Allow arel to use tables that contain a column type that it doesn't k…

…now how to query against.

lib/arel/engines/sql/attributes.rb

@@ -15,7 +15,7 @@ def self.for(column)
         when :binary    then String
         when :boolean   then Boolean
         else
-          raise NotImplementedError, "Column type `#{column.type}` is not currently handled"
+          Undefined
         end
       end
 
@@ -28,6 +28,11 @@ def type_cast(value)
         @column.type_cast(value)
       end
 
+      # Attribute type for column types that Arel doesn't know how to handle.
+      class Undefined < Arel::Attribute
+        include Attributes
+      end
+
       %w(Boolean Decimal Float Integer String Time).each do |klass|
         class_eval <<-R
           class #{klass} < Arel::Attributes::#{klass}