Controllers

Requirement: This guide expects that you have gone through the introductory guides and got a Phoenix application up and running.

Requirement: This guide expects that you have gone through the request life-cycle guide.

Phoenix controllers act as intermediary modules. Their functions — called actions — are invoked from the router in response to HTTP requests. The actions, in turn, gather all the necessary data and perform all the necessary steps before invoking the view layer to render a template or returning a JSON response.

Phoenix controllers also build on the Plug package, and are themselves plugs. Controllers provide the functions to do almost anything we need to in an action. If we do find ourselves looking for something that Phoenix controllers don't provide, we might find what we're looking for in Plug itself. Please see the Plug guide or the Plug documentation for more information.

A newly generated Phoenix app will have a single controller named PageController, which can be found at lib/hello_web/controllers/page_controller.ex which looks like this:

defmodule HelloWeb.PageController do
  use HelloWeb, :controller

  def index(conn, _params) do
    render(conn, "index.html")
  end
end

The first line below the module definition invokes the __using__/1 macro of the HelloWeb module, which imports some useful modules.

PageController gives us the index action to display the Phoenix welcome page associated with the default route Phoenix defines in the router.

Actions

Controller actions are just functions. We can name them anything we like as long as they follow Elixir's naming rules. The only requirement we must fulfill is that the action name matches a route defined in the router.

For example, in lib/hello_web/router.ex we could change the action name in the default route that Phoenix gives us in a new app from index:

get "/", PageController, :index

to home:

get "/", PageController, :home

as long as we change the action name in PageController to home as well, the welcome page will load as before.

defmodule HelloWeb.PageController do
  ...

  def home(conn, _params) do
    render(conn, "index.html")
  end
end

While we can name our actions whatever we like, there are conventions for action names which we should follow whenever possible. We went over these in the routing guide, but we'll take another quick look here.

index - renders a list of all items of the given resource type
show - renders an individual item by ID
new - renders a form for creating a new item
create - receives parameters for one new item and saves it in a data store
edit - retrieves an individual item by ID and displays it in a form for editing
update - receives parameters for one edited item and saves the item to a data store
delete - receives an ID for an item to be deleted and deletes it from a data store

Each of these actions takes two parameters, which will be provided by Phoenix behind the scenes.

The first parameter is always conn, a struct which holds information about the request such as the host, path elements, port, query string, and much more. conn comes to Phoenix via Elixir's Plug middleware framework. More detailed information about conn can be found in the Plug.Conn documentation.

The second parameter is params. Not surprisingly, this is a map which holds any parameters passed along in the HTTP request. It is a good practice to pattern match against parameters in the function signature to provide data in a simple package we can pass on to rendering. We saw this in the request life-cycle guide when we added a messenger parameter to our show route in lib/hello_web/controllers/hello_controller.ex.

defmodule HelloWeb.HelloController do
  ...

  def show(conn, %{"messenger" => messenger}) do
    render(conn, "show.html", messenger: messenger)
  end
end

In some cases — often in index actions, for instance — we don't care about parameters because our behavior doesn't depend on them. In those cases, we don't use the incoming parameters, and simply prefix the variable name with an underscore, calling it _params. This will keep the compiler from complaining about the unused variable while still keeping the correct arity.

Rendering

Controllers can render content in several ways. The simplest is to render some plain text using the text/2 function which Phoenix provides.

For example, let's rewrite the show action from HelloController to return text instead. For that, we could do the following.

def show(conn, %{"messenger" => messenger}) do
  text(conn, "From messenger #{messenger}")
end

Now /hello/Frank in your browser should display From messenger Frank as plain text without any HTML.

A step beyond this is rendering pure JSON with the json/2 function. We need to pass it something that the Jason library can decode into JSON, such as a map. (Jason is one of Phoenix's dependencies.)

def show(conn, %{"messenger" => messenger}) do
  json(conn, %{id: messenger})
end

If we again visit /hello/Frank in the browser, we should see a block of JSON with the key id mapped to the string "Frank".

{"id": "Frank"}

The json/2 function is useful for writing APIs and there is also the html/2 function for rendering HTML, but most of the times we use Phoenix views to build our responses. For this, Phoenix includes the render/3 function. It is specially important for HTML responses, as Phoenix Views provide performance and security benefits.

