May 08, 2015

Testing Ecto Validations

I recently was playing around with Phoenix and Ecto, Elixir’s database library, and I wanted to test my validations. In the process, I wrote a little library along the lines of Shoulda from Ruby. However, when José Valim saw it, he suggested a much better approach which I think illustrates what makes Elixir great.

Validating Ecto Models

In Ecto, a model looks like this:

defmodule MyApp.User do
  use Ecto.Model

  schema "users" do
    field :username, :string
    field :email, :string
    field :encrypted_password, :string

If you want to validate that a new user has a username and email address before inserting it into the database, you can define a changeset/2 function:

@required_params ~w(username, email)
@optional_params ~w()

def changeset(model, params) do
  |> cast(params, @required_params, @optional_params)

cast/4 will return a Changeset struct which has valid? attribute, which can be true or false. It can perform basic presence validation, using a list of required and optional fields. Everything not in those fields will be ignored.

If you want to perform more advanced validation, Ecto has helpers for that as well, such as validate_unique:

def changeset(model, params) do
  |> cast(params, @required_params, @optional_params)
  |> validate_unique(:email)
  |> validate_format(:email, ~r/@/)

Since all the validators take a changeset as their first argument, they can be made into pipelines quite easily, like the above.

With a changeset/2 function in place, you can then design your controller action like so:

def create(conn, params) do
  user = MyApp.User.changeset(%MyApp.User{}, params)

  if user.valid?
    # return success
    # render error

There are several reasons this is great.

  1. There are no global validations gumming things up in places they shouldn’t. If you do Rails long enough, you’ll have a time when its global validations cause you pain.
  2. Testing models is easy, because you don’t have to insert valid models in every test. Changesets are an opt-in behavior.
  3. If you need different validations depending on context, just define multiple changesets, like this:
# :create is an action name, or whatever else you want
def changeset(:create, model, params) do
  |> cast(params, ["username"], [])

def changeset(:update, model, params) do
  |> cast(params, ["email"], [])

Testing Validations

Testing an Ecto validation is a simple process:

# Generate a changeset for a given field/value pair
changeset = MyApp.User.changeset(%MyApp.User{}, %{username: nil})

# Assert that an error is present
assert {:username, "can't be blank"} in changeset.errors

# Or, assert that the validation passed
refute {:username, "can't be blank"} in changeset.errors

As José suggested, this can be cleaned up more by adding a simple private function to your test case module:

defp errors_on(model \\ %MyApp.User{}, params) do
  model.__struct__.changeset(model, params).errors

You can then write the above like this:

assert {:username, "can't be blank"} in errors_on(%{username: nil})
refute {:email, "can't be blank"} in errors_on(%{email: nil})

The code is explicit, and there is no magic. What’s more, there’s no need for a gem like shoulda to clean things up here, because it’s simple enough!

This errors_on/2 function can easily be extracted to a module and reused, just like any other Elixir code. New developers looking at your code will easily be able to figure out what is going on, unlike shoulda, where they must accept what it’s doing on faith.

A Simpler Way to Code

I love how Elixir encourages this simpler way of coding. There are more moving parts than there are in Ruby land, perhaps, but they’re conceptually simpler. Everything is made up of state and functions when you get down to it. It’s certainly possible to do complex things, but more often than not, you don’t have to.

It all comes down to the difference between explicitness and implicitness. Elixir code is meant to be mostly explicit, whereas Ruby code often is implicit. Explicit code doesn’t hide what it’s doing. Implicit code hides its implementation and thus has a steeper learning curve should you need to dig into its innards.

If you write code explicitly, it may be a little longer than the implicit version, but it will be easier to change, making for a more maintainable system.

I’m still a recovering Ruby developer, (where we DSL all the things!) but learning this explicit style has been an enlightening experience so far, and one I’d recommend to any Rubyist.


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