postgresql怎么用change.column

def up    connection.execute(%q{        alter table contests        alter column prize type integer using cast(prize as integer),        alter column price set default 200    })enddef up    case ActiveRecord::Base.connection    when ActiveRecord::ConnectionAdapters::PostgreSQLAdapter        connection.execute(%q{            alter table contests            alter column prize type integer using cast(prize as integer),            alter column price set default 200        })    when ActiveRecord::ConnectionAdapters::MySQLAdapter        # MySQL version...    endend

ActiveRecord::Base.connection.execute("TRUNCATE TABLE #{table_name}")