Module | ActionController::Filters::ClassMethods
In: | lib/action_controller/filters.rb

Filters enable controllers to run shared pre- and post-processing code for its actions. These filters can be used to do authentication, caching, or auditing before the intended action is performed. Or to do localization or output compression after the action has been performed. Filters have access to the request, response, and all the instance variables set by other filters in the chain or by the action (in the case of after filters).

## Filter inheritance

Controller inheritance hierarchies share filters downwards, but subclasses can also add or skip filters without affecting the superclass. For example:

      class BankController < ActionController::Base
        before_filter :audit

        private
          def audit
            # record the action and parameters in an audit log
          end
      end

      class VaultController < BankController
        before_filter :verify_credentials

        private
          def verify_credentials
            # make sure the user is allowed into the vault
          end
      end

Now any actions performed on the BankController will have the audit method called before. On the VaultController, first the audit method is called, then the verify_credentials method. If the audit method renders or redirects, then verify_credentials and the intended action are never called.

## Filter types

A filter can take one of three forms: method reference (symbol), external class, or inline method (proc). The first is the most common and works by referencing a protected or private method somewhere in the inheritance hierarchy of the controller by use of a symbol. In the bank example above, both BankController and VaultController use this form.

Using an external class makes for more easily reused generic filters, such as output compression. External filter classes are implemented by having a static `filter` method on any class and then passing this class to the filter method. Example:

      class OutputCompressionFilter
        def self.filter(controller)
          controller.response.body = compress(controller.response.body)
        end
      end

      class NewspaperController < ActionController::Base
        after_filter OutputCompressionFilter
      end

The filter method is passed the controller instance and is hence granted access to all aspects of the controller and can manipulate them as it sees fit.

The inline method (using a proc) can be used to quickly do something small that doesn‘t require a lot of explanation. Or just as a quick test. It works like this:

      class WeblogController < ActionController::Base
        before_filter { |controller| head(400) if controller.params["stop_action"] }
      end

As you can see, the block expects to be passed the controller after it has assigned the request to the internal variables. This means that the block has access to both the request and response objects complete with convenience methods for params, session, template, and assigns. Note: The inline method doesn‘t strictly have to be a block; any object that responds to call and returns 1 or -1 on arity will do (such as a Proc or an Method object).

Please note that around_filters function a little differently than the normal before and after filters with regard to filter types. Please see the section dedicated to around_filters below.

## Filter chain ordering

Using `before_filter` and `after_filter` appends the specified filters to the existing chain. That‘s usually just fine, but some times you care more about the order in which the filters are executed. When that‘s the case, you can use `prepend_before_filter` and `prepend_after_filter`. Filters added by these methods will be put at the beginning of their respective chain and executed before the rest. For example:

      class ShoppingController < ActionController::Base
        before_filter :verify_open_shop

      class CheckoutController < ShoppingController
        prepend_before_filter :ensure_items_in_cart, :ensure_items_in_stock

The filter chain for the CheckoutController is now `:ensure_items_in_cart, :ensure_items_in_stock,` `:verify_open_shop`. So if either of the ensure filters renders or redirects, we‘ll never get around to see if the shop is open or not.

You may pass multiple filter arguments of each type as well as a filter block. If a block is given, it is treated as the last argument.

## Around filters

Around filters wrap an action, executing code both before and after. They may be declared as method references, blocks, or objects responding to `filter` or to both `before` and `after`.

To use a method as an `around_filter`, pass a symbol naming the Ruby method. Yield (or `block.call`) within the method to run the action.

      around_filter :catch_exceptions

      private
        def catch_exceptions
          yield
        rescue => exception
          logger.debug "Caught exception! #{exception}"
          raise
        end

To use a block as an `around_filter`, pass a block taking as args both the controller and the action block. You can‘t call yield directly from an `around_filter` block; explicitly call the action block instead:

      around_filter do |controller, action|
        logger.debug "before #{controller.action_name}"
        action.call
        logger.debug "after #{controller.action_name}"
      end

To use a filter object with `around_filter`, pass an object responding to `:filter` or both `:before` and `:after`. With a filter method, yield to the block as above:

      around_filter BenchmarkingFilter

      class BenchmarkingFilter
        def self.filter(controller, &block)
          Benchmark.measure(&block)
        end
      end

With `before` and `after` methods:

      around_filter Authorizer.new

      class Authorizer
        # This will run before the action. Redirecting aborts the action.
        def before(controller)
          unless user.authorized?
            redirect_to(login_url)
          end
        end

        # This will run after the action if and only if before did not render or redirect.
        def after(controller)
        end
      end

If the filter has `before` and `after` methods, the `before` method will be called before the action. If `before` renders or redirects, the filter chain is halted and `after` will not be run. See Filter Chain Halting below for an example.

