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.

spec/cancan/data_mapper_additions_spec.rb

@@ -0,0 +1,59 @@
+require "spec_helper"
+require 'cancan/data_mapper_additions'
+
+describe CanCan::DataMapperAdditions do
+  before(:each) do
+    @model_class = Class.new
+    @model_class.class_eval do
+      include DataMapper::Resource
+    end
+    stub(@model_class).all(:conditions => ['true=false']) { 'no-match:' }
+
+    @ability = Object.new
+    @ability.extend(CanCan::Ability)
+  end
+
+  it "should return no records when no ability is defined so no records are found" do
+    @model_class.accessible_by(@ability, :read).should == 'no-match:'
+  end
+
+  it "should call all with matching ability conditions" do
+    @ability.can :read, @model_class, :foo => {:bar => 1}
+    stub(@model_class).all(:conditions => {:foo => {:bar => 1}}) { 'found-records:' }
+    @model_class.accessible_by(@ability, :read).should == 'no-match:found-records:'
+  end
+
+  it "should merge association joins and sanitize conditions" do
+    @ability.can :read, @model_class, :foo => {:bar => 1}
+    @ability.can :read, @model_class, :too => {:car => 1, :far => {:bar => 1}}
+
+    stub(@model_class).all(:conditions => {:foo => {:bar => 1}}) { 'foo:' }
+    stub(@model_class).all(:conditions => {:too => {:car => 1, :far => {:bar => 1}}}) { 'too:' }
+
+    @model_class.accessible_by(@ability).should == 'no-match:too:foo:'
+  end
+
+  it "should allow to define sql conditions by not hash" do
+    @ability.can :read, @model_class, :foo => 1
+    @ability.can :read, @model_class, ['bar = ?', 1]
+
+    stub(@model_class).all(:conditions => {:foo => 1}) { 'foo:' }
+    stub(@model_class).all(:conditions => ['bar = ?', 1]) { 'bar:' }
+
+    @model_class.accessible_by(@ability).should == 'no-match:bar:foo:'
+  end
+
+  it "should not allow to fetch records when ability with just block present" do
+    @ability.can :read, @model_class do false end
+    lambda {
+      @model_class.accessible_by(@ability)
+    }.should raise_error(CanCan::Error)
+  end
+
+  it "should not allow to check ability on object when nonhash sql ability definition without block present" do
+    @ability.can :read, @model_class, ['bar = ?', 1]
+    lambda {
+      @ability.can? :read, @model_class.new
+    }.should raise_error(CanCan::Error)
+  end
+end