How to get the value of the counter of an AutoField, such as the usual id field of most models?
At the moment, I do:
MyModel.objects.latest('id').id
But that does not work when all the objects have been deleted from the database.
Of course, a database-agnostic answer would be best.
EDIT
The accepted answer in Model next available primary key is not very relevant to my question, as I do not intend to use the counter value to create a new object. Also I don't mind if the value I get is not super accurate.
Background.
AFAIK there isn't a database agnostic query. Different databases handle auto increment differently and there rarely is a use case for django to find out what the next possible auto increment ID is.
To elaborate further, in postgresql you could do select nextval('my_sequence') while in mysql you would need to use the last_insert_id() but what this returns is the ID for the last insert and not the next one these two may actually be very different! To get the actual value you would need to use 'SHOW TABLE STATUS'
Solution.
Create a record, save it, inspect it's ID and delete it.
This will change the next id but you have indicated that you need only an approximation.
The alternative is to do a manual transaction with a rollback. This too would alter the next id in case of mysql.
from django.db import transaction
#transaction.atomic
def find_next_val(mymodel):
try:
# ...
obj = mymoel.objects.create(....)
print obj.id
raise IntegrityError
except IntegrityError:
pass
Related
Given RangeA of auto-incrementing primary key integers (1 to 32767) and RangeB (32768 to 2147483647). If a condition is true, save an object with a primary key assigned in RangeA, else save it in RangeB.
The admin (me) would be the only user saving to RangeA. If the above is not possible: It would be not ideal but still usable if Django always saved in RangeB and it required going into shell to save in RangeA.
How can this be done using Django and Postgres?
Quite possible. First thing is to change your model so that it doesn't use the standard AutoField as the primary key
class MyModel(models.Model):
id = models.IntegerField(primary_key=True)
Then you need to connect to postgresql and create two different sequences.
CREATE SEQUENCE small START 1;
CREATE SEQUENCE big START 32768;
Instead of typing that into the PSQL console, you might also consider editing the django migration (using a RunSQL directive) to create create these.
Next step is to override the save method
def save(self,*args, **kwargs)
if not self.id :
cursor = connection.cursor()
if small condition:
cursor.execute("select nextval('small')")
else:
cursor.execute("select nextval('big')")
self.id = cursor.fetchone()[0]
super(MyModel,self).save(*args,**kwargs)
Alternative to overriding the save method is to create a postgresql BEFORE INSERT Trigger. The round trip to get the nextval isn't very costly but if this is a concern creating a trigger is the option to choose. In that case you don't need to over ride the save method but you have to implement the same logic inside the trigger.
I have a model like
class Item(models.Model):
site = Site()
id_on_site = PositiveIntegerField()
Now i want to create an instance Item(current_site, next_id_on_site) with
next_id_on_site = Item.objects.filter(site=current_site).aggregate(current_id=Max("id_on_site"))['current_id']+1
The problem is, that the operation of generating the ID and creating the Item is not atomic, so there is a race condition which creates duplicate IDs, so .get(site=current_site, id_on_site=someid) will raise a MultipleObjectsReturned exception.
Using unique_together in the model does not help with the generation of the auto increment ID and doesn't seem to be implemented at the DB level at all.
unique_together is definitely implemented in the database, but since it generates a unique database index, you may need to run a migration to see its effects.
If all you want is for id_on_site to be some unique identifier for the item at the site, it might be easier to use something like a UUIDField(default=uuid.uuid4), which has a near-certain guarantee of uniqueness. If you need the ID to be an auto-incrementing integer, that's a bit harder.
One option to avoid the race condition would be to lock the row with the highest id_on_site value. (This will only work on certain database backends, e.g. postgres):
from django.db.transaction import atomic
with atomic():
next_id_on_site = (
Item.objects
.filter(site=current_site)
.select_for_update(nowait=False)
.latest('id_on_site').id_on_site)
Item.objects.create(current_site, next_id_on_site)
This should cause other transactions to block if they're trying to get the highest id_on_site, and once the transaction is committed, the item you just inserted will be returned to the other transaction. This could be problematic if the transaction is long-lived for some reason.
I'm reading django doc and see how django knows to do the update or insert method when calling save(). The doc says:
If the object’s primary key attribute is set to a value that evaluates to True (i.e. a value other than None or the empty string), Django executes an UPDATE.
