Rails - Jun 2006 - Using the database schema to help validate data

I started working on this in response to Dave Thomas'' keynote as 
RailsConf 2006.  This little patch will validate that your text fields 
will fit into the database.  Obviously this is just a start, and many 
more validations could be added.  Also, this code could probably be 
extracted into a plugin instead of patch to ActiveRecord.

Index: test/validations_test.rb
==================================================================---
test/validations_test.rb    (revision 4495)
+++ test/validations_test.rb    (working copy)
@@ -461,6 +461,13 @@
     t.title = nil
     assert t.valid?
   end
+
+  def test_validates_database_schema
+    Topic.validates_database_schema
+    t = Topic.create("title" => "a" * 256,
"content" => "whatever")
+
+    assert !t.valid?, "validation should have failed because the title 
is too long"
+  end

   def test_validates_length_of_using_maximum
     Topic.validates_length_of :title, :maximum => 5
Index: lib/active_record/validations.rb
==================================================================---
lib/active_record/validations.rb    (revision 4491)
+++ lib/active_record/validations.rb    (working copy)
@@ -250,6 +250,21 @@
         write_inheritable_set(:validate_on_update, methods)
       end

+      def validates_database_schema()
+        columns.each {|column|
+          column.primary = column.name == primary_key
+          # Setup default validation for text columns
+          if column.text? then
+            # varchars have a limit, while text columns generally do 
not
+            if column.limit then
+              # this is a hack for attributes which are reserved words 
like ''type''
+              name = "#{column.name}_before_type_cast"
+              validates_length_of name, :maximum=>column.limit
+            end
+          end
+        }
+      end
+
       def condition_block?(condition)
         condition.respond_to?("call") && (condition.arity ==
1 ||
condition.arity == -1)
       end

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