Let's rollback our show action to what we originally wrote in the request life-cycle guide:

defmodule HelloWeb.HelloController do
  use HelloWeb, :controller

  def show(conn, %{"messenger" => messenger}) do
    render(conn, "show.html", messenger: messenger)
  end
end

In order for the render/3 function to work correctly, the controller and view must share the same root name (in this case Hello), and it also must have the same root name as the template directory (in this case hello) where the show.html.heex template lives. In other words, HelloController requires HelloView, and HelloView requires the existence of the lib/hello_web/templates/hello directory, which must contain the show.html.heex template.

render/3 will also pass the value which the show action received for messenger from the parameters as an assign.

If we need to pass values into the template when using render, that's easy. We can pass a keyword like we've seen with messenger: messenger, or we can use Plug.Conn.assign/3, which conveniently returns conn.

  def show(conn, %{"messenger" => messenger}) do
    conn
    |> Plug.Conn.assign(:messenger, messenger)
    |> render("show.html")
  end

Note: Using Phoenix.Controller imports Plug.Conn, so shortening the call to assign/3 works just fine.

Passing more than one value to our template is as simple as connecting assign/3 functions together:

  def show(conn, %{"messenger" => messenger}) do
    conn
    |> assign(:messenger, messenger)
    |> assign(:receiver, "Dweezil")
    |> render("show.html")
  end

Generally speaking, once all assigns are configured, we invoke the view layer. The view layer then renders show.html alongside the layout and a response is sent back to the browser.

Views and templates have their own guide, so we won't spend much time on them here. What we will look at is how to assign a different layout, or none at all, from inside a controller action.

Assigning layouts

Layouts are just a special subset of templates. They live in the templates/layout folder (lib/hello_web/templates/layout). Phoenix created three for us when we generated our app. The default root layout is called root.html.heex, and it is the layout into which all templates will be rendered by default.

Since layouts are really just templates, they need a view to render them. This one is LayoutView which is defined in lib/hello_web/views/layout_view.ex. Since Phoenix generated this view for us, we won't have to create a new one as long as we put the layouts we want to render inside the lib/hello_web/templates/layout directory.

Before we create a new layout, though, let's do the simplest possible thing and render a template with no layout at all.

The Phoenix.Controller module provides the put_root_layout/2 function for us to switch root layouts. This takes conn as its first argument and a string for the basename of the layout we want to render. It also accepts false to disable the layout altogether.

You can edit the index action of PageController in lib/hello_web/controllers/page_controller.ex to look like this.

def index(conn, _params) do
  conn
  |> put_root_layout(false)
  |> render("index.html")
end

After reloading http://localhost:4000/, we should see a very different page, one with no title, logo image, or CSS styling at all.

Now let's actually create another layout and render the index template into it. As an example, let's say we had a different layout for the admin section of our application which didn't have the logo image. To do this, let's copy the existing root.html.heex to a new file admin.html.heex in the same directory lib/hello_web/templates/layout. Then let's replace the lines in admin.html.heex that displays the logo with the word "Administration".

Remove these lines:

<a href="https://phoenixframework.org/" class="phx-logo">
  <img src={Routes.static_path(@conn, "/images/phoenix.png")} alt="Phoenix Framework Logo"/>
</a>

Replace them with:

<p>Administration</p>

Then, pass the basename of the new layout into put_root_layout/2 in our index action in lib/hello_web/controllers/page_controller.ex.

def index(conn, _params) do
  conn
  |> put_root_layout("admin.html")
  |> render("index.html")
end

When we load the page, we should be rendering the admin layout without a logo and with the word "Administration".

Overriding rendering formats

Rendering HTML through a template is fine, but what if we need to change the rendering format on the fly? Let's say that sometimes we need HTML, sometimes we need plain text, and sometimes we need JSON. Then what?

Phoenix allows us to change formats on the fly with the _format query string parameter. To make this happen, Phoenix requires an appropriately named view and an appropriately named template in the correct directory.

As an example, let's take PageController's index action from a newly generated app. Out of the box, this has the right view PageView, the right templates directory (lib/hello_web/templates/page), and the right template for rendering HTML (index.html.heex.)

def index(conn, _params) do
  render(conn, "index.html")
end

What it doesn't have is an alternative template for rendering text. Let's add one at lib/hello_web/templates/page/index.text.eex. Here is our example index.text.eex template.

OMG, this is actually some text.

