Add support and tests for datamapper.

This broke some of the mongoid tests and I don't know how to fix them.  Both packages
  define Symbol#in, and when you load them both things don't behave properly.  Hopefully
  someone more versed in mongoid can rewrite the spec to not depend on the Symbol extensions.

lib/cancan/data_mapper_additions.rb

@@ -0,0 +1,69 @@
+require 'dm-core'
+
+module CanCan
+  module Ability
+    # could use alias_method_chain, but it's not worth adding activesupport as a gem dependency
+    alias_method :query_without_data_mapper_support, :query
+    def query(action, subject)
+      if Object.const_defined?('DataMapper') && subject <= DataMapper::Resource
+        query_with_data_mapper_support(action, subject)
+      else
+        query_without_data_mapper_support(action, subject)
+      end
+    end
+
+    def query_with_data_mapper_support(action, subject)
+      DataMapperQuery.new(subject, relevant_rules_for_query(action, subject))
+    end
+  end
+
+  class DataMapperQuery
+    def initialize(sanitizer, rules)
+      @sanitizer = sanitizer
+      @rules = rules
+    end
+
+    def conditions
+      @rules.map {|r| r.instance_variable_get(:@conditions) }
+    end
+  end
+
+  # This module is automatically included into all Active Record models.
+  module DataMapperAdditions
+    module ClassMethods
+      # Returns a scope which fetches only the records that the passed ability
+      # can perform a given action on. The action defaults to :read. This
+      # is usually called from a controller and passed the +current_ability+.
+      #
+      #   @articles = Article.accessible_by(current_ability)
+      #
+      # Here only the articles which the user is able to read will be returned.
+      # If the user does not have permission to read any articles then an empty
+      # result is returned. Since this is a scope it can be combined with any
+      # other scopes or pagination.
+      #
+      # An alternative action can optionally be passed as a second argument.
+      #
+      #   @articles = Article.accessible_by(current_ability, :update)
+      #
+      # Here only the articles which the user can update are returned. This
+      # internally uses Ability#conditions method, see that for more information.
+      def accessible_by(ability, action = :read)
+        query = ability.query(action, self)
+
+        scope = all(:conditions => ['true=false'])
+        query.conditions.each do |condition|
+          scope += all(:conditions => condition)
+        end
+
+        return scope
+      end
+    end
+  end
+end
+
+if Object.const_defined?('DataMapper')
+  DataMapper::Model.class_eval do
+    include CanCan::DataMapperAdditions::ClassMethods
+  end
+end