I wanted a #login method in test_helper that would allow me to easily login from any of my functional tests. However, the #post method won't allow you to set a different controller than the one in the @controller instance variable that's defined in your test's #setup. Well, by looking at how the #process method works, you can see that it just grabs the controller from @controller. Redefine that, and you're good to go:

old_controller = @controller
@controller = LoginController.new
post(
  :attempt_login,
  {:user => {:name => 'joe', :password => 'password'}}
)
@controller = old_controller

If you have several login methods, such as a #login_admin and #login_regular, you could make a wrapper to reduce duplication:

def wrap_with_controller( new_controller = LoginController )
  old_controller = @controller
  @controller = new_controller.new
  yield
  @controller = old_controller
end

def login_admin
  wrap_with_controller do
    post(
      :attempt_login,
      {:user => {:name => 'root', :password => 'password'}}
    )
  end
end

def login_regular
  wrap_with_controller do
    post(
      :attempt_login,
      {:user => {:name => 'joe', :password => 'password'}}
    )
  end
end

As found at http://media.pragprog.com/titles/fr_rr/code/CreateFixturesFromLiveData/lib/tasks/extract_fixtures.rake

desc 'Create YAML test fixtures from data in an existing database.  
Defaults to development database.  Set RAILS_ENV to override.'

task :extract_fixtures => :environment do
  sql  = "SELECT * FROM %s"
  skip_tables = ["schema_info"]
  ActiveRecord::Base.establish_connection
  (ActiveRecord::Base.connection.tables - skip_tables).each do |table_name|
    i = "000"
    File.open("#{RAILS_ROOT}/test/fixtures/#{table_name}.yml", 'w') do |file|
      data = ActiveRecord::Base.connection.select_all(sql % table_name)
      file.write data.inject({}) { |hash, record|
        hash["#{table_name}_#{i.succ!}"] = record
        hash
      }.to_yaml
    end
  end
end