keep ivars private, do not manipulate them outside their owner object

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -236,6 +236,10 @@ def column(name, type, options = {})
         self
       end
 
+      def remove_column(name)
+        @columns_hash.delete name.to_s
+      end
+
       [:string, :text, :integer, :float, :decimal, :datetime, :timestamp, :time, :date, :binary, :boolean].each do |column_type|
         define_method column_type do |*args|
           options = args.extract_options!

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -458,7 +458,7 @@ def add_column(table_name, column_name, type, options = {}) #:nodoc:
 
       def remove_column(table_name, column_name, type = nil, options = {}) #:nodoc:
         alter_table(table_name) do |definition|
-          definition.columns.delete(definition[column_name])
+          definition.remove_column column_name
         end
       end