Prependers are a way to easily and cleanly extend third-party code via
Add this line to your application's Gemfile:
And then execute:
Or install it yourself as:
$ gem install prependers
To define a prepender manually, simply include the
Prependers::Prepender module. For instance,
if you have installed an
animals gem and you want to extend the
Animals::Dog class, you can
define a module like the following:
module Animals::Dog::AddBarking include Prependers::Prepender def bark puts 'Woof!' end end Animals::Dog.new.bark # => 'Woof!'
Extending class methods
If you want to extend a module's class methods, you can define a
ClassMethods module in your
module Animals::Dog::AddFamily include Prependers::Prepender module ClassMethods def family puts 'Canids' end end end Animals::Dog.family # => 'Canids'
As you can see, the
ClassMethods module has automagically been
Using a namespace
It can be useful to have a prefix namespace for your prependers. That way, you don't have to worry about accidentally overriding any vendor modules. This is actually the recommended way to define your prependers.
You can accomplish this by passing the
:namespace option when including
module MyApp module Animals module Dog module AddBarking include Prependers::Prepender[namespace: MyApp] def bark puts 'Woof!' end end end end end
Verifying original sources
One issue you may run into when extending third-party code is that, when the original implementation is updated, it's not always obvious whether you have to update any of your extensions.
Prependers make this a bit easier with the concept of original source verification: you can compute a SHA1 hash of the original implementation, store it along with your prepender, and then verify it against the current hash when your application loads. If the original source changes, you get an error asking you to ensure your prepender is still relevant.
To use original source verification in your prependers, pass the
module Animals::Dog::AddBarking include Prependers::Prepender[verify: nil] # ... end
When you load your application now, you will get an error with instructions on how to set the proper hash:
Prependers::OutdatedPrependerError: You have not defined an original hash for Animals::Dog in Animals::Dog::AddBarking. You can define the hash by updating your include statement as follows: include Prependers::Prepender[verify: 'f7175533215c39f3f3328aa5829ac6b1bb168218']
At this point, you should update your prepender with the correct hash:
module Animals::Dog::AddBarking include Prependers::Prepender[verify: 'f7175533215c39f3f3328aa5829ac6b1bb168218'] # ... end
Now, when the underlying implementation of
Animals::Dog changes because of a dependency update or
other reasons, Prependers will raise an error such as the following:
Prependers::OutdatedPrependerError: The stored hash for Animals::Dog in Animals::Dog::AddBarking is f7175533215c39f3f3328aa5829ac6b1bb168218, but the current hash is 2f05682e4f46b509c23a8418d9427a9eeaa8a79e instead. This most likely means that the original source has changed. Check that your prepender is still valid, then update the stored hash: include Prependers::Prepender[verify: '2f05682e4f46b509c23a8418d9427a9eeaa8a79e']
Original source verification also works when a module is defined in multiple locations.
NOTE: Due to limitations in Ruby's API, it is not possible to use source verification with modules that don't define any methods. Prependers will raise an error if you try to do this.
If you don't want to include
Prependers::Prepender, you can also autoload prependers from a
path, they will be loaded in alphabetical order.
Here's the previous example, but with autoloading:
# app/prependers/animals/dog/add_barking.rb module Animals::Dog::AddBarking def bark puts 'Woof!' end end # somewhere in your initialization code Prependers.load_paths(File.expand_path('app/prependers'))
Note that, in order for autoprepending to work, the paths of your prependers must match the names of the prependers you defined.
You can pass multiple arguments to
#load_paths, which is useful if you have subdirectories in
Prependers.load_paths( File.expand_path('app/prependers/controllers'), File.expand_path('app/prependers/models'), # ... )
You can pass the
:namespace option to
#load_paths to have it forwarded to all prependers:
Prependers.load_paths( File.expand_path('app/prependers/controllers'), File.expand_path('app/prependers/models'), namespace: Acme, )
Integrating with Rails
To use prependers in your Rails app, simply create them under
app/prependers/controllers etc. and add the following to your
#setup_for_rails accepts the same options as
After checking out the repo, run
bin/setup to install dependencies. Then, run
rake spec to run
the tests. You can also run
bin/console for an interactive prompt that will allow you to
To install this gem onto your local machine, run
bundle exec rake install. To release a new
version, update the version number in
version.rb, and then run
bundle exec rake release, which
will create a git tag for the version, push git commits and tags, and push the
.gem file to
Bug reports and pull requests are welcome on GitHub at https://github.com/nebulab/prependers.
The gem is available as open source under the terms of the MIT License.