Tram::Examiner

The gem allows to add a standalone validator to Ruby classes.

This is a part of collection of patterns extracted from Rails projects with a special focus on separation and composability of data validators.

Installation

Add this line to your application's Gemfile:

gem 'tram-examiner'

And then execute:

$ bundle

Or install it yourself as:

$ gem install tram-examiner

Usage

Include the module to the class and define validation rules inside the schema block.

require "tram-examiner"

class User < Struct.new(:name, :age)
  include Tram::Examiner

  examiner do
    validates :name, presence: true
  end
end

Inside the block you should use standard ActiveModel validations. The trick is that instead of including ActiveModel::Validations directly, the examiner adds validations to a separate decorator, wrapped around validated object.

It adds a variable @__examiner__, which refers to that standalone decorator. Access to validation result is provided via delegators valid?, validate! and errors.

user = User.new '', 46

user.valid?          # => false
user.validate!       # Boom with #<<ActiveModel::ValidationError: Validation failed: Name can't be blank>
user.errors.messages # => { name: ["cannot be blank"] }

Under the hood:

user.is_a? ActiveModel::Validations
# => false

user.instance_variable_get(:@__examiner__).is_a? ActiveModel::Validations
# => true

For syntax sugar you can use let memoizer helper inside the examiner block:

class User < Struct.new(:first_name, :last_name, :age)
  include Tram::Examiner

  examiner do
    let(:name) { [first_name, last_name].compact.join(" ") }
    validates :name, presence: true
  end
end

user = User.new '', '', 51
user.errors.messages # => { name: ["cannot be blank"] }

Notice the block is evaluated in the scope of standalone validator, not the current class! That is why neither let, nor validates are available outside of the block.

License

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