## Filter chain skipping

Declaring a filter on a base class conveniently applies to its subclasses, but sometimes a subclass should skip some of its superclass’ filters:

      class ApplicationController < ActionController::Base
        before_filter :authenticate
        around_filter :catch_exceptions
      end

      class WeblogController < ApplicationController
        # Will run the :authenticate and :catch_exceptions filters.
      end

      class SignupController < ApplicationController
        # Skip :authenticate, run :catch_exceptions.
        skip_before_filter :authenticate
      end

      class ProjectsController < ApplicationController
        # Skip :catch_exceptions, run :authenticate.
        skip_filter :catch_exceptions
      end

      class ClientsController < ApplicationController
        # Skip :catch_exceptions and :authenticate unless action is index.
        skip_filter :catch_exceptions, :authenticate, :except => :index
      end

## Filter conditions

Filters may be limited to specific actions by declaring the actions to include or exclude. Both options accept single actions (`:only => :index`) or arrays of actions (`:except => [:foo, :bar]`).

      class Journal < ActionController::Base
        # Require authentication for edit and delete.
        before_filter :authorize, :only => [:edit, :delete]

        # Passing options to a filter with a block.
        around_filter(:except => :index) do |controller, action_block|
          results = Profiler.run(&action_block)
          controller.response.sub! "</body>", "#{results}</body>"
        end

        private
          def authorize
            # Redirect to login unless authenticated.
          end
      end

## Filter Chain Halting

`before_filter` and `around_filter` may halt the request before a controller action is run. This is useful, for example, to deny access to unauthenticated users or to redirect from HTTP to HTTPS. Simply call render or redirect. After filters will not be executed if the filter chain is halted.

Around filters halt the request unless the action block is called. Given these filters

      after_filter :after
      around_filter :around
      before_filter :before

The filter chain will look like:

      ...
      .      #   .  #around (code before yield)
      .  .       #   .  .  #before (actual filter code is run)
      .  .  .       #   .  .  .  execute controller action
      .  .  .  /
      .  .  ...
      .  .  /
      .  #around (code after yield)
      . /
      #after (actual filter code is run, unless the around filter does not yield)

If `around` returns before yielding, `after` will still not be run. The `before` filter and controller action will not be run. If `before` renders or redirects, the second half of `around` and will still run but `after` and the action will not. If `around` fails to yield, `after` will not be run.

### Methods

after_filter append_after_filter append_around_filter append_before_filter around_filter before_filter filter_chain prepend_after_filter prepend_around_filter prepend_before_filter skip_after_filter skip_before_filter skip_filter

### Public Instance methods

after_filter(*filters, &block)

Alias for append_after_filter

append_after_filter(*filters, &block)

The passed `filters` will be appended to the array of filters that run after actions on this controller are performed.

append_around_filter(*filters, &block)

If you `append_around_filter A.new, B.new`, the filter chain looks like

      B#before
        A#before
          # run the action
        A#after
      B#after

With around filters which yield to the action block, `before` and `after` are the code before and after the yield.

append_before_filter(*filters, &block)

The passed `filters` will be appended to the filter_chain and will execute before the action on this controller is performed.

around_filter(*filters, &block)

Alias for append_around_filter

before_filter(*filters, &block)

Alias for append_before_filter

filter_chain()

Returns an array of Filter objects for this controller.

prepend_after_filter(*filters, &block)

The passed `filters` will be prepended to the array of filters that run after actions on this controller are performed.

prepend_around_filter(*filters, &block)

If you `prepend_around_filter A.new, B.new`, the filter chain looks like:

      A#before
        B#before
          # run the action
        B#after
      A#after

With around filters which yield to the action block, `before` and `after` are the code before and after the yield.

prepend_before_filter(*filters, &block)

The passed `filters` will be prepended to the filter_chain and will execute before the action on this controller is performed.

skip_after_filter(*filters)

Removes the specified filters from the `after` filter chain. Note that this only works for skipping method-reference filters, not procs. This is especially useful for managing the chain in inheritance hierarchies where only one out of many sub-controllers need a different hierarchy.

You can control the actions to skip the filter for with the `:only` and `:except` options, just like when you apply the filters.

skip_before_filter(*filters)

Removes the specified filters from the `before` filter chain. Note that this only works for skipping method-reference filters, not procs. This is especially useful for managing the chain in inheritance hierarchies where only one out of many sub-controllers need a different hierarchy.

You can control the actions to skip the filter for with the `:only` and `:except` options, just like when you apply the filters.

skip_filter(*filters)

Removes the specified filters from the filter chain. This only works for method reference (symbol) filters, not procs. This method is different from skip_after_filter and skip_before_filter in that it will match any before, after or yielding around filter.

You can control the actions to skip the filter for with the `:only` and `:except` options, just like when you apply the filters.

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