I wouldn’t save alpha in the database. I would use a property on the model or
annotate the field to the object manager.
Property:
class Guest(models.Model):
@property
def alpha(self):
return self.lname[0] if self.lname else ''
Or on the object manager:
class GuestManager(models.Manager):
def get_queryset(self):
return
super().get_queryset().annotate(alpha=Left('lname', 1))
class Guest(models.Model):
objects = GuestManager()
From: [email protected] [mailto:[email protected]] On
Behalf Of rmschne
Sent: Monday, April 15, 2019 10:57 AM
To: Django users
Subject: How to properly use MySQL Generated Fields in Django
I have a generated field in a MySQL table "alpha = upper(substr(`lname`,1,1))"
where the field lname is a last name of a person. Idea is that I get a field of
the first letter of the last name.
In Django, I have in the table 'attendees' a field alpha =
models.CharField("alpha", max_length=1, db_column='alpha', help_text='leading
alphabetic letter of lastname.')
This works ok when I a new record into the database and the field "alpha" is
computed correctly. It works where I can query out the data using Django.
Now I have discovered that if in Django I adjust one of the the other fields
(not lname), when I do a "save() on the record", Django throws an error:
"The value specified for generated column 'alpha' in table 'attendees' is not
allowed.")
The one field i am changing in the "attendee" table is a foreign key field; and
I'm giving Django the foreign key object. So I think I'm doing that correctly;
but the new attendee object is not being saved.
I've searched Google for "Django Mysql generated field" and come up with
nothing relevant on anything special needed; but I suspect I'm missing
something hence this query. Looking for something special about generated
fields in MySQL and Django, if anything special?
Some code extracts:
class Guest(models.Model):
lname = models.CharField(max_length=150, db_column='Lname',
blank=True,help_text="Last name")
alpha = models.CharField("alpha", max_length=1, db_column='alpha',
help_text='leading alphabetic letter of lastname.')
contact = models.ForeignKey(Contact, related_name='adcontact',
db_column='ContactID',blank=True, null=True, help_text="Name of linked contact
for this guest")
timestamp =
models.DateTimeField(db_column='timestamp',auto_now=True,blank=False)
cad=Guest.objects.filter(contact__id=idfrom)
print "Guest: ",len(cad)
if cad:
for i in cad:
print "i.contact:",i.id,i.contact
print "contactto:",contactto.id,contactto
i.contact=contactto
try:
i.save(["contact"])
except Exception as e:
print ">>>>Exception. cad contact save failed.",e
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