I have a below model,
class Entry(models.Model):
blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
head = models.CharField(max_length=255)
authors = models.ManyToManyField(Author)
I have created an object in Entry model, when I try to check if there is any objects in Entry model it shows error as attached image
It is Entry.objects.filter(pk=1).exists(), since .get() is not a QuerySet, but an Entry object in this case. So you check with:
if Entry.objects.filter(pk=1).exists():
# …
else:
# …
Here however, it is probably simpler to work with a try-except clause, and thus work in an EAFP style [wiki]:
try:
entry = Entry.objects.get(pk=1)
print('k')
except Entry.DoesNotExists:
print('false')
If you need the object you can do:
entry = Entry.objects.filter(pk=1).first()
if entry:
# …
else:
# …
The .first() method returns None if the queryset is empty.
I have the following 2 models:
class Note(models.Model):
name= models.CharField(max_length=35)
class ActionItem(models.Model):
note = models.models.OneToOneField(Note, on_delete=models.CASCADE)
target = models.CharField(max_length=35)
category = models.ForeignKey(Category, blank=True, null=True, on_delete=models.CASCADE)
In other models(based on some conditions) I trigger an utility function that create a Note:
def create_note(target=None, action=None):
note = Note(target=target, name=name).save()
transaction.on_commit(
ActionItem(note=note, target=target).save())
I get the following error:
null value in column "note_id" violates not-null constraint
DETAIL: Failing row contains (6, null).
If I use:
So, I presume the error appears because save, doesn't return anything.
I need the Note to pass it as a FK to ActionItem, an be sure it was saved.
The .save() method of a model does not return anything, hence your note variable is None, and as a result the creation of an ActionItem object gets a None for note reference, and thus raises na error.
We can solve it by using Note.objects.create(..) which .saves() and returns the object:
def create_note(target=None, action=None):
note = Note.object.create(target=target, name=name)
transaction.on_commit(lambda: ActionItem.object.create(note=note, target=target))
Alternatively, we can first construct the object, and then .save() it:
def create_note(target=None, action=None):
note = Note(target=target, name=name)
note.save()
transaction.on_commit(lambda: ActionItem.object.create(note=note, target=target))
You can use create function instead of save function
def create_note(target=None, action=None):
note = Note.objects.create(name=name)
actionItem = ActionItem.object.create(note=note, target=target)
I am going through Tango with Django and I can't solve this excercise.
I get django.db.utils.IntegrityError: UNIQUE constraint failed: rango_category.name error. This is after I try to implement views attribute to Category object. This is a excerpt from my database population script. I though maybe that I should makemigrations and then migrate to update models for DB. However, this didn't help.
cats = {
'Python' : {'pages': python_pages, 'views':128},
'Django': {'pages': django_pages, 'views':128},
"Other Frameworks": {'pages':other_pages, 'views':128},
}
for cat, cat_data in cats.items():
c = add_cat(cat, cat_data['views'])
for p in cat_data["pages"]:
add_page(c, p['title'], p['url'])
for c in Category.objects.all():
for p in Page.objects.filter(category=c):
print("- {0} - {1}".format(str(c), str(p)))
def add_cat(name, views):
c = Category.objects.get_or_create(name=name, views=views)[0]
c.views=views
c.save()
return c
Adding Category model:
class Category(models.Model):
name = models.CharField(max_length=128, unique=True)
views = models.IntegerField(default=0)
class Meta:
verbose_name_plural = 'Categories'
def __str__(self):
return self.name
You got the error because unique = True in name = models.CharField(max_length=128, unique=True) means that Django will raise constraint errror UNIQUE constraint failed in case you trying to save a new instance with the same name value; A violation of the unique constraint
get_or_create doesn't work because views=views that may be different even though name equals name
When you create or get your instance, you can do so with only the name field
def add_cat(name, views):
c = Category.objects.get_or_create(name=name, views=views)[0]
c.views=views
c.save()
return c
As mentioned in comments, the error is caused by violation of the unique constraint on Category.name. The reason that this is happening might not be completely obvious, however. When you call Category.objects.get_or_create, you are asking django to look for an object with both the given name and the given number of views. If one is not found, then django tries to create one, which violates the unique constraint if there is already a Category of the same name, but with a different number of views. Depending on desired behavior, you can fix by either:
remove unique constraint altogether
change unique constraint to a unique_together constraint
change the model reference to first get or create by name, then set (or modify) the views attribute (don't forget to save)
I'm trying to write an internal API in my application without necessarily coupling it with the database.
