--- layout: default title: "Features" current: "features" ---


Flexible and Plugable

Merb has a comprehensive set of features, implemented in it's core and through its plugin system.


Plugins in Merb are implemented as simple gems, which are distributed under the merb github account and by others too. That means that plugins can take advantage, out of the box, for Rubygems’ versioning and dependency control. Plugins can either be installed to the system’s repository or bundled into the gems directory in an application’s distribution; Merb applications simply add the /gems folder as an alternate repository.

Plugins for Merb already include support for ActiveRecord, DataMapper, and Sequel, with support for SQL sessions, model generation, and database.yml baked in to all three. Merb-plugins Git repository also includes a helpers plugin (to add support for Rails-style form helpers).


Merb’s controllers are made up of two components. First, an AbstractController, which handles layout- and template-finding, instance variable assignment, and before/after filters. Second, a Merb::Controller, which handles request/response semantics. Because the components are separate, it is possible to inherit from AbstractController, which Merb does for Mailers and Parts (again, more on that later).

Controllers also support excellent content-type negotiation. You can specify in your controllers, or in individual actions, what MIME-types should be supported via provides :xml, :html. Once that information is provided, the controller has a number of ways to automatically render the appropriate content. If a template called foo.html.erb exists, it will automatically be rendered for all content-type text/html, and so on.

Additionally, calling render @object, will call @object.to_mime_type (for instance, @object.to_xml). The mime-type chosen in both cases is based upon either the file extension (foo.html maps to the :html type), or the Accepts header (the first acceptable content-type that’s also in the provides list). If the object doesn’t have the appropriate method, render will fall-back to rendering a template (so if the user requests, say, HTML, the lack of #to_html on the object will cause the foo.html.erb template to be loaded).


Merb’s Mailers are implemented on top of AbstractController, so you get all the default controller behavior (including templates, assigns, and before/after filters) for free in the Mailer. But instead of calling render, you call render_mail, which takes options like: render_mail :html => :foo, :text => :bar.

A number of options are supported, including attachments via an #attach method, so you can build up your multi-part mails with attachments and site-wide layouts fairly trivially.

Mailers have their own root directory, which contains controller classes inside it, as well as a views directory (which contains layouts, just like a regular controller), and an optional helpers directory. Because AbstractController can specify its layout root trivially, it’s easy to create new controller types and drop them in.

Mailers are called from a regular controller via send_mail Klass, :action, options, where options is a hash of options such as from, to, subject, and cc.


Like Mailers, parts take advantage of the flexibility of the AbstractController to allow simple Controller/View delegation. Parts have a directory structure that’s identical to the Mailer structure, and you can use them to separate out logic about partials that are used throughout your app.

For instance, you might have a tag-cloud that appears app-wide. You could create a TagCloud part, and have actions in the part set up the controller logic for the template. Like controllers and Mailers, parts can have layouts, templates, and before/after filters. Parts are called via part TagCloud => :show. Parts can also be used just to segment out the logic for sections of partials, so you have discrete components instead of one massive controller.


Merb also handles exceptions interestingly. Instead of exception raising throwing an error in your application, Merb catches certain types of exceptions and allows you to handle them in a controller/view fashion. For instance, raising NotFound will call the Exception#not_found action, which you can customize as appropriate. Raising an error in this way will also send the appropriate error-code back to the browser.

All HTTP error codes are defined in Merb as Exception classes, so you can raise NotAcceptable, which will call Exception#not_acceptable, and return a 406 error to the client.

Tests and Specs

Merb is testing-framework-agnostic: you can use Test::Unit, Rspec, or test/spec. All three testing frameworks have built-in support for mock objects that allow you to micro-target your tests exactly as you like. And because Merb is so modular, it’s easy to test your controller without a request object at all, if you’d want to.


Merb has a series of generators that allow you get up and running quickly. The Merb application generator is started via merb-gen app application-name. It’ll build a skeleton app that includes folders for controllers, parts, and Mailers. Building a plugin is easy as well, merb-gen plugin merb_plugin_name will produce a very simple plugin skeleton with a few rake tasks to help you deploy the plugin (such as rake package and rake install).

In addition, Merb has controller generators, which create an empty controller file, a view directory with an empty index.html.erb, an empty helper file, and a test file in your chosen spec framework. Merb also has model generators, which are implemented by the ORM plugins, and support a special syntax: merb-gen Product name:string price_in_cents:integer will generate a new model that implements those attributes using its own syntax. For instance, ActiveRecord would generate a migration, while DataMapper would generate new model using its property syntax.