Strukt provides an extended defstruct macro which builds on top of Ecto.Schema and Ecto.Changeset to remove the boilerplate of defining type specifications, implementing validations, generating changesets from parameters, JSON serialization, and support for autogenerated fields.

This builds on top of Ecto embedded schemas, so the same familiar syntax you use today to define schema'd types in Ecto, can now be used to define structs for general purpose usage.

The functionality provided by the defstruct macro in this module is strictly a superset of the functionality provided both by Kernel.defstruct/1, as well as Ecto.Schema. If you import it in a scope where you use Kernel.defstruct/1 already, it will not interfere. Likewise, the support for defining validation rules inline with usage of field/3, embeds_one/3, etc., is strictly additive, and those additions are stripped from the AST before field/3 and friends ever see it.

def deps do
  [
    {:strukt, "~> 0.3"}
  ]
end

The following is an example of using defstruct/1 to define a struct with types, autogenerated primary key, and inline validation rules.

defmodule Person do
  use Strukt

  @derives [Jason.Encoder]
  @primary_key {:uuid, Ecto.UUID, autogenerate: true}
  @timestamps_opts [autogenerate: {NaiveDateTime, :utc_now, []}]

  defstruct do
    field :name, :string, required: true
    field :email, :string, format: ~r/^.+@.+$/

    timestamps()
  end
end

And an example of how you would create and use this struct:

# Creating from params, with autogeneration of fields
iex> {:ok, person} = Person.new(name: "Paul", email: "[email protected]")
...> person
%Person{
  uuid: "d420aa8a-9294-4977-8b00-bacf3789c702",
  name: "Paul",
  email: "[email protected]",
  inserted_at: ~N[2021-06-08 22:21:23.490554],
  updated_at: ~N[2021-06-08 22:21:23.490554]
}

# Validation (Create)
iex> {:error, %Ecto.Changeset{valid?: false, errors: errors}} = Person.new(email: "[email protected]")
...> errors
[name: {"can't be blank", [validation: :required]}]

# Validation (Update)
iex> {:ok, person} = Person.new(name: "Paul", email: "[email protected]")
...> {:error, %Ecto.Changeset{valid?: false, errors: errors}} = Person.change(person, email: "foo")
...> errors
[email: {"has invalid format", [validation: :format]}]

# JSON Serialization/Deserialization
...> person == person |> Jason.encode!() |> Person.from_json()
true

There are a few different ways to express and customize validation rules for a struct.

• Inline (as shown above, these consist of the common validations provided by Ecto.Changeset)
• Validators (with module and function variants, as shown below)
• Custom (by overriding the validate/1 callback)

The first two are the preferred method of expressing and controlling validation of a struct, but if for some reason you prefer a more manual approach, overriding the validate/1 callback is an option available to you and allows you to completely control validation of the struct.

NOTE: Be aware that if you override validate/1 without calling super/1 at some point in your implementation, none of the inline or module/function validators will be run. It is expected that if you are overriding the implementation, you are either intentionally disabling that functionality, or are intending to delegate to it only in certain circumstances.

This is the primary method of implementing reusable validation rules:

There are two callbacks, init/1 and validate/2. You can choose to omit the implementation of init/1 and a default implementation will be provided for you. The default implementation returns whatever it is given as input. Whatever is returned by init/1 is given as the second argument to validate/2. The validate/2 callback is required.

defmodule MyValidator.ValidPhoneNumber do
  use Strukt.Validator

  @pattern ~r/^(\+1 )[0-9]{3}-[0-9]{3}-[0-9]{4}$/

  @impl true
  def init(opts), do: Enum.into(opts, %{})

  @impl true
  def validate(changeset, %{fields: fields}) do
    Enum.reduce(fields, changeset, fn field, cs ->
      case fetch_change(cs, field) do
        :error ->
          cs

        {:ok, value} when value in [nil, ""] ->
          add_error(cs, field, "phone number cannot be empty")

        {:ok, value} when is_binary(value) ->
          if value =~ @pattern do
            cs
          else
            add_error(cs, field, "invalid phone number")
          end

        {:ok, _} ->
          add_error(cs, field, "expected phone number to be a string")
      end
    end)
  end
end

These are useful for ad-hoc validators that are specific to a single struct and aren't likely to be useful in other contexts. The function is expected to received two arguments, the first is the changeset to be validated, the second any options passed to the validation/2 macro:

defmodule File do
  use Strukt

  @allowed_content_types []

  defstruct do
    field :filename, :string, required: true
    field :content_type, :string
    field :content, :binary
  end

  validation :validate_filename_matches_content_type, @allowed_content_types

  defp validate_filename_matches_content_type(changeset, allowed) do
    # ...
  end
end

As with module validators, the function should always return an Ecto.Changeset.

You may express validation rules that apply only conditionally using guard clauses. For example, extending the example above, we could validate that the filename and content type match only when either of those fields are changed:

  # With options
  validation :validate_filename_matches_content_type, @allowed_content_types
    when is_map_key(changeset.changes, :filename) or is_map_key(changeset.changes, :content_type)

  # Without options
  validation :validate_filename_matches_content_type
    when is_map_key(changeset.changes, :filename) or is_map_key(changeset.changes, :content_type)

By default validation rules have an implicit guard of when true if one is not explicitly provided.

Using the :source option allows you to express that a given field may be provided as a parameter using a different naming scheme than is used in idiomatic Elixir code (i.e. snake case):

defmodule Person do
  use Strukt

  @derives [Jason.Encoder]
  @primary_key {:uuid, Ecto.UUID, autogenerate: true}
  @timestamps_opts [autogenerate: {NaiveDateTime, :utc_now, []}]

  defstruct do
    field :name, :string, required: true, source: :NAME
    field :email, :string, format: ~r/^.+@.+$/

    timestamps()
  end
end

# in iex
iex> {:ok, person} = Person.new(%{NAME: "Ivan", email: "[email protected]"})
...> person
%Person{
  uuid: "f8736f15-bfdc-49bd-ac78-9da514208464",
  name: "Ivan",
  email: "[email protected]",
  inserted_at: ~N[2021-06-08 22:21:23.490554],
  updated_at: ~N[2021-06-08 22:21:23.490554]
}

NOTE: This does not affect serialization/deserialization via Jason.Encoder when derived.

For more, see the usage docs