Required field doesn't get unique default values when setting it to required later on

[Hannes (Neobis)]

I created a field with a _defaults method. (see code below)
In this method was a uuid4() value.
After a while I decided that this field should become required. So, i added required=1 on the field definition and updated the database.
I expected that every existing record would get a unique UUID.
What I got was the same unique UUID for every record.

After a bit of investigation I found out where this happens:
http://bazaar.launchpad.net/~openerp/openobject-server/7.0/view/head:/openerp/osv/orm.py#L3169

And indeed, the UPDATE SQL just writes 1 default value to all records.

In my case (UUID) this was a problem and something I didn't expect.

In other cases (like datetime.now()) this can be a less important problem.

Just a simple code example below:

# -*- coding: utf-8 -*-
from openerp.osv import fields, orm
from uuid import uuid4

class MyModel(orm.Model):
    _name = 'my.model'
    _columns = {
        'reference': fields.char('Request reference', required=1),
        'state': fields.selection([('draft', 'Draft'), ('sent', 'Sent')], 'state', required=1)
    }

    def _get_default_reference(self, cr, uid, context=None):
        #TODO: use openerp sequences
        return '%s_%s' % (cr.dbname, str(uuid4()))

    _defaults = {
        'reference': _get_default_reference,
        'state': 'draft',
    }

Later I added:
    _sql_constraints = [
        ('unique_reference', 'unique(reference)', 'A record with the same reference already exists.')
    ]

Then i got:

2014-04-23 10:05:38,042 5014 ERROR db openerp.sql_db: bad query: ALTER TABLE "my_model" ADD CONSTRAINT "my_model_unique_name" unique(reference)
Traceback (most recent call last):
  File "/home/hannes/openerp/openobject-server/7.0/openerp/sql_db.py", line 226, in execute
    res = self._obj.execute(query, params)
IntegrityError: could not create unique index "my_model_unique_name"
DETAIL: Key (reference)=(db_d9833065-58df-43c9-b5db-3bb867d9102b) is duplicate