Formalism

Ruby gem for forms with validations and nesting.

Why

I need for service-like objects.

I've explored these projects:

• Reform
• Mutations
• Interactor
• dry-rb

But nothing of them supports all features I need for:

• nesting (into unlimited levels) of themselves;
• simple syntax;
• custom validations and coercions;
• unified output.

So, I've tried to combine these all into one library and got Formalism.

Why here are forms and what about service objects?

I've discovered that form object, only with validations, are useless without service objects. So, I've combined them: service objects include validations.

If these are service objects, why they called forms?

Because if we're combining them — it's more like forms with logic inside for me than service objects built-in forms. Even in HTML we're writing <form>. So, Formalism can accept all data from any-difficult <form> and process it, also with nested forms (for example, if you have some request form with contact data and want to pass contacts into something like user form).

And if I need for simple service object without validation?

You can use Formalism::Action, a parent of Formalism::Form.

Installation

Add this line to your application's Gemfile:

gem 'formalism'

And then execute:

bundle install

Or install it yourself as:

gem install formalism

Usage

Basic example

class FindArtistForm < Formalism::Form
  field :name

  private

  def validate
    if name.to_s.empty?
      errors.add 'Name is not provided'
    end
  end

  def execute
    Artist.first(fields_and_nested_forms)
  end
end

class CreateAlbumForm < Formalism::Form
  field :name, String
  fiels :tags, Array, of: String
  nested :artist, FindArtistForm

  private

  def validate
    if name.to_s.empty?
      errors.add 'Name is not provided'
    end
  end

  def execute
    Album.create(fields_and_nested_forms)
  end
end

form = CreateAlbumForm.new(
  name: 'Hits', tags: %w[Indie Rock Hits], artist: { name: 'Alex' }
)
form.run

Running

Usually you need to initialize a form and execute #run method. Internally, it runs #valid? (public) and #execute (private) methods. #valid? runs #validate (private) of a form itself and nested forms. #run can be redefined for database transaction, for example.

Also you can call .run with arguments for #initialize, it's the alias for #initialize + #run.

Form outcome

Any call of run returns Form::Outcome instance which has #success?, #result and #errors methods. Result is a result of #execute method. Be careful: calling #result for failed outcome will raise ValidationError.

Field type

Field receives type as the second argument. It's not required. It can be a constant, String or Symbol. If specified — there is a coercion to specified type, if not — data remains unchanged.

Nested forms — their class, as constant. Type or :initialize block is required.

Formalism also supports Array type with the optional :of option (type of elements). Coercion will be applied to a data itself and to its elements.

Coercion

There is built-in coercion into some types, if you try to coerce to undefined type — you'll get Formalism::Form::NoCoercionError.

You can define a coercion to some type via definition of such class:

# frozen_string_literal: true

module Formalism
  class Form < Action
    class Coercion
      ## Class for coercion to String
      class String < Base
        private

        def execute
          @value&.to_s
        end
      end
    end
  end
end

Default value

field and nested accepts :default option. It can be any value, if it's an instance of Proc — it'll be executed in the form instance scope.

Different keys

field supports :key option (Symbol) to receive data by a different key, not as a field name.

Custom initialization of nested forms

By default, nested forms initialized with data by key as their name in parent data. So, if a parent receive { foo: 1, bar: { baz: 2 } }, it's nested form :bar will receive { baz: 2 }.

If you want to prevent initialization at all, or pass custom arguments — you should use :initialize option which accepts a proc with a form class argument.

If you want to just refine incoming data (add or remove) — you should define #params_for_nested_* private method, where * is a nested form name. You can use super inside.

Order of filling with data

Fields and nested forms are filling in order of their definition. But sometimes you want to change this order, for example, if you have a nested forms in ancestors which depends on data in children forms. For such cases you can use :depends_on option, which accepts fields and nested forms names as Symbol or Array of symbols. They will be filled (and initialized) before dependent.

Merging into final data

There is Form#fields_and_nested_forms as final data (after coercion, defaults, etc). But you may want to not include some fields or nested forms into this data. You can do it via :merge option, which can be true, false or Proc (executed in form's instance scope).

For example:

field :bar, merge: true
nested :only_valid, nested_form_class, merge: ->(form) { form.valid? }

Runnable

You can disable #valid? and #run of forms (including nested ones) by setting form.runnable = false. It can be helpful for some cases, for example, with policies (permissions):

def initialize_nested_form(name, options)
  return unless (form = super)

  form.runnable = allowed_to_change?(name)
  form
end

Inheritance

Any class ChildForm < ParentForm will have all fields and nested forms from ParentForm.

Removing (inherited) field

But you're able to remove (usually inherited) fields by:

class ChildForm < ParentForm
  remove_field :field_from_parent
end

Modules

You can define modules and use them later like this:

module CommonFields
  include Formalism::Form::Fields

  field :base_field
  nested :base_nested
end

class SomeForm < Formalism::Form
  include CommonFields

  field :another_field
end

Convert to params

You can convert a Form back to (processed) params, for example, for view render:

form = CreateAlbumForm.new(
  name: 'Hits', tags: %w[Indie Rock Hits], artist: { name: 'Alex' }
)

form.to_params
# {
#   name: 'Hits',
#   tags: %w[Indie Rock Hits],
#   artist: { name: 'Alex' }
# }

Actions

For actions without fields, nesting and validation you can use Formalism::Action (the parent of Formalism::Form).

Plugins

There is a few plugins which I personally need for:

• formalism-model_forms Default CRUD forms for Sequel DB models. Can be renamed!

• formalism-sequel_transactions Sequel transactions inside forms.

• formalism-r18n_errors R18n errors inside forms, including validation helpers. Can be separated!

Development

After checking out the repo, run bundle install to install dependencies.

Then, run toys rspec to run the tests.

To install this gem onto your local machine, run toys gem install.

To release a new version, run toys gem release %version%. See how it works here.

Contributing

Bug reports and pull requests are welcome on GitHub.

License

The gem is available as open source under the terms of the MIT License.