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https://github.com/tram-rb/tram-policy

Policy Object Pattern
https://github.com/tram-rb/tram-policy

errors patterns policy validation validator validators

Last synced: 4 months ago
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Policy Object Pattern

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README 

          
# Tram::Policy



Policy Object Pattern





Sponsored by Evil Martians



[![Gem Version][gem-badger]][gem]

[![Inline docs][inch-badger]][inch]



## Intro



Policy objects are responsible for context-related validation of objects, or mixes of objects. Here **context-related** means a validation doesn't check whether an object is valid by itself, but whether it is valid for some purpose (context). For example, we could ask if some article is ready (valid) to be published, etc.



There are several well-known interfaces exist for validation like [ActiveModel::Validations][active-model-validation], or its [ActiveRecord][active-record-validation] extension for Rails, or PORO [Dry::Validation][dry-validation]. All of them focus on providing rich DSL-s for **validation rules**.



**Tram::Policy** follows another approach -- it uses simple Ruby methods for validation, but focuses on building both *customizable* and *composable* results of validation, namely their errors.



- By **customizable** we mean adding any number of *tags* to errors -- to allow filtering and sorting validation results.

- By **composable** we mean a possibility to merge errors provided by one policy into another, and build nested sets of well-focused policies.



Keeping this reasons in mind, let's go to some examples.



## Synopsis



The gem uses [Dry::Initializer][dry-initializer] interface for defining params and options for policy object instanses:



```ruby

require "tram-policy"



class Article::ReadinessPolicy < Tram::Policy

  # required param for article to validate

  param  :article



  # memoized attributes of the article (you can set them explicitly in specs)

  option :title,    proc(&:to_s), default: -> { article.title }

  option :subtitle, proc(&:to_s), default: -> { article.subtitle }

  option :text,     proc(&:to_s), default: -> { article.text }



  # define what methods and in what order we should use to validate an article

  validate :title_presence

  validate :subtitle_presence

  validate do # use anonymous lambda

    return unless text.empty?

    errors.add :empty, field: "text", level: "error"

  end



  private



  def title_presence

    return unless title.empty?

    # Adds an error with a unique key and a set of additional tags

    # You can use any tags, not only an attribute/field like in ActiveModel

    errors.add :blank_title, field: "title", level: "error"

  end



  def subtitle_presence

    return unless subtitle.empty?

    # Notice that we can set another level

    errors.add :blank_subtitle, field: "subtitle", level: "warning"

  end

end

```



Because validation is the only responsibility of a policy, we don't need to call it explicitly.



Policy initializer will perform all the checks immediately, memoizing the results into `errors` array. The methods `#valid?`, `#invalid?` and `#validate!` just check those `#errors`.



You should treat an instance immutable.



```ruby

article = Article.new title: "A wonderful article", subtitle: "", text: ""

policy  = Article::ReadinessPolicy[article] # syntax sugar for constructor `new`



# Simple checks

policy.errors.any? # => true

policy.valid?      # => false

policy.invalid?    # => true

policy.validate!   # raises Tram::Policy::ValidationError



# And errors

policy.errors.count # => 2 (no subtitle, no text)

policy.errors.filter { |error| error.tags[:level] == "error" }.count # => 1

policy.errors.filter { |error| error.level == "error" }.count # => 1

```



## Validation Results



Let look at those errors closer. We define 3 representation of errors:



- error objects (`policy.errors`)

- error items (`policy.items`, `policy.errors.items`, `policy.errors.map(&:item)`)

- error messages (`policy.messages`, `policy.errors.messages`, `policy.errors.map(&:message)`)



Errors by themselves are used for composition (see the next chapter), while `items` and `messages` represent errors for translation.



The difference is the following.



- The `messages` are translated immediately using the current locale.



- The `items` postpone translation for later (for example, you can store them in a database and translate them to the locale of UI by demand).



### Items



Error items contain arrays that could be send to I18n.t for translation. We add the default scope from the name of policy, preceeded by the `["tram-policy"]` root namespace.



```ruby

policy.items # or policy.errors.items, or policy.errors.map(&:item)

# => [

#      [

#        :blank_title,

#        {

#          scope: ["tram-policy", "article/readiness_policy"]],

#          field: "title",

#          level: "error"

#        }

#      ],

#      ...

#    ]



I18n.t(*policy.items.first)

# => "translation missing: en.tram-policy.article/readiness_policy.blank_title"

```



You can change the root scope if you will (this could be useful in libraries):



```ruby

class MyGemPolicy < Tram::Policy

  root_scope "mygem", "policies" # inherited by subclasses

end



class Article::ReadinessPolicy < MyGemPolicy

  # ...

end



# ...

I18n.t(*policy.items.first)

# => "translation missing: en.mygem.policies.article/readiness_policy.blank_title"

```



### Messages



Error messages contain translation of `policy.items` in the current locale:



```ruby

policy.messages # or policy.errors.messages, or policy.errors.map(&:message)

# => [

#      "translation missing: en.tram-policy.article/readiness_policy.blank_title",

#      "translation missing: en.tram-policy.article/readiness_policy.blank_subtitle"

#    ]

