As Elixir developers, we typically use Ecto.Changeset
to validate proposed database records changes in Elixir. It has comprehensive field validation capabilities and standardized error reporting, both of which we can customize.
In this post, I will share how you can use Ecto.Changeset
beyond the database context, and use it as an API validation mechanism. Combine it with Gettext
, and you get easy error translation for your APIs.
Regular Ecto
If you’ve worked with Elixir, then you’ve probably seen Ecto schemas and changesets like this:
@primary_key {:user_id, :binary_id, autogenerate: true}
schema "users" do
field :username, :string, unique: true
field :password, :string, virtual: true, redact: true
field :password_hash, :string
field :first_name, :string
field :last_name, :string
field :is_banned, :boolean, default: false
field :is_deleted, :boolean, default: false
embeds_one :photo, Photo
timestamps()
end
def changeset(user, attrs) do
user
|> cast(attrs, [:username, :first_name, :last_name, :password])
|> validate_required([:username, :first_name, :password])
|> shared_validations()
end
This is a regular Ecto.Schema
for a users
table, with a changeset/2
function that receives the User
struct and the proposed database record changes as attrs
.
Passing valid data to changeset/2
When passing valid data to the changeset function changeset/2
, it will return the tuple {:ok, %Ecto.Changeset{valid?: true, changes: changes, ...}}
. Where changes
is a map
of validated database record changes, like:
%{first_name: "Jane", last_name: "Doe", username: "janedoe"}
Passing invalid data to changeset/2
When passing invalid data to the changeset/2
function, it will return the tuple {:error, %Ecto.Changeset{valid?: false, errors: errors, ...}}
. Where errors
is a keyword list
, like:
errors: [
first_name: {"should be at least %{count} character(s)",
[count: 3, validation: :length, kind: :min, type: :string]},
last_name: {"should be at most %{count} character(s)",
[count: 20, validation: :length, kind: :max, type: :string]},
username: {"can't be blank", [validation: :required]}
]
Which is a bit unreadable, but with the help of Ecto.Changeset.traverse_errors(changeset, &translate_error/1)
we can translate these errors in a human-readable map
, like:
%{
"first_name" => ["should be at least 3 character(s)"],
"last_name" => ["should be at most 20 character(s)"],
"username" => ["can't be blank"]
}
We’ll be using the changes
and errors
fields later in this post, so let’s keep them in mind. 😉
When creating a Phoenix project,
translate_error/1
is provided by default. If you’ve chosen to installGettext
(installed by default for a Phoenix app), you get easy language translation of these errors for free.
API validation
Using Ecto.Changeset
for database record changes is nice, but let’s take a look at API validation. I’ll use a JSON-based API in the following examples, because that’s what I’m most familiar with.
Basic controller
Below is an example of a basic Phoenix controller, which creates a User
and an EmailAddress
, and verifies and updates an existing EmailConfirmation
. Each action has a function call. These three function calls happen in succession, and each must succeed before another can be called.
That means we will want to validate the request parameters before we start any of the function calls; because if one succeeds, a later one may fail when it receives invalid data.
# API.AccountController
action_fallback(API.FallbackController)
def create(conn, %{"account" => params}) do
with {:ok, email_confirmation} <- verify_email_confirmation(params),
{:ok, email_address} <- create_email_address(params),
{:ok, user} <- create_user(params) do
conn
|> put_status(:created)
|> render("create.json", user: user, email_address: email_address)
end
end
def create(_conn, _params), do: {:error, :bad_request}
Let’s see how we can validate the input parameters, before we call any of the functions.
Basic validation
We could add a pattern match in the controller arguments, and add some manual validation in private functions in the same controller file.
With the pattern match def create(conn, %{"email" => _, "code" => _, "username” => _, "first_name" => _ "last_name" => _} = params) do
we can guarantee all the fields that we need are present. There’s no validation of the values yet.
Because we lack an API validation library, we need to manually confirm the validity of each field in the controller:
-
email
must be a valid email address, e.g. “john@example.com” -
code
must be a valid confirmation code, e.g. “123456” -
username
must be a valid string without spaces and special characters, e.g. “johndoe” -
first_name
andlast_name
must be strings with a minimum length of 3 characters and a maximum length of 20 characters.