class Product(models.Model):
name=models.CharField(max_length=4000)
price=models.IntegerField(default=-1)
currency=models.CharField(max_length=3, default='INR')
class Image(models.Model):
# NOTE -- Have changed the table name to products_images
width=models.IntegerField(default=-1)
height=models.IntegerField(default=-1)
url=models.URLField(max_length=1000, verify_exists=False)
product=models.ForeignKey(Product)
def create_product:
p=Product()
i=Image(height=100, widght=100, url='http://something/something')
p.image_set.add(i)
return p
Now, when I call create_product() Django throws up an error:
IntegrityError: products_images.product_id may not be NULL
However, if I call p.save() & i.save() before calling p.image_set.add(i) it works. Is there any way that I can add objects to a related object set without saving both to the DB first?
def create_product():
product_obj = Product.objects.create(name='Foobar')
image_obj = Image.objects.create(height=100, widght=100, url='http://something/something', product=product_obj)
return product_obj
Explanation:
Product object has to be created first and then assign it to the Image object because id and name here is required field.
I am wondering why wouldn't you not require to make a product entry in DB in first case? If there is any specific reason then i may suggest you some work around?
EDIT: Okay! i think i got you, you don't want to assign a product to an image object initially. How about creating a product field as null is equal to true.
product = models.ForeignKey(Product, null=True)
Now, your function becomes something like this:
def create_product():
image_obj = Image.objects.create(height=100, widght=100, url='http://something/something')
return image_obj
Hope it helps you?
I got same issue with #Saurabh Nanda
I am using Django 1.4.2. When I read in django, i see that
# file django/db/models/fields/related.py
def get_query_set(self):
try:
return self.instance._prefetched_objects_cache[rel_field.related_query_name()]
except (AttributeError, KeyError):
db = self._db or router.db_for_read(self.model, instance=self.instance)
return super(RelatedManager,self).get_query_set().using(db).filter(**self.core_filters)
# file django/db/models/query.py
qs = getattr(obj, attname).all()
qs._result_cache = vals
# We don't want the individual qs doing prefetch_related now, since we
# have merged this into the current work.
qs._prefetch_done = True
obj._prefetched_objects_cache[cache_name] = qs
That 's make sese, we only need to set property _prefetched_objects_cache for the object.
p = Product()
image_cached = []
for i in xrange(100):
image=Image(height=100, widght=100, url='http://something/something')
image_cached.append(image)
qs = p.images.all()
qs._result_cache = image_cached
qs._prefetch_done = True
p._prefetched_objects_cache = {'images': qs}
Your problem is that the id isn't set by django, but by the database (it's represented in the database by an auto-incremented field), so until it's saved there's no id. More about this in the documentation.
I can think of three possible solutions:
Set a different field of your Image model as the primary key (documented here).
Set a different field of your Production model as the foreign key (documented here).
Use django's database transactions API (documented here).
I have model Foo which has field bar. The bar field should be unique, but allow nulls in it, meaning I want to allow more than one record if bar field is null, but if it is not null the values must be unique.
Here is my model:
class Foo(models.Model):
name = models.CharField(max_length=40)
bar = models.CharField(max_length=40, unique=True, blank=True, null=True, default=None)
And here is the corresponding SQL for the table:
CREATE TABLE appl_foo
(
id serial NOT NULL,
"name" character varying(40) NOT NULL,
bar character varying(40),
CONSTRAINT appl_foo_pkey PRIMARY KEY (id),
CONSTRAINT appl_foo_bar_key UNIQUE (bar)
)
When using admin interface to create more than 1 foo objects where bar is null it gives me an error: "Foo with this Bar already exists."
However when I insert into database (PostgreSQL):
insert into appl_foo ("name", bar) values ('test1', null)
insert into appl_foo ("name", bar) values ('test2', null)
This works, just fine, it allows me to insert more than 1 record with bar being null, so the database allows me to do what I want, it's just something wrong with the Django model. Any ideas?
EDIT
The portability of the solution as far as DB is not an issue, we are happy with Postgres.
I've tried setting unique to a callable, which was my function returning True/False for specific values of bar, it didn't give any errors, however seamed like it had no effect at all.
So far, I've removed the unique specifier from the bar property and handling the bar uniqueness in the application, however still looking for a more elegant solution. Any recommendations?
Django has not considered NULL to be equal to NULL for the purpose of uniqueness checks since ticket #9039 was fixed, see:
http://code.djangoproject.com/ticket/9039
The issue here is that the normalized "blank" value for a form CharField is an empty string, not None. So if you leave the field blank, you get an empty string, not NULL, stored in the DB. Empty strings are equal to empty strings for uniqueness checks, under both Django and database rules.
