When saving in admin, readonly fields are potentially overwritten

[Fingal Plumpton]

On the admin page for a model, if a field is in readonly_fields or has editable=False, then saving that model from the admin will create an SQL query that sets the readonly field to it's value at the time the save button was clicked. This field might have been changed (e.g. by a command or some other process) during the time that the model is being saved, and with current behaviour it would then be overwritten to it's old value.

Correct behaviour would be to not set the readonly/uneditable field at all in the SQL query.

The exception to this is a DateTimeField with auto_now or auto_now_add, which have editable=False but should be updated when the SQL query is made.