We could do this with a private function:
# API.AccountController
defp validate_params(%{"email" => email, "first_name" => first_name, ...}) do
with true <- is_email(email),
true <- String.length(first_name) >= 3,
true <- String.length(last_name) <= 20,
... do
:ok
else
false -> :error
end
end
But that gets dirty, fast. ❌
Using Ecto
Instead of doing validations manually, let’s create an Ecto.Schema
and an accompanying changeset called AccountController.CreateAction
that contains our validations, in the controller namespace:
# API.AccountController.CreateAction
embedded_schema do
field(:email, :string)
field(:code, :string)
field(:username, :string)
field(:first_name, :string)
field(:last_name, :string)
end
def changeset(attrs) do
%CreateAction{}
|> cast(attrs, [:email, :code, :first_name, :last_name])
|> validate_required([:email, :code, :first_name, :last_name])
|> validate_length(:code, is: 6)
|> validate_length(:username, min: 3, max: 10)
|> validate_length(:first_name, min: 3, max: 20)
|> validate_length(:last_name, min: 1, max: 20)
|> validate_format(:email_address, @email_regex)
|> validate_format(:username, @username_regex)
|> validate_format(:first_name, @name_regex)
|> validate_format(:last_name, @name_regex)
|> update_change(:email, &String.downcase/1)
|> update_change(:username, &String.downcase/1)
end
def validate_params(params) do
case changeset(params) do
%Ecto.Changeset{valid?: false} = changeset ->
{:error, changeset}
%Ecto.Changeset{valid?: true, changes: changes} ->
{:ok, changes}
end
end
I’m going a bit overboard with the validations, to show the extent to which you can validate API fields and values. It can be very fine-grained. 👌
In my own projects, I tend to abstract these validations into shared functions. For example, the first_name
and last_name
validations happen in both the CreateAction
and in User
schemas, so they share a separate validation function in User
.
For example:
def validate_name(changeset, field) do
changeset
|> validate_format(field, @name_regex)
|> validate_length(field, min: 3, max: 20)
end
Very nice. ✅
Implementation
OK. Let’s implement the validate_params/1
function of API.AccountController.CreateAction
in the controller:
# API.AccountController
def create(conn, params) do
with {:ok, attrs} <- CreateAction.validate_params(params),
{:ok, email_confirmation} <- verify_email_confirmation(attrs),
{:ok, email_address} <- create_email_address(attrs),
{:ok, user} <- create_user(attrs) do
conn
|> put_status(:created)
|> render("create.json", user: user, email_address: email_address)
end
end
Much cleaner. So what happens here?
We call CreateAction.validate_params/1
before any other function gets involved. validate_params/1
receives the request parameters as a map
, and validates them using changeset/1
, returning either {:ok, attrs}
or {:error, changeset}
.
If the request parameters are valid, then the Ecto.Changeset
struct
contains the valid?: true
and changes: changes
fields. changes
is the map
of validated request parameters that we want to pass to our subsequent function calls as {:ok, attrs}
.
If the request parameters are invalid, then the Ecto.Changeset
struct
contains the valid?: false
field, and we pass the Ecto.Changeset
back to our controller function as {:error, changeset}
, where it gets picked up by the FallbackController
.
Error messages
So when the API request body contains invalid parameters, we receive {:error, %Ecto.Changeset{}}
. To process this error, we need a FallbackController
. Luckily, this is provided by default in a Phoenix project.
If you’re missing
FallbackController
, then you can run one of themix phx.gen
tasks from https://hexdocs.pm/phoenix/Mix.Tasks.Phx.Gen.html and it will be generated for you.
Catching errors with FallbackController
The default FallbackController
contains a fallback function like this:
# API.FallbackController
def call(conn, {:error, %Ecto.Changeset{} = changeset}) do
conn
|> put_status(:unprocessable_entity)
|> put_view(ChangesetView)
|> render("error.json", changeset: changeset)
end
Whenever a function inside a controller returns {:error, %Ecto.Changeset{}}
, it is caught by this fallback function inside FallbackController
. The function then renders the changeset
errors as a message, and returns the connection with a 422
HTTP status code (:unprocessable_entity
).
For example, it returns error messages like this:
%{
"errors" => %{
"code" => ["should be 6 character(s)"],
"email_address" => ["has invalid format"]
}
}
You can customize the error parsing with Ecto.Changeset.traverse_errors/2
, but the default provided by Phoenix is a nice format for a frontend system to handle.
Translating error messages
If you have Gettext
installed (which is installed by default in a Phoenix project), then you can add custom error translations for any language you need.
Since the error messages returned from Ecto.Changeset
are always in a simple and specific format, like "is in valid"
"can't be blank"
and "should be at least 8 character(s)"
, we can easily add error translations for our API.
I won’t dive into the details of Gettext
, but the previous example of a rendered error could easily be translated into this, in Spanish 🇪🇸:
%{
"errors" => %{
"code" => ["debe tener 6 caracter(es)"],
"email_address" => ["tiene un formato inválido"]
}
}
Not sure if this translates correctly, because I don't speak much Spanish. But it's a nice feature to have, right? 🤷
I hope you learned something today, and that this post will help you build better APIs in Elixir. The next post will be about Dialyzer, and why you should (always) use it for development. Godspeed, alchemists! ⚗️
More information
- Read about
Ecto.Schema
andembedded_schema
here: https://hexdocs.pm/ecto/Ecto.Schema.html - Read about
Ecto.Changeset
and error messages from changeset validations here: https://hexdocs.pm/ecto/Ecto.Changeset.html - Read about
FallbackController
here: https://hexdocs.pm/phoenix/Phoenix.Controller.html#action_fallback/1 - Read about
Plug
HTTP status codes here: https://hexdocs.pm/plug/Plug.Conn.Status.html - Read about Phoenix generators here: https://hexdocs.pm/phoenix/Mix.Tasks.Phx.Gen.html
- Read about
Gettext
here: https://hexdocs.pm/gettext/Gettext.html
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