Plyushkin

Plyushkin provides a way to capture historical data in an ActiveRecord class.

Add this line to your application's Gemfile:

gem 'plyushkin'

And then execute:

$ bundle

Or install it yourself as:

$ gem install plyushkin

Configure the backend service that plyushkin will use in your environments/<environment>.rb file.
For example, to configure the stub service for running specs, the following code would go in your config/environments/test.rb file of a Rails application.

config.before_initialize do |c|
  Plyushkin::Service.service = Plyushkin::Service::Stub.new
end

To use plyushkin against a live web service,

config.before_initialize do |c|
  Plyushkin::Service.service =
    Plyushkin::Service::Web.new(:url => 'http://yourservice.com')
end

To add a property, use the hoards class macro on a class that inherits from ActiveRecord::Base.

class Vehicle < ActiveRecord::Base
  hoards :mechanic
end

This will add a mechanic attribute to the Vehicle class. To set the mechanic for the vehicle

v = Vehicle.new
v.mechanic.create(:value => 'Mike')

This will update the in-memory version of the vehicle, but has not yet persisted the change. To save the change to the plyushkin-service

v.save

To get the value of the vehicle's mechanic

v.mechanic.last.value  # => 'Mike'

To get the date that the mechanic was last set

v.mechanic.last.date   # => Wed, 09 Jul 2014 11:52:12 -0500

Every Plyushkin value has a date attribute.

Once we change the mechanic on the vehicle, we will now have historical data and can view all the mechanics that worked on the vehicle.

v.mechanic.create(:value => 'Joe')
v.mechanic.all.map(&:value) # => ['Mike', 'Joe']

To store a property using a DateTime other than the current time

v.mechanic.create(:value => 'Sally', :date => 3.days.ago)
v.mechanic.last.date # => Sun, 06 Jul 2014 11:52:12 -0500

In the example above, we needed to use last and all methods to see the mechanic. This is because the property getter returns a Plyushkin::Property. A property consists of instances of classes that derive from Plyushkin::BaseValue.

Plyushkin::BaseValue provides basic value functionality. It provides a date attribute, that is required for all Plyushkin values. And, it provides four formatters, to_i, to_f, to_date and to_bool.

When not specifying a value type when calling hoards, Plyushkin will use a Plyushkin::StringValue as the value type. In addition to the base implementation, this basic implementation includes one attribute, value, which uses no formatter.

In most applications, you will not want to use Plyushkin::StringValue, and instead would want to create your own value type implementation that derives from Plyushkin::BaseValue and use custom value types.

In our vehicle example, if we wanted to capture oil change history, we would start by creating a custom value type.

class OilChangeValue < Plyushkin::BaseValue
  persisted_attr :mileage, :formatter => :to_i
  persisted_attr :oil_type
end

The OilChangeValue will have a mileage attribute that uses the to_i formatter. Formatters attempt to convert the attribute assignment to a specified format. If no formatter is provided, the attribute will be stored as a string.

We can now add this to our vehicle

class Vehicle < ActiveRecord::Base
  hoards :mechanic
  hoards :oil_change, :type => OilChangeValue
end

To set the oil_change

v.oil_change.create(:mileage => '1234', :oil_type => '10W30')

To get the latest oil change details

v.oil_change.last.mileage  # => 1234
v.oil_change.last.oil_type # => '10W30'

When defining a hoards property, you can set a callback for after the value is persisted. For example, if the vehicle table contains a next_oil_change_mileage column, we might want to update it whenever an oil_change is saved.

class Vehicle < ActiveRecord::Base
  hoards :oil_change, :type => OilChangeValue,
         :after_create => :calculate_next_oil_change_mileage

  def calculate_next_oil_change_mileage
    next_oil_change_mileage = oil_change.last.mileage + 3000
  end
end

Sometimes it is useful to filter hoarded values. For example, we may want the ability to soft-delete values from the historical properties (hard deletes is against the phylosophy of plyushkin), but we can obtain similar results by:

class OilChangeValue < Plyushkin::BaseValue
  persisted_attr :mileage, :formatter => :to_i
  persisted_attr :oil_type
  persisted_attr :is_deleted, :formatter => :to_bool
end

class Vehicle < ActiveRecord::Base
  hoards :oil_change, :type => OilChangeValue, :filter => :soft_delete_filter

  def soft_delete_filter(value)
    value.is_deleted != true
  end
end

Now all values for the oil_change property will be filtered to exclude any values where is_deleted == true.

