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

[endymion]

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_format/4, but for our :integer and :float values we simply rely on the base cast/4 for those types without the need for more validation. When given "foo" to input type :float the changeset error message returns "is invalid". Using Gettext and traverse_errors/2 we can fix this error message on the UI, but that ends up in needing to have branching error message logic and makes it more difficult with a dozen types to clearly see where the error message definitions exist.

Instead we have a function custom_cast_error_message(changeset, field, message) that replaces the given field error only for cast/4 validations to be our custom message

  @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

A sample usage of this looks 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)

Posting this because I saw numerous posts about this same issues and didn’t love the solutions found.

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

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 laye…

Collecting and formatting changeset errors?

I’m using changesets to validate various API submissions and I’m wondering if someone has come up with an easy way to format the errors? It’s a lot of busy work to traverse the changeset errors and assemble a meaningful error message, so I thought I’d ask the forum people to see if someone had already solved that particular problem (yes, I’m fishing). Thank you!

[cjbottaro]

My initial thought was to do this in translate_errors using the error metadata, but I like this better since it keeps the error messages all in one place and not spread out…

[LostKobrakai]

Ecto types can already return custom error messages. For cast customizing the error doesn‘t really make sense as you‘d never really use it for individual fields.

[endymion]

That doesn’t work in our case. We have a union type that contains multiple embeds_one which delegates the specific validation logic for each type.

defmodule UnionStruct do
  use Ecto.Schema
  embedded_schema do
    embeds_one :url, UnionStruct.Url
    embeds_one :ssn, UnionStruct.Ssn
    embeds_one :phone_numer, UnionStruct.PhoneNumer
    embeds_one :date, UnionStruct.Date
    embeds_one :float, UnionStruct.Float
    embeds_one :Integer, UnionStruct.Integer

    field :other_data, :any
  end
end

# Example definition for PhoneNumber
defmodule UnionStruct.PhoneNumber do
  use Ecto.Schema

  embedded_schema do
    field :other_data, :any
    field :value, :string
  end

  def changeset(changeset, params, opts) do
    Ecto.Changeset.cast(changeset, params, [:value], opts)
    |> App.Changeset.custom_cast_error_message(:value, "Our custom phone number error message here")
    |> validate_phone_number(:value)
  end
end

For the fields like [:url, :phone_number, :ssn] those are all :string Ecto types. Hence why we can not have a single custom error message for :string when casting a value to :url, as the message we want to present with the input :url or :phone_number is different.

[LostKobrakai]

defmodule PhoneNumberType do
  use Ecto.Type

  def type, do: :string

  def cast(value) do
    case Ecto.Type.cast(:string, value) do
      {:ok, value} -> {:ok, value}
      :error -> {:error, "Our custom phone number error message here"}
    end
  end

  def dump(value), do: Ecto.Type.dump(:string, value)
  
  def load(value), do: Ecto.Type.load(:string, value)
end

defmodule UnionStruct.PhoneNumber do
  use Ecto.Schema

  embedded_schema do
    field :other_data, :any
    field :value, PhoneNumberType
  end

  def changeset(changeset, params, opts) do
    Ecto.Changeset.cast(changeset, params, [:value], opts)
    |> validate_phone_number(:value)
  end
end

You can wrap generic types like this to get more specific types. If you have many similar ones you can even abstract this using Ecto.ParameterizedType.

[endymion]

This was my first thought at addressing this issue, we have several custom Ecto types that such as our own date format that we can do this for. But our total number of types we would need to do this for is around 12. Maintaining those custom 12 types just to set a cast message didn’t feel like a good trade off for the long run.

[LostKobrakai]

@endymion

Maintaining those custom 12 types just to set a cast message didn’t feel like a good trade off for the long run.

defmodule CustomString do
  use Ecto.ParameterizedType

  def init(opts) do
    %{error: opts[:validation_message]}
  end

  def type(_), do: :string

  def cast(value, params) do
    case Ecto.Type.cast(:string, value) do
      {:ok, value} -> {:ok, value}
      :error -> {:error, params.error}
    end
  end

  def dump(value, _, _), do: Ecto.Type.dump(:string, value)
  def load(value, _, _), do: Ecto.Type.load(:string, value)
end

defmodule UnionStruct.PhoneNumber do
  use Ecto.Schema

  embedded_schema do
    field :value, CustomString, validation_message: "abc"
  end

  def changeset(changeset, params, opts) do
    Ecto.Changeset.cast(changeset, params, [:value], opts)
    |> validate_phone_number(:value)
  end
end