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Extends defstruct with schemas, changeset validation, and more

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# Strukt

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.

## Installation

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

## Example

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

``` elixir
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:

``` elixir
# Creating from params, with autogeneration of fields
iex> {:ok, person} = Person.new(name: "Paul", email: "bitwalker@example.com")
...> person
%Person{
  uuid: "d420aa8a-9294-4977-8b00-bacf3789c702",
  name: "Paul",
  email: "bitwalker@example.com",
  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: "bitwalker@example.com")
...> errors
[name: {"can't be blank", [validation: :required]}]

# Validation (Update)
iex> {:ok, person} = Person.new(name: "Paul", email: "bitwalker@example.com")
...> {: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
```

## Validation

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.

### Module Validators

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.

```elixir
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
```

### Function Validators

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:

```elixir
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`.

### Conditional Rules

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:

```elixir
  # 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.

## Custom Fields

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):

``` elixir
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: "ivan@example.com"})
...> person
%Person{
  uuid: "f8736f15-bfdc-49bd-ac78-9da514208464",
  name: "Ivan",
  email: "ivan@example.com",
  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](https://hexdocs.pm/strukt/usage.html)

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