There are just a few more things we need to do to make this work. We need to tell our router that it should accept the text format. We do that by adding text to the list of accepted formats in the :browser pipeline. Let's open up lib/hello_web/router.ex and change plug :accepts to include text as well as html like this.

defmodule HelloWeb.Router do
  use HelloWeb, :router

  pipeline :browser do
    plug :accepts, ["html", "text"]
    plug :fetch_session
    plug :fetch_live_flash
    plug :put_root_layout, {HelloWeb.LayoutView, :root}
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end
...

We also need to tell the controller to render a template with the same format as the one returned by Phoenix.Controller.get_format/1. We do that by substituting the name of the template "index.html" with the atom version :index.

def index(conn, _params) do
  render(conn, :index)
end

If we go to http://localhost:4000/?_format=text, we will see "OMG, this is actually some text.".

Sending responses directly

If none of the rendering options above quite fits our needs, we can compose our own using some of the functions that Plug gives us. Let's say we want to send a response with a status of "201" and no body whatsoever. We can do that with the Plug.Conn.send_resp/3 function.

Edit the index action of PageController in lib/hello_web/controllers/page_controller.ex to look like this:

def index(conn, _params) do
  send_resp(conn, 201, "")
end

Reloading http://localhost:4000 should show us a completely blank page. The network tab of our browser's developer tools should show a response status of "201" (Created). Some browsers (Safari) will download the response, as the content type is not set.

To be specific about the content type, we can use put_resp_content_type/2 in conjunction with send_resp/3.

def index(conn, _params) do
  conn
  |> put_resp_content_type("text/plain")
  |> send_resp(201, "")
end

Using Plug functions in this way, we can craft just the response we need.

Setting the Content Type

Analogous to the _format query string param, we can render any sort of format we want by modifying the HTTP Content-Type Header and providing the appropriate template.

If we wanted to render an XML version of our index action, we might implement the action like this in lib/hello_web/page_controller.ex.

def index(conn, _params) do
  conn
  |> put_resp_content_type("text/xml")
  |> render("index.xml", content: some_xml_content)
end

We would then need to provide an index.xml.eex template which created valid XML, and we would be done.

For a list of valid content mime-types, please see the MIME library.

Setting the HTTP Status

We can also set the HTTP status code of a response similarly to the way we set the content type. The Plug.Conn module, imported into all controllers, has a put_status/2 function to do this.

Plug.Conn.put_status/2 takes conn as the first parameter and as the second parameter either an integer or a "friendly name" used as an atom for the status code we want to set. The list of status code atom representations can be found in Plug.Conn.Status.code/1 documentation.

Let's change the status in our PageController index action.

def index(conn, _params) do
  conn
  |> put_status(202)
  |> render("index.html")
end

The status code we provide must be a valid number.

Redirection

Often, we need to redirect to a new URL in the middle of a request. A successful create action, for instance, will usually redirect to the show action for the resource we just created. Alternately, it could redirect to the index action to show all the things of that same type. There are plenty of other cases where redirection is useful as well.

Whatever the circumstance, Phoenix controllers provide the handy redirect/2 function to make redirection easy. Phoenix differentiates between redirecting to a path within the application and redirecting to a URL — either within our application or external to it.

In order to try out redirect/2, let's create a new route in lib/hello_web/router.ex.

defmodule HelloWeb.Router do
  ...

  scope "/", HelloWeb do
    ...
    get "/", PageController, :index
    get "/redirect_test", PageController, :redirect_test
    ...
  end
end

Then we'll change PageController's index action of our controller to do nothing but to redirect to our new route.

defmodule HelloWeb.PageController do
  use HelloWeb, :controller

  def index(conn, _params) do
    redirect(conn, to: "/redirect_test")
  end
end

Actually, we should make use of the path helpers, which are the preferred approach to link to any page within our application, as we learned about in the routing guide.

def index(conn, _params) do
  redirect(conn, to: Routes.page_path(conn, :redirect_test))
end

Finally, let's define in the same file the action we redirect to, which simply renders the index, but now under a new address:

def redirect_test(conn, _params) do
  render(conn, "index.html")
end

When we reload our welcome page, we see that we've been redirected to /redirect_test which shows the original welcome page. It works!

If we care to, we can open up our developer tools, click on the network tab, and visit our root route again. We see two main requests for this page - a get to / with a status of 302, and a get to /redirect_test with a status of 200.

Notice that the redirect function takes conn as well as a string representing a relative path within our application. For security reasons, the :to helper can only redirect for paths within your application. If you want to redirect to a fully-qualified path or an external URL, you should use :external instead:

def index(conn, _params) do
  redirect(conn, external: "https://elixir-lang.org/")
end

Flash messages

Sometimes we need to communicate with users during the course of an action. Maybe there was an error updating a schema, or maybe we just want to welcome them back to the application. For this, we have flash messages.

