dm-is-versioned¶ ↑

DataMapper plugin enabling simple versioning of models.

When a model is versioned, and updated, instead of the previous version being lost in the mists of time, it is saved in a subsidiary table, so that it can be restored later if needed.

Please Note! this gem behaves differently to how AR’s :acts_as_versioned works. There is currently no multi version storage possible.

Install the dm-is-versioned gem.

$ (sudo)? gem install dm-is-versioned

Download or clone dm-is-versioned from Github.

$ cd /path/to/dm-is-versioned

$ rake install            # will install dm-is-versioned

To start using this gem, just require dm-is-versioned in your app.

Lets say we have a Post class, and we want to retain the previous version of a post. Just do this:

class Post
  include DataMapper::Resource

  property :id,         Serial
  property :title,      String
  property :body,       Text
  property :updated_at, DateTime

  is_versioned :on => :updated_at

  ## this syntax also works
  # is :versioned, :on => :updated_at

end

That simple snippet will automatically create a second Post::Version model and a (post_versions) table in your DB when you auto migrate or auto upgrade:

Post.auto_migrate! # => will run auto_migrate! on Post::Version, too
Post.auto_upgrade! # => will run auto_upgrade! on Post::Version, too

# or
DataMapper.auto_migrate! # => will run auto_migrate! on Post::Version, too
DataMapper.auto_upgrade! # => will run auto_upgrade! on Post::Version, too

# don't forget to require 'dm-migrations' before you migrate.

OK, now that we have a versioned model, let’s see what we can do with it.

We start with creating a new Post:

post = Post.create(:title => 'A versioned Post title', :body => "A versioned Post body")
# automatically saved

When we create this Post, no secondary version is created.

The versioning only takes place when we update our Post:

post.title = "An updated & versioned Post title"
post.save

In the Post::Version (post_versions) table we would now find the previous version of the Post.

This is how it would look like:

# db.post table
---------------------------------------------------------------------------------------
| id | title                                | body                       | updated_at |
---------------------------------------------------------------------------------------
| 1 | 'An updated & versioned Post title'   | 'A versioned Post body'    | DateTime   |

# db.post_versions table
---------------------------------------------------------------------------------------
| id | title                                | body                       | updated_at |
---------------------------------------------------------------------------------------
| 1 | 'A versioned Post title'              | 'A versioned Post body'    | DateTime   |

Once you have a versioned model, you can retrieve the previous version, like this:

old_post = post.versions.first
  => #<Post::Version @id=1 @title="A versioned Post title" @body=... @updated_at=...>

Please Note! that the #versions method returns an array by default.

That’s basically what dm-is-versioned does.

Now there are some gotcha’s that might not be entirely obvious to everyone, so let’s clarify them here.

In this type of scenario:

class Post
  <snip...>
  property :updated_at, DateTime

  is_versioned :on => :updated_at
end

You must ensure that the versioned trigger always has a value, either through:

# using the dm-timestamps gem, which automatically updates the :updated_at attribute
timestamps :at

# or a callback method, that updates the value before save
before(:save) { self.updated_at = Time.now }

Without this, things just don’t work.

The post_versions table is NOT created unless you migrate your models through:

DataMapper.auto_migrate! / Post.auto_migrate!
DataMapper.auto_upgrade! / Post.auto_upgrade!

That’s about it.

If something is not behaving intuitively, it is a bug, and should be reported. Report it here: datamapper.lighthouseapp.com/

• Make it work more like AR’s :acts_as_versioned plugin, with support for multiple versions and so on.

• Enable replacing a current version with an old version.

• Anything else missing?

• Fork the project.

• Make your feature addition or bug fix.

• Add tests for it. This is important so we don’t break it in a future version unintentionally.

• Commit, do not mess with rakefile, version, or history.

  • (if you want to have your own version, that is fine but bump version in a commit by itself we can ignore when we pull)

• Send us a pull request. Bonus points for topic branches.

Copyright © 2011 Timothy Bennett. Released under the MIT License.

See LICENSE for details.

Credit also goes to these contributors.