Read only columns

[dmitchell]

We use two types of read only columns and it would be nice to compile time (or run time if necessary) enforce that they're read only. One is supposed to be read-only but unenforced: that's

    created_at              datetime   default CURRENT_TIMESTAMP not null,
    updated_at              datetime   default CURRENT_TIMESTAMP not null on update CURRENT_TIMESTAMP,

where any code setting these is likely to screw up timezone and thus create unintentionally incomparable values.

The other is computed columns which will give a runtime error if the code tries to set them.

It'd be great to be able to declare these readonly in the Table<> definition.