How do you provide custom error message for Ecto's cast errors?

[Athunnea]

If you provide a string to an :integer field you get
errors: [count: {"is invalid", [type: :integer, validation: :cast]}]
in a changeset, which is not helpful for a user. Unfortunately Ecto.Schema.field/3 doesn’t accept custom error messages (as of v3.9 at least).

I want to return something like “should be a number” to a user instead of “is invalid”.

One solution could be a create a custom integer type, which can return a custom error message. Another way could be processing this in web layer in some kind of humanize_error function, replacing default “is invalid” with “should be a number”.

For now I’ve settled with custom ecto types, but as their number started to grow, I want to re-evaluate my approach. So I wonder how do others solve similar issues? I would appreciate your thoughts.

PS. What I like about custom types, is that I can also use them to clean up values before casting them. I can trim strings, remove double spaces, drop “.0” and “,0” from integers.

[D4no0]

Have you tried using gettext for this? You should have some examples in a generated phoenix project.

[LostKobrakai]

The metadata in the error would allow you to do the transformation you mention to the error in Ecto.Changeset.traverse_errors (which is likely already called somewhere in your codebase). You can ignore the existing error string and match on metadata[:validation] + metadata[:type].

[endymion]

I made a post discussing how we solved this issue.

Cleanly add custom errors for Ecto.Changeset.cast/4

Found a clean solution for adding custom error messages when using Ecto.Changeset.cast/4. The issue presented is cast/4 does not allow :message for an opts unlike other changeset functions like validate_format/4 and other custom validator functions. This presented a problem for us as we have a number of custom types and messages that we use to validate our inputs. This works fine when validating things like a phone number because we can simply pass our custom error message directly to validate…

It boils down to creating a function

  @doc """
  Overwrites the error message for `Ecto.Changeset.cast/4` from `"is invalid"` to the
  custom supplied error message.
  """
  def custom_cast_error_message(changeset, field, message) do
    errors = Enum.map(changeset.errors, fn {error_field, {_msg, opts}} = error ->
      if field == error_field && opts[:validation] == :cast do
        {field, {message, opts}}
      else
        error
      end
    end)

    %{changeset | errors: errors}
  end

and calling it like

message = "This is our custom error message for cast/4"

Ecto.Changeset.cast(answer, params, [:value])
|> Aw.Changeset.custom_cast_error_message(:value, message)