You can also specify a class level filter for all hoards that a class might implement by using:

class Vehicle < ActiveRecord::Base
  filter_hoards_by :soft_delete_filter
end

This would cause all hoards to be filtered using the soft_delete_filter, unless this behavior was overridden using a :filter parameter on the individual hoards.

If at anytime you wish to get unfiltered data from a property, you can still do that using the :unfiltered option:

vehicle.oil_change.all(:unfiltered => true) #=> unfiltered data

There may be a case where we don't need to track history when a value is set, but is the same as the previous value. For example, if the mechanic for your last maintenance is Mike and Mike again performs the maintenance, we don't need two data points recorded.

In the current configuration, calling create twice would create two values

v.mechanic.create(:value => 'Joe')
v.mechanic.create(:value => 'Mike')
v.mechanic.create(:value => 'Mike')
v.mechanic.all.map(&:value) # => [ 'Joe', 'Mike', 'Mike' ]

By setting the :ignore_unchanged_values option, we can change this behavior.

class Vehicle < ActiveRecord::Base
  hoards :mechanic, :ignore_unchanged_values => true
end

v.mechanic.create(:value => 'Joe')
v.mechanic.create(:value => 'Mike')
v.mechanic.create(:value => 'Mike')
v.mechanic.all.map(&:value) # => [ 'Joe', 'Mike' ]

In this example, the date of the last value will be the date that mechanic was first assigned 'Mike'.

Validation is done with ActiveModel::Validations. This is included in Plyushkin::BaseValue.

class OilChangeValue < Plyushkin::BaseValue
  persisted_attr :oil_type
  persisted_attr :mileage

  validates :oil_type, :inclusion    => { :in => ['10W30', '5W40'] }
  validates :mileage,  :numericality => { :only_integer             => true,
                                          :greater_than_or_equal_to => 0 }
end

Plyushkin::BaseValue and it's subclasses are classes and additional behavior can be added to encapsulate functionality.

class OilChange < Plyushkin::BaseValue
  persisted_attr :mileage

  def mileage_as_km
    mileage / 0.6214
  end
end

v.oil_change.create(:mileage => 10000)
v.oil_change.last.mileage       # => 10000
v.oil_change.last.mileage_as_km # => 6214

Plyushkin::NilValue is a special case used when no value has been assigned to a property yet.

v = Vehicle.new
v.mechanic.last            # => Plyushkin::NilValue
v.mechanic.last.value      # => nil
v.oil_change.last          # => Plyushkin::NilValue
v.oil_change.last.mileage  # => nil
v.oil_change.last.oil_type # => nil

This is necessary so that consumers of the property do not need to check if a value is nil before trying to access an attribute of the value.

In addition, Plyushkin::Property has a property empty? to indicate whether any values have been assigned

v.mechanic.empty?   # => true
v.oil_change.empty? # => true

Plyushkin provides RSpec matchers for testing class macros. To use these matchers, add config.include Plyushkin::Test::Matchers to your RSpec.configure in spec_helper.

To test Plyushkin configuration in your custom value type:

describe OilChangeValue do
  it { should persist_attribute(:mileage) }
  it { should persist_attribute(:mileage).with_format(:to_i) }
  it { should_not persist_attribute(:air_filter) }
  it { should persist_attribute(:oil_type) }
  it { should_not persist_attribute(:oil_type).with_format(:to_f) }

  # RSpec Shoulda matchers also work to test validations here.
end

To test Plyushkin configuration in your model:

describe Vehicle do
  it { should hoard(:mechanic) }
  it { should hoard(:mechanic).and_ignore_unchanged_values } 
  it { should hoard(:oil_change).of_type(OilChangeValue) }
  it { should hoard(:oil_change).of_type(OilChangeValue).
    and_after_create_call(:calculate_next_oil_change_mileage) }
end

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request