Multiselect checkbox input for existing records

Hey there! I think the most straightforward way to go about this is to use the prepare_params option of AshPhoenix.Form. I put together a little example project that you can run, and go to /posts. There you can create a post with two category options each toggled with checkboxes. In your case, the label would be the name of the group, and instead of #{category} it would be something like #{group.id}.

This is a link to the relevant form_component.ex.

https://github.com/zachdaniel/checkbox_example/blob/main/lib/example_web/live/post_live/form_component.ex

Thank you. This works!
However, I am unable to get form validation to work. Whenever I validate the form with:

  @impl true
  def handle_event("validate", %{"invitation" => params}, socket) do
    form = Form.validate(socket.assigns.form, params, errors: true)
    {:noreply, assign(socket, :form, form)}
  end

All of the selected groups get unchecked.

For anyone struggling with this, we solved it by getting the value of groups:

def render(assigns) do
  assigns =
    assigns
    |> assign(:checked_groups, AshPhoenix.Form.value(assigns.form, :groups) || [])

and setting checked attribute depending on whether a group is inside that list:

<%= for group <- @groups do %>
  <.input
    type="checkbox"
    label={group.name}
    name={@form.name <> "[groups][#{group.id}]"}
    id={@form.id <> "_groups_#{group.id}"}
    checked={group.id in @checked_groups}
    />

The groups stay checked after validation.
We just need to make sure we’re assigning the form during validation, as always:

def handle_event("validate", %{"invitation" => params}, socket) do
  form = AshPhoenix.Form.validate(socket.assigns.form, params)
  {:noreply, assign(socket, :form, form)}
end