If the object’s primary key attribute is not set or if the UPDATE didn’t update anything, Django executes an INSERT link.
But in practice, when I create a new instance of a Model and set its "id" property to a value that already exist in my database records. For example: I have a Model class named "User" and have a propery named "name".Just like below:
class User(model.Model):
name=model.CharField(max_length=100)
Then I create a new User and save it:
user = User(name="xxx")
user.save()
now in my database table, a record like id=1, name="xxx" exists.
Then I create a new User and just set the propery id=1:
newuser = User(id=1）
newuser.save()
not like the doc says.when I had this down.I checked out two records in my database table.One is id = 1 ,another is id=2
So, can anyone explain this to me? I'm confused.Thanks!
Because in newer version of django ( 1.5 > ), django does not check whether the id is in the database or not. So this could depend on the database. If the database report that this is duplicate, then it will update and if the database does not report it then it will insert. Check the doc -
In Django 1.5 and earlier, Django did a SELECT when the primary key
attribute was set. If the SELECT found a row, then Django did an
UPDATE, otherwise it did an INSERT. The old algorithm results in one
more query in the UPDATE case. There are some rare cases where the
database doesn’t report that a row was updated even if the database
contains a row for the object’s primary key value. An example is the
PostgreSQL ON UPDATE trigger which returns NULL. In such cases it is
possible to revert to the old algorithm by setting the select_on_save
option to True.
https://docs.djangoproject.com/en/1.8/ref/models/instances/#how-django-knows-to-update-vs-insert
But if you want this behavior, set select_on_save option to True.
You might wanna try force_update if that is required -
https://docs.djangoproject.com/en/1.8/ref/models/instances/#forcing-an-insert-or-update
Suppose you have a model Entry, with a field "author" pointing to another model Author. Suppose this field can be null.
If I run the following QuerySet:
Entry.objects.filter(author=X)
Where X is some value. Suppose in MySQL I have setup a compound index on Entry for some other column and author_id, ideally I'd like the SQL to just use "author_id" on the Entry model, so that it can use the compound index.
It turns out that Entry.objects.filter(author=5) would work, no join is done. But, if I say author=None, Django does a join with Author, then add to the Where clause Author.id IS NULL. So in this case, it can't use the compound index.
Is there a way to tell Django to just check the pk, and not follow the link?
The only way I know is to add an additional .extra(where=['author_id IS NULL']) to the QuerySet, but I was hoping some magic in .filter() would work.
Thanks.
(Sorry I was not clearer earlier about this, and thanks for the answers from lazerscience and Josh).
Does this not work as expected?
Entry.objects.filter(author=X.id)
You can either use a model or the model id in a foreign key filter. I can't check right yet if this executes a separate query, though I'd really hope it wouldn't.
If do as you described and do not use select_related() Django will not perform any join at all - no matter if you filter for the primary key of the related object or the related itself (which doesn't make any difference).
You can try:
print Entry.objects.(author=X).query
Assuming that the foreign key to Author has the name author_id, (if you didn't specify the name of the foreign key column for ForeignKey field, it should be NAME_id, if you specified the name, then check the model definition / your database schema),
Entry.objects.filter(author_id=value)
should work.
Second Attempt:
http://docs.djangoproject.com/en/dev/ref/models/querysets/#isnull
Maybe you can have a separate query, depending on whether X is null or not by having author__isnull?
Pretty late, but I just ran into this. I'm using Q objects to build up the query, so in my case this worked fine:
~Q(author_id__gt=0)
This generates sql like
NOT ("author_id" > 0 AND "author_id" IS NOT NULL)
You could probably solve the problem in this question by using
Entry.objects.exclude(author_id__gt=0)
I have a model that contains a foreign key value, then in the form generated from this model, I want to auto select the record's key according to the record I'm adding the form's contents to...I've tried the below code, but it tells me QuerySet doesn't contain vehicle
stock = Issues.objects.filter(vehicle=id)
form = IssuesForm(initial={'id_vehicle': stock.vehicle})
I'm a bit new to django btw so any ideas are highly appreciated
filter always gives a QuerySet, which is a set of values. If you just want a single object, you should use get.
However I don't really understand why you need to do the lookup at all. You have the id value already, since you are using it to look up stock. So why don't you just pass id as the value for id_vehicle?