The Phoenix.Controller module provides the put_flash/3 and get_flash/2 functions to help us set and retrieve flash messages as a key-value pair. Let's set two flash messages in our HelloWeb.PageController to try this out.

To do this we modify the index action as follows:

defmodule HelloWeb.PageController do
  ...
  def index(conn, _params) do
    conn
    |> put_flash(:info, "Welcome to Phoenix, from flash info!")
    |> put_flash(:error, "Let's pretend we have an error.")
    |> render("index.html")
  end
end

In order to see our flash messages, we need to be able to retrieve them and display them in a template layout. One way to do the first part is with get_flash/2 which takes conn and the key we care about. It then returns the value for that key.

For our convenience, the application layout, lib/hello_web/templates/layout/app.html.heex, already has markup for displaying flash messages.

<p class="alert alert-info" role="alert"><%= get_flash(@conn, :info) %></p>
<p class="alert alert-danger" role="alert"><%= get_flash(@conn, :error) %></p>

When we reload the welcome page, our messages should appear just above "Welcome to Phoenix!"

The flash functionality is handy when mixed with redirects. Perhaps you want to redirect to a page with some extra information. If we reuse the redirect action from the previous section, we can do:

  def index(conn, _params) do
    conn
    |> put_flash(:info, "Welcome to Phoenix, from flash info!")
    |> put_flash(:error, "Let's pretend we have an error.")
    |> redirect(to: Routes.page_path(conn, :redirect_test))
  end

Now if you reload the welcome page, you will be redirected and the flash messages will be shown once more.

Besides put_flash/3 and get_flash/2, the Phoenix.Controller module has another useful function worth knowing about. clear_flash/1 takes only conn and removes any flash messages which might be stored in the session.

Phoenix does not enforce which keys are stored in the flash. As long as we are internally consistent, all will be well. :info and :error, however, are common and are handled by default in our templates.

Action fallback

Action fallback allows us to centralize error handling code in plugs which are called when a controller action fails to return a %Plug.Conn{} struct. These plugs receive both the conn which was originally passed to the controller action along with the return value of the action.

Let's say we have a show action which uses with to fetch a blog post and then authorize the current user to view that blog post. In this example we might expect fetch_post/1 to return {:error, :not_found} if the post is not found and authorize_user/3 might return {:error, :unauthorized} if the user is unauthorized. We could use ErrorView which is generated by Phoenix for every new application to handle these error paths accordingly:

defmodule HelloWeb.MyController do
  use Phoenix.Controller

  def show(conn, %{"id" => id}, current_user) do
    with {:ok, post} <- fetch_post(id),
         :ok <- authorize_user(current_user, :view, post) do
      render(conn, "show.json", post: post)
    else
      {:error, :not_found} ->
        conn
        |> put_status(:not_found)
        |> put_view(HelloWeb.ErrorView)
        |> render(:"404")

      {:error, :unauthorized} ->
        conn
        |> put_status(403)
        |> put_view(HelloWeb.ErrorView)
        |> render(:"403")
    end
  end
end

Now imagine you may need to implement similar logic for every controller and action handled by your API. This would result in a lot of repetition.

Instead we can define a module plug which knows how to handle these error cases specifically. Since controllers are module plugs, let's define our plug as a controller:

defmodule HelloWeb.MyFallbackController do
  use Phoenix.Controller

  def call(conn, {:error, :not_found}) do
    conn
    |> put_status(:not_found)
    |> put_view(HelloWeb.ErrorView)
    |> render(:"404")
  end

  def call(conn, {:error, :unauthorized}) do
    conn
    |> put_status(403)
    |> put_view(HelloWeb.ErrorView)
    |> render(:"403")
  end
end

Then we can reference our new controller as the action_fallback and simply remove the else block from our with:

defmodule HelloWeb.MyController do
  use Phoenix.Controller

  action_fallback HelloWeb.MyFallbackController

  def show(conn, %{"id" => id}, current_user) do
    with {:ok, post} <- fetch_post(id),
         :ok <- authorize_user(current_user, :view, post) do
      render(conn, "show.json", post: post)
    end
  end
end

Whenever the with conditions do not match, HelloWeb.MyFallbackController will receive the original conn as well as the result of the action and respond accordingly.

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