```



The messages are translated if the keys are symbolic. Strings are treated as already translated:



```ruby

class Article::ReadinessPolicy < Tram::Policy

  # ...

  def title_presence

    return unless title.empty?

    errors.add "Title is absent", field: "title", level: "error"

  end

end



# ...

policy.messages

# => [

#      "Title is absent",

#      "translation missing: en.tram-policy.article/readiness_policy.blank_subtitle"

#    ]

```



## Partial Validation



You can use tags in checkers -- to add condition for errors to ignore



```ruby

policy.valid? { |error| !%w(warning error).include? error.level } # => false

policy.valid? { |error| error.level != "disaster" }               # => true

```



Notice the `invalid?` method takes a block with definitions for errors to count (not ignore)



```ruby

policy.invalid? { |error| %w(warning error).include? error.level } # => true

policy.invalid? { |error| error.level == "disaster" }              # => false



policy.validate! { |error| error.level != "disaster" } # => nil (seems ok)

```



## Composition of Policies



You can use errors in composition of policies:



```ruby

class Article::PublicationPolicy < Tram::Policy

  param  :article

  option :selected, proc { |value| !!value } # enforce booleans



  validate :article_readiness

  validate :article_selection



  private



  def article_readiness

    # Collects errors tagged by level: "error" from "nested" policy

    readiness_errors = Article::ReadinessPolicy[article].errors.filter(level: "error")



    # Merges collected errors to the current ones.

    # New errors are also tagged by source: "readiness".

    errors.merge(readiness_errors, source: "readiness")

  end



  def article_selection

    errors.add "Not selected", field: "selected", level: "info" unless selected

  end

end

```



## Exceptions



When you use `validate!` it raises `Tram::Policy::ValidationError` (subclass of `RuntimeError`). Its message is built from selected errors (taking into account a `validation!` filter).



The exception also carries a backreference to the `policy` that raised it. You can use it to extract either errors, or arguments of the policy during a debugging:



```ruby

begin

  policy.validate!

rescue Tram::Policy::ValidationError => error

  error.policy == policy # => true

end

```



## Additional options



Class method `.validate` supports several options:



### `stop_on_failure`



If a selected validation will fail (adds an error to the collection), the following validations won't be executed.



```ruby

require "tram-policy"



class Article::ReadinessPolicy < Tram::Policy

  # required param for article to validate

  param  :article



  validate :title_presence, stop_on_failure: true

  validate :title_valid # not executed if title is absent



  # ...

end

```



## RSpec matchers



RSpec matchers defined in a file `tram-policy/matcher` (not loaded in runtime).



Use `be_invalid_at` matcher to check whether a policy has errors with given tags.



```ruby

# app/policies/user/readiness_policy.rb

class User::ReadinessPolicy < Tram::Policy

  option :name,  proc(&:to_s), optional: true

  option :email, proc(&:to_s), optional: true



  validate :name_presence



  private



  def name_presence

    return unless name.empty?

    errors.add "Name is absent", level: "error"

  end

end

```



```ruby

# spec/spec_helper.rb

require "tram/policy/rspec"

```



```ruby

# spec/policies/user/readiness_policy_spec.rb

RSpec.describe User::ReadinessPolicy do

  subject(:policy) { described_class[email: "user@example.com"] }



  let(:user) { build :user } # <- expected a factory



  it { is_expected.to be_invalid }

  it { is_expected.to be_invalid_at level: "error" }

  it { is_expected.to be_valid_at   level: "info" }

end

```



The matcher checks not only the presence of an error, but also ensures that you provided translation of any message to any available locale (`I18n.available_locales`).



## Generators



The gem provides simple tool for scaffolding new policy along with its RSpec test template and translations.



```shell

$ tram-policy user/readiness_policy -p user -o admin -v name_present:blank_name email_present:blank_email

```



This will generate a policy class with specification compatible to both [RSpec][rspec] and [FactoryBot][factory_bot].



Under the keys `-p` and `-o` define params and options of the policy.

Key `-v` should contain validation methods along with their error message keys.



## Installation



Add this line to your application's Gemfile:



```ruby

gem 'tram-policy'

```



And then execute:



```shell

$ bundle

```



Or install it yourself as:



```shell

$ gem install tram-policy

```



## License



The gem is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT).



[codeclimate-badger]: https://img.shields.io/codeclimate/github/tram-rb/tram-policy.svg?style=flat

[codeclimate]: https://codeclimate.com/github/tram-rb/tram-policy

[gem-badger]: https://img.shields.io/gem/v/tram-policy.svg?style=flat

[gem]: https://rubygems.org/gems/tram-policy

[inch-badger]: http://inch-ci.org/github/tram-rb/tram-policy.svg

[inch]: https://inch-ci.org/github/tram-rb/tram-policy

[travis-badger]: https://img.shields.io/travis/tram-rb/tram-policy/master.svg?style=flat

[active-model-validation]: http://api.rubyonrails.org/classes/ActiveModel/Validations.html

[active-record-validation]: http://guides.rubyonrails.org/active_record_validations.html

[dry-validation]: http://dry-rb.org/gems/dry-validation/

[dry-initializer]: http://dry-rb.org/gems/dry-initializer/

[i18n]: https://github.com/svenfuchs/i18n

[rspec]: http://rspec.info/

[factory_bot]: https://github.com/thoughtbot/factory_bot