Form validation with Phoenix LiveView

This post was updated 01 May - 2020


Form validation with Phoenix LiveView

One thing that has always been problematic is when providing live form validation for a user that interacts with a form.

The problem has always been that you need to have validation logic in both backend and frontend. That is of course not optimal.

However, with Phoenix LiveView, the problem with providing form validation is to a large extent solved. Let me explain how.


In this tutorial I already have a project setup but I want to generate a resource for products.

mix phx.gen.html Products Product products name description:text price:float

When that is generated, follow the instructions and add the resources in the routes and run

mix ecto.migrate

I can now start the server with

mix phx.server

And navigate to http://localhost:4000/products/new and see the form.

Step 2

Next step is to customize some validation rules. For example, I want to make sure that the name needs to be of a specific length and the price needs to be larger than zero.

In the file lib/tutorial/products/product.ex, I can add the extra validation rules in the bottom of the changeset function.

  def changeset(product, attrs) do
    |> cast(attrs, [:name, :description, :price])
    |> validate_required([:name, :description, :price])
    |> validate_length(:name, min: 2)
    |> validate_number(:price, greater_than: 0)

Step 3

Now, I can stop the server and create a folder called live inside the /tutorial_web/ folder. Then create the file lib/tutorial_web/live/product_form_live.ex.

The initial content will be a very basic LiveView component.

defmodule TutorialWeb.ProductFormLive do
  use Phoenix.LiveView

  def mount(%{"action" => action, "csrf_token" => csrf_token}, socket) do
    assigns = [
      conn: socket,
      action: action,
      csrf_token: csrf_token

    {:ok, assign(socket, assigns)}

  def render(assigns) do
    TutorialWeb.ProductView.render("form.html", assigns)

Note that I expect %{"action" => action, "csrf_token" => csrf_token} to be included when I eventially call the LiveView component.

As you can see, I am referencing a template that I have yet to create. I need to rename the form.html.eex to the LiveView compatible form.html.leex.

The LiveView component are included in both when you are creating a product and editing a product. So I need to update the code in both lib/tutorial_web/templates/product/new.html.eex and lib/tutorial_web/templates/product/edit.html.eex

I need to update those file to call the actual LiveView component. Replace:

<%= render "form.html", Map.put(assigns, :action, Routes.product_path(@conn, :create)) %>


<%= live_render @conn, TutorialWeb.ProductFormLive,
  session: %{"action" => Routes.product_path(@conn, :create), "csrf_token" => Plug.CSRFProtection.get_csrf_token()}

When I start the server now, and navigate to http://localhost:4000/products/new I can see the error:

I need to add the changeset to the LiveView component.

In the top add:

  alias Videos.Products
  alias Videos.Products.Product

  def mount(%{a"action" => action, "csrf_token" => csrf_token}, socket) do
    assigns = [
      conn: socket,
      action: action,
      csrf_token: csrf_token,
      changeset: Products.change_product(%Product{})

    {:ok, assign(socket, assigns)}

Now, when refreshing the page the form should be visible. However, if I enter something, I dont get the form validations. For that to work, I need to tell the component that it should validate.

in lib/tutorial_web/templates/product/form.html.leex I need to make 2 changes. First its the phx_change: :validate and csrf_token: @csrf_token.

<%= form_for @changeset, @action, [phx_change: :validate, class: "block", csrf_token: @csrf_token], fn f -> %>
<% end %>

The csrf_token needs to be specified so that Phoenix recognises and allows the form to be submittet.

And back in the LiveView component, I need to add the event handler for the validation:

  def handle_event("validate", %{"product" => product_params}, socket) do
    changeset =
      |> Product.changeset(product_params)
      |> Map.put(:action, :insert)

    {:noreply, assign(socket, changeset: changeset)}

If i go to http://localhost:4000/products/new and test this out by typing a character in the name field I can see that it works. Kind of..

First, one issue is that the alert error is displayed. Second, it displays the error messages for the fields I havent even touched.

To fix the first issue, I will just remove the alert from the file lib/tutorial_web/templates/product/form.html.leex.

Actually, I dont even want anyone to be able to submit the form if invalid, so I will change the submit to:

<%= submit "Save", class: "btn btn-primary mr-2", disabled: !@changeset.valid? %>

and update CSS to:

.btn.disabled, .btn:disabled, input[type="submit"]:disabled {
  opacity: .65;
  @apply pointer-events-none;

To fix the second issue with displaying errors in all fields, I need to open lib/tutorial_web/views/error_helpers.ex and in the function def error_tag(form, field) do and add , data: [phx_error_for: input_id(form, field):

  def error_tag(form, field) do
    Enum.map(Keyword.get_values(form.errors, field), fn error ->
      field_name = field |> Atom.to_string() |> String.capitalize()
      content_tag(:span, "#{field_name} #{translate_error(error)}", class: "block mt-1 text-sm text-red-700", data: [phx_error_for: input_id(form, field)])

Final result and comments

So, this now works when adding a new resource.

However, the code needs to change slightly to make it work with edit a resource so i have a full example on Github

But basically we just need to make sure we have an existing product or a new %Product{}.

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