class ActiveRecord::ConnectionAdapters::MySQL::SchemaCreation

def visit_ChangeColumnDefaultDefinition(o)
  sql = +"ALTER COLUMN #{quote_column_name(o.column.name)} "
  if o.default.nil? && !o.column.null
    sql << "DROP DEFAULT"
  else
    sql << "SET DEFAULT #{quote_default_expression(o.default, o.column)}"
  end
end