As of 3.0, Ash uses Splode
to as our basis for errors. The documentation below still applies, but it is powered by Splode
under the hood.
There is a difficult balance to cut between informative errors and enabling simple reactions to those errors. Since many extensions may need to work with and/or adapt their behavior based on errors coming from Ash, we need rich error messages. However, when you have a hundred different exceptions to represent the various kinds of errors a system can produce, it becomes difficult to say something like "try this code, and if it is invalid, do x, if it is forbidden, do y. To this effect, exceptions in Ash have one of four classes mapping to the top level exceptions.
- forbidden -
Ash.Error.Forbidden
- invalid -
Ash.Error.Invalid
- framework -
Ash.Error.Framework
- unknown -
Ash.Error.Unknown
Since many actions can be happening at once, we want to support the presence of multiple errors as a result of a request to Ash. We do this by grouping up the errors into one before returning or raising.
We choose an exception based on the order of the exceptions listed above. If there is a single forbidden, we choose Ash.Error.Forbidden
, if there is a single invalid, we choose Ash.Error.Invalid
and so on. The actual errors will be included in the errors
key on the exception. The exception's message will contain a bulleted list of all the underlying exceptions that occurred. This makes it easy to react to specific kinds of errors, as well as to react to any/all of the errors present.
An example of a single error being raised, representing multiple underlying errors:
AshExample.Representative
|> Ash.Changeset.for_create(:create, %{employee_id: "the best"})
|> Ash.create!()
** (Ash.Error.Invalid) Invalid Error
* employee_id: must be absent.
* first_name, last_name: at least 1 must be present.
This allows easy rescuing of the major error classes, as well as inspection of the underlying cases
try do
AshExample.Representative
|> Ash.Changeset.for_create(:create, %{employee_id: "dabes"})
|> Ash.create!()
rescue
e in Ash.Error.Invalid ->
"Encountered #{Enum.count(e.errors)} errors"
end
"Encountered 2 errors"
This pattern does add some additional overhead when you want to rescue specific kinds of errors. For example, you may need to do something like this:
try do
AshExample.Representative
|> Ash.Changeset.for_create(:create, %{employee_id: "dabes"})
|> Ash.create!()
rescue
e in Ash.Error.Invalid ->
case Enum.find(e.errors, &(&1.__struct__ == A.Specific.Error)) do
nil ->
...handle errors
error ->
...handle specific error you found
end
end
When returning errors in your application, you can a few different things:
A shortcut for creating errors is to return a keyword list containing field
and message
. This works in changes and validations. For example:
# in a change, you use `Ash.Changeset.add_error/2`
def change(changeset, _, _) do
if under_21?(changeset) do
Ash.Changeset.add_error(changeset, field: :age, message: "must be 21 or older")
else
changeset
end
end
# in a validation, you return the error in an `{:error, error}` tuple.
def change(changeset, _, _) do
if under_21?(changeset) do
{:error, field: :age, message: "must be 21 or older"}
else
:ok
end
end
These are all modules under Ash.Error.*
. You can create a new one with error.exception(options)
, and the options are documented in each exception. This documentation is missing in some cases. Go to the source code of the exception to see its special options. All of them support the vars
option, which are values to be interpolated into the message, useful for things like translation.
For example:
def change(changeset, _, _) do
if some_condition(changeset) do
error = Ash.Error.Changes.Required.new(
field: :foo,
type: :attribute,
resource: changeset.resource
)
Ash.Changeset.add_error(changeset, error)
else
changeset
end
end
You can create a custom exception like so. This is an example of a builtin exception that you could mirror to build your own
defmodule Ash.Error.Action.InvalidArgument do
@moduledoc "Used when an invalid value is provided for an action argument"
use Splode.Error, fields: [:field, :message, :value], class: :invalid
def message(error) do
"""
Invalid value provided#{for_field(error)}#{do_message(error)}
#{inspect(error.value)}
"""
end
end