of any Mix task. It's the function that does all the work when users invoke our task. In ours, all we do is send a greeting from our app, but we can implement our run/1 function to do whatever we need it to. Note that Mix.shell().info/1 is the preferred way to print text back out to the user.

Of course, our task is just a module, so we can define other private functions as needed to support our run/1 function.

Now that we have our task module defined, our next step is to compile the application.

$ mix compile
Compiled lib/tasks/hello.greeting.ex
Generated hello.app

Now our new task should be visible to mix help.

$ mix help --search hello
mix hello.greeting # Sends a greeting to us from Hello Phoenix

Notice that mix help displays the text we put into the @shortdoc along with the name of our task.

So far, so good, but does it work?

$ mix hello.greeting
Greetings from the Hello Phoenix Application!

Indeed it does.

If you want to make your new Mix task to use your application's infrastructure, you need to make sure the application is started and configure when Mix task is being executed. This is particularly useful if you need to access your database from within the Mix task. Thankfully, Mix makes it really easy for us via the @requirements module attribute:

  @requirements ["app.start"]

  @impl Mix.Task
  def run(_args) do
    Mix.shell().info("Now I have access to Repo and other goodies!")
    Mix.shell().info("Greetings from the Hello Phoenix Application!")
  end

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View Source Mix tasks

Phoenix tasks

mix phx.gen.channel and mix phx.gen.socket

Ecto tasks

Creating our own Mix task

`mix phx.gen.channel` and `mix phx.gen.socket`