Phoenix.Component.to_form/1 seems to be confused by boolean field defaults

Phoenix Forum Questions / Help
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1

I have a schema with a boolean field with a default value of true. I’m using this in a LiveView, wrapped in a changeset and converted to a Phoenix.HTML.Form with to_form/1. As well as rendering an input for the field, there’s another part of the form that displays conditionally depending on whether the boolean field is set (<div :if={not @form[my_field].value ...>}. This works on page load, and when the field is toggled to false, but when I toggle it back to true the view crashes with an argument error, because it turns out the value is now set to the string "true" instead of the boolean true.

I can work around this with :if={@form[:my_field].value in [false, "false"], but I wondered whether this is expected behaviour or a bug.

I’ve reduced it to a simple demo in a LiveBook with just a struct and changeset, which you can find in this gist. Here are the key bits:

defmodule Person do
  alias Ecto.Changeset

  defstruct name: "", member: true

  def changeset(person, changes \\ %{}) do
    Changeset.cast(
      {person,
       %{name: :string, member: :boolean}},
      changes,
      [:name, :member]
    )
  end
end

The changeset itself handles the casting correctly:

changeset = %Person{} |> Person.changeset()
IO.inspect(Changeset.fetch_field!(changeset, :member), label: "Default")

changeset = %Person{} |> Person.changeset(%{"member" => "false"})
IO.inspect(Changeset.fetch_field!(changeset, :member), label: "From default to false")

changeset = %Person{} |> Person.changeset(%{"member" => "true"})
IO.inspect(Changeset.fetch_field!(changeset, :member), label: "From default to true")

changeset = %Person{member: true} |> Person.changeset(%{"member" => "false"})
IO.inspect(Changeset.fetch_field!(changeset, :member), label: "From true to false")

changeset = %Person{member: false} |> Person.changeset(%{"member" => "true"})
IO.inspect(Changeset.fetch_field!(changeset, :member), label: "From false to true")

Default: true
From default to false: false
From default to true: true
From true to false: false
From false to true: true

But when I turn it into a form, it treats the toggle back to true as a change to the string "true":

form = %Person{} |> Person.changeset() |> to_form()
IO.inspect(form[:member].value, label: "Default")

form = %Person{} |> Person.changeset(%{"member" => "false"}) |> to_form()
IO.inspect(form[:member].value, label: "From default to false")

form = %Person{} |> Person.changeset(%{"member" => "true"}) |> to_form()
IO.inspect(form[:member].value, label: "From default to true")

form = %Person{member: true} |> Person.changeset(%{"member" => "false"}) |> to_form()
IO.inspect(form[:member].value, label: "From true to false")

form = %Person{member: false} |> Person.changeset(%{"member" => "true"}) |> to_form()
IO.inspect(form[:member].value, label: "From false to true")

Default: true
From default to false: false
From default to true: "true"
From true to false: false
From false to true: true

If it turns out to be a bug I’ll raise an issue, but it’s much more likely I’m just doing something wrong!

2

This is expected behaviour. You want to be able to handle the string format, because the fallback in case a value wasn’t able to be cast (when using changeset) is to use the string value as supplied by the submit. E.g. consider a date input, where the user was in the middle of a date. You don’t want to clear out their input value just because it’s not yet a valid date.

3

Thanks for the clarification @LostKobrakai