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acts_as_versioned is a gem for Rails 3.1, 3.2 & 4 to enable easy versioning of models. As a versioned model is updated revisions are kept in a seperate table, providing a record of what changed.

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In your Gemfile simply include:

gem 'acts_as_versioned_jw', '~> 3.2.2'

The next time you run bundle install you'll be all set to start using acts_as_versioned.

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By default acts_as_versioned is unobtrusive. You will need to explicitly state which models to version. To do so, add the line acts_as_versioned to your model, like so:

class MyModel < ActiveRecord::Base
  acts_as_versioned
  #...
end

Next we need to create a migration to setup our versioning tables:

bundle exec rails generate migration AddVersioningToMyModel

Once that is completed, edit the generated migration. acts_as_versioned patches your model to add a create_versioned_table and drop_versioned_table method. A migration for MyModel (assuming MyModel already existed) might look like:

class AddVersioningToMyModel < ActiveRecord::Migration
  def self.up
    MyModel.create_versioned_table
  end

  def self.down
    MyModel.drop_versioned_table
  end
end

Execute your migration:

bundle exec rake db:migrate

And you're finished! Without any addition work, MyModel is being versioned.

Sometime you want to exclude an attribute of a model from being versioned. That can be accomplished with the :except paramter to acts_as_versioned:

class MyMode < ActiveRecord::Base
  acts_as_versioned :except => :some_attr_i_dont_want_versioned
  
end

Recording a history of changes to a model is only useful if you can do something with that data. With acts_as_versioned there are several ways you can interact with a model's revisions.

To determine what the current version number for a model is:

model.version

The version attribute is available for both the actual model, and also any revisions of a model. Thusly, the following is valid:

model.versions.last.version

As alluded to above, you can get an array of revisions of a model via the versions attribute:

model.versions

The returned objects are of a type MyModel::Version where MyModel is the model you are working with. These objects have identical fields to MyModel. So, if MyModel had a name attribute, you could also say:

model.versions.last.name

To revert a model to an older revision, simply call revert_to with the version number you desire to rever to:

model.revert_to(version_number)

Occasionally you might need to save a model without necessary creating revisions. To do so, use the save_without_revision method:

model.save_without_revision

Adding a field to your model does not automatically add it to the versioning table. So, when you add new fields, be sure to add them to both:

class AddNewFieldToMyModel < ActiveRecord::Migration
  def change
    add_column :my_models, :new_field_, :string
    add_column :my_model_versions, :new_field_, :string
  end
end

As has been stated, the versioned data is stored seperately from the main class. This also implies that model.versions returns an area of object of a class other than MyModel (where model is an instance of MyModel, keeping with our working example). The instances returned are actually of type MyModel::Version. With this, comes the fact that any methods, associations, etc. defined on MyModel are not present on MyModel::Version.

While this sounds obvious, it can some times be unexpected. Especially when acts_as_versioned make it so easy to grab historical records from a live record. A common scenario where this can come up is associations.

Say MyModel belongs to TheMan. Also, assume that you want to find out where (in the past) a particular instance of MyModel was updated in regards to it's association to TheMan. You could write that as:

model.versions.keep_if { |m| m.the_man != current_man }.last

However, this will not work. This is because MyModel::Version does not belong to TheMan. You could compare ids here, or you could patch MyModel::Version to belong to TheMan like:

class MyModel
  acts_as_versioned
  belongs_to :the_men
  #some stuff
  
  class Version
    belongs_to :the_men
  end
end