Attestor

Validations and policies for immutable Ruby objects

Motivation

I like the ActiveModel::Validations more than any other part of the whole Rails. The more I like it the more painful the problem that it mutates validated objects.

Every time you run validations, the collection of object's #errors is cleared and populated with new messages. So you can't validate frozen (immutable) objects without magic tricks.

To solve the problem, the attestor gem:

• Provides a simplest API for validating immutable objects.
• Makes it possible to isolate validators (as policy objects) from their targets.
• Allows policy objects to be composed by logical operations to provide complex policies.

Approach

Instead of collecting errors inside the object, the module defines two instance methods:

• validate! raises an exception (Attestor::InvalidError), that carries errors outside of the object.
• validate - the safe version of validate!. It rescues from the exception and returns a report object, that carries the exception as well as its error messages.

In both cases the inspected object stays untouched (and can be made immutable).

Installation

Add this line to your application's Gemfile:

# Gemfile
gem "attestor"

Then execute:

bundle

Or add it manually:

gem install attestor

Base Use

Declare validation in the same way as ActiveModel's .validate method does:

Transfer = Struct.new(:debet, :credit) do
  include Attestor::Validations

  validate :consistent

  private

  def consistent
    fraud = credit.sum - debet.sum
    invalid :inconsistent, fraud: fraud if fraud != 0
  end
end

Alternatively, you can describe validation in a block, executed in an instance's scope:

class Transfer
  # ...
  validate { invalid :inconsistent if credit.sum != debet.sum }
end

The #invalid method translates its argument and raises an exception with the resulting message.

# config/locales/en.yml
---
en:
  attestor:
    errors:
      transfer:
        inconsistent: "Credit differs from debet by %{fraud}"

To validate an object, use its #validate! method:

debet  = OpenStruct.new(sum: 100)
credit = OpenStruct.new(sum: 90)
fraud_transfer = Transfer.new(debet, credit)

begin
  transfer.validate!       # with the bang
rescue Attestor::InvalidError => error
  error.object == transfer # => true
  error.messages           # => ["Credit differs from debet by 10"]
end

Alternatively use the safe version #validate. It rescues from an exception and returns a corresponding report:

report = transfer.validate  # without the bang

report.valid?               # => false
report.invalid?             # => true
report.object == transfer   # => true
report.messages             # => ["Credit differs from debet by 10"]
report.error                # => <Attestor::InvalidError ...>

Use of Contexts

Sometimes you need to validate the object agaist the subset of validations, not all of them.

To do this use :except and :only options of the .validate class method.

class Transfer
  # ...
  validate :consistent, except: :steal_of_money
end

Then call a #validate!/#validate methods with that context:

fraud_transfer.validate!                 # => InvalidError
fraud_transfer.validate! :steal_of_money # => PASSES!

You can use the same validator several times with different contexts. They will be used independently from each other.

class Transfer
  # ...

  validate :consistent, only: :fair_trade, :consistent
  validate :consistent, only: :legal

end

You can group validations that uses shared context:

class Transfer

  # This is the same as:
  #
  # validate :consistent, only: :fair_trade
  # validate :limited, only: :fair_trade
  validations only: :fair_trade do
    validate :consistent
    validate :limited
  end
end

Delegation

Extract validator to an external object (policy), that responds to validate!.

ConsistentTransfer = Struct.new(:debet, :credit) do
  include Attestor::Validations

  def validate!
    invalid :inconsistent unless debet.sum == credit.sum
  end
end

Then use validates helper (with an "s" at the end):

class Transfer 
  # ...
  validates { ConsistentTransfer.new(:debet, :credit) }
end

or by method name:

class Transfer
  # ...
  validates :consistent_transfer

  def consistent_transfer
    ConsistentTransfer.new(:debet, :credit)
  end

The difference between .validate :something and .validates :something methods is that:

• .validate expects #something to make checks and raise error by itself
• .validates expects #something to respond to #validate!

Policy Objects

Basically the policy includes Attestor::Validations with additional methods to allow logical compositions.

To create a policy as a Struct use the builder:

ConsistencyPolicy = Attestor::Policy.new(:debet, :credit) do
  def validate!
    fraud = credit - debet
    invalid :inconsistent, fraud: fraud if fraud != 0
  end
end

If you doesn't need Struct, include Attestor::Policy to the class and initialize its arguments somehow else:

class ConsistencyPolicy
  include Attestor::Policy
  # ...
end

Policy objects can be used by validates method like other objects that respond to #validate!:

class Transfer
  # ...
  validates { ConsistencyPolicy.new(debet, credit) }
end

Complex Policies

Policies (assertions) can be combined by logical methods.

Suppose we have two policy objects:

valid_policy.validate.valid?   # => true
invalid_policy.validate.valid? # => false

Use factory methods to provide compositions:

complex_policy = valid_policy.not
complex_policy.validate! # => fails

complex_policy = valid_policy.and(valid_policy, invalid_policy)
complex_policy.validate! # => fails

complex_policy = invalid_policy.or(invalid_policy, valid_policy)
complex_policy.validate! # => passes

complex_policy = valid_policy.xor(valid_poicy, valid_policy)
complex_policy.validate! # => fails

complex_policy = valid_policy.xor(valid_poicy, invalid_policy)
complex_policy.validate! # => passes

The or, and and xor methods called without argument(s) don't provide a policy object. They return lazy composer, expecting #not method.

complex_policy = valid_policy.and.not(invalid_policy, invalid_policy)
# this is the same as:
valid_policy.and(invalid_policy.not, invalid_policy.not)

If you prefer wrapping to chaining, use the Policy factory methods instead:

Policy.and(valid_policy, invalid_policy)
# this is the same as: valid_policy.and(invalid_policy)

Policy.or(valid_policy, invalid_policy)
# this is the same as: valid_policy.or(invalid_policy)

Policy.xor(valid_policy, invalid_policy)
# this is the same as: valid_policy.xor(invalid_policy)

Policy.not(valid_policy)
# this is the same as: valid_policy.not

As before, you can use any number of policies (except for negation of a single policy) at any number of nesting.

RSpec helpers

In a RSpec tests you can use spies for valid and invalid objects:

• valid_spy is a spy that returns nil in response to #validate! and valid report in responce to #validate.
• invalid_spy raises on #validate! and returns invalid report in responce to #validate method call.

require "attestor/rspec"

describe "something" do

  let(:valid_object)   { valid_spy   }
  let(:invalid_object) { invalid_spy }

  # ...
end

To check whether an arbitrary object is valid, simply use #validate method's result:

expect(object.validate).to be_valid
expect(object.validate).to be_invalid

Compatibility

Tested under rubies compatible to rubies with API 1.9.3+:

• MRI 1.9.3+
• Rubinius-2 (modes 1.9+)
• JRuby 9.0.0.0.pre1+

Uses RSpec 3.0+ for testing and hexx-suit for dev/test tools collection.

Contributing

• Read the STYLEGUIDE.
• Fork the project
• Create your feature branch (git checkout -b my-new-feature)
• Add tests for it
• Commit your changes (git commit -am '[UPDATE] Add some feature')
• Push to the branch (git push origin my-new-feature)
• Create a new Pull Request

Latest Changes

See the CHANGELOG

License

See the MIT LICENSE.