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 filtermethod 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_filterand after_filterappends 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 filteror to both beforeand 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_filterblock; 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 :filteror both :beforeand :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 beforeand aftermethods:

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 beforeand aftermethods, the beforemethod will be called before the action. If beforerenders or redirects, the filter chain is halted and afterwill 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_filterand around_filtermay 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 aroundreturns before yielding, afterwill still not be run. The beforefilter and controller action will not be run. If beforerenders or redirects, the second half of aroundand will still run but afterand the action will not. If aroundfails to yield, after will not be run.

Methods
A
B
F
P
S
Instance Public methods
after_filter(*filters, &block)

Shorthand for #append_after_filter since it's the most common.

append_after_filter(*filters, &block)

The passed filterswill be appended to the array of filters that run afteractions on this controller are performed.

Also aliased as: after_filter
# File actionpack/lib/action_controller/filters.rb, line 498
def append_after_filter(*filters, &block)
  filter_chain.append_filter_to_chain(filters, :after, &block)
end
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 afterare the code before and after the yield.

Also aliased as: around_filter
# File actionpack/lib/action_controller/filters.rb, line 521
def append_around_filter(*filters, &block)
  filter_chain.append_filter_to_chain(filters, :around, &block)
end
append_before_filter(*filters, &block)

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

Also aliased as: before_filter
# File actionpack/lib/action_controller/filters.rb, line 483
def append_before_filter(*filters, &block)
  filter_chain.append_filter_to_chain(filters, :before, &block)
end
around_filter(*filters, &block)

Shorthand for append_around_filtersince it's the most common.

before_filter(*filters, &block)

Shorthand for #append_before_filter since it's the most common.

filter_chain()

Returns an array of Filter objects for this controller.

# File actionpack/lib/action_controller/filters.rb, line 573
def filter_chain
  if chain = read_inheritable_attribute('filter_chain')
    return chain
  else
    write_inheritable_attribute('filter_chain', FilterChain.new)
    return filter_chain
  end
end
prepend_after_filter(*filters, &block)

The passed filterswill be prepended to the array of filters that run afteractions on this controller are performed.

# File actionpack/lib/action_controller/filters.rb, line 504
def prepend_after_filter(*filters, &block)
  filter_chain.prepend_filter_to_chain(filters, :after, &block)
end
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 afterare the code before and after the yield.

# File actionpack/lib/action_controller/filters.rb, line 535
def prepend_around_filter(*filters, &block)
  filter_chain.prepend_filter_to_chain(filters, :around, &block)
end
prepend_before_filter(*filters, &block)

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

# File actionpack/lib/action_controller/filters.rb, line 489
def prepend_before_filter(*filters, &block)
  filter_chain.prepend_filter_to_chain(filters, :before, &block)
end
skip_after_filter(*filters)

Removes the specified filters from the afterfilter 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 :onlyand :exceptoptions, just like when you apply the filters.

# File actionpack/lib/action_controller/filters.rb, line 558
def skip_after_filter(*filters)
  filter_chain.skip_filter_in_chain(*filters, &:after?)
end
skip_before_filter(*filters)

Removes the specified filters from the beforefilter 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 :onlyand :exceptoptions, just like when you apply the filters.

# File actionpack/lib/action_controller/filters.rb, line 548
def skip_before_filter(*filters)
  filter_chain.skip_filter_in_chain(*filters, &:before?)
end
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 :onlyand :exceptoptions, just like when you apply the filters.

# File actionpack/lib/action_controller/filters.rb, line 568
def skip_filter(*filters)
  filter_chain.skip_filter_in_chain(*filters)
end