You can force the admin interface to store NULL for an empty string by providing your own customized model form for Foo with a clean_bar method that turns the empty string into None:
class FooForm(forms.ModelForm):
class Meta:
model = Foo
def clean_bar(self):
return self.cleaned_data['bar'] or None
class FooAdmin(admin.ModelAdmin):
form = FooForm
** edit 11/30/2015: In python 3, the module-global __metaclass__ variable is no longer supported.
Additionaly, as of Django 1.10 the SubfieldBase class was deprecated:
from the docs:
django.db.models.fields.subclassing.SubfieldBase has been deprecated and will be removed in Django 1.10.
Historically, it was used to handle fields where type conversion was needed when loading from the database,
but it was not used in .values() calls or in aggregates. It has been replaced with from_db_value().
Note that the new approach does not call the to_python() method on assignment as was the case with SubfieldBase.
Therefore, as suggested by the from_db_value() documentation and this example, this solution must be changed to:
class CharNullField(models.CharField):
"""
Subclass of the CharField that allows empty strings to be stored as NULL.
"""
description = "CharField that stores NULL but returns ''."
def from_db_value(self, value, expression, connection, contex):
"""
Gets value right out of the db and changes it if its ``None``.
"""
if value is None:
return ''
else:
return value
def to_python(self, value):
"""
Gets value right out of the db or an instance, and changes it if its ``None``.
"""
if isinstance(value, models.CharField):
# If an instance, just return the instance.
return value
if value is None:
# If db has NULL, convert it to ''.
return ''
# Otherwise, just return the value.
return value
def get_prep_value(self, value):
"""
Catches value right before sending to db.
"""
if value == '':
# If Django tries to save an empty string, send the db None (NULL).
return None
else:
# Otherwise, just pass the value.
return value
I think a better way than overriding the cleaned_data in the admin would be to subclass the charfield - this way no matter what form accesses the field, it will "just work." You can catch the '' just before it is sent to the database, and catch the NULL just after it comes out of the database, and the rest of Django won't know/care. A quick and dirty example:
from django.db import models
class CharNullField(models.CharField): # subclass the CharField
description = "CharField that stores NULL but returns ''"
__metaclass__ = models.SubfieldBase # this ensures to_python will be called
def to_python(self, value):
# this is the value right out of the db, or an instance
# if an instance, just return the instance
if isinstance(value, models.CharField):
return value
if value is None: # if the db has a NULL (None in Python)
return '' # convert it into an empty string
else:
return value # otherwise, just return the value
def get_prep_value(self, value): # catches value right before sending to db
if value == '':
# if Django tries to save an empty string, send the db None (NULL)
return None
else:
# otherwise, just pass the value
return value
For my project, I dumped this into an extras.py file that lives in the root of my site, then I can just from mysite.extras import CharNullField in my app's models.py file. The field acts just like a CharField - just remember to set blank=True, null=True when declaring the field, or otherwise Django will throw a validation error (field required) or create a db column that doesn't accept NULL.
You can add UniqueConstraint with condition of nullable_field=null and not to include this field in fields list.
If you need also constraint with nullable_field wich value is not null, you can add additional one.
Note: UniqueConstraint was added since django 2.2
class Foo(models.Model):
name = models.CharField(max_length=40)
bar = models.CharField(max_length=40, unique=True, blank=True, null=True, default=None)
class Meta:
constraints = [
# For bar == null only
models.UniqueConstraint(fields=['name'], name='unique__name__when__bar__null',
condition=Q(bar__isnull=True)),
# For bar != null only
models.UniqueConstraint(fields=['name', 'bar'], name='unique__name__when__bar__not_null')
]
Because I am new to stackoverflow I am not yet allowed to reply to answers, but I would like to point out that from a philosophical point of view, I can't agree with the most popular answer tot this question. (by Karen Tracey)
The OP requires his bar field to be unique if it has a value, and null otherwise. Then it must be that the model itself makes sure this is the case. It cannot be left to external code to check this, because that would mean it can be bypassed. (Or you can forget to check it if you write a new view in the future)
Therefore, to keep your code truly OOP, you must use an internal method of your Foo model. Modifying the save() method or the field are good options, but using a form to do this most certainly isn't.
Personally I prefer using the CharNullField suggested, for portability to models I might define in the future.
The quick fix is to do :
def save(self, *args, **kwargs):
if not self.bar:
self.bar = None
super(Foo, self).save(*args, **kwargs)
This is fixed now that https://code.djangoproject.com/ticket/4136 is resolved. In Django 1.11+ you can use models.CharField(unique=True, null=True, blank=True) without having to manually convert blank values to None.
Another possible solution
class Foo(models.Model):
value = models.CharField(max_length=255, unique=True)
class Bar(models.Model):
foo = models.OneToOneField(Foo, null=True)
I recently had the same requirement. Instead of subclassing different fields, I chose to override the save() metod on my model (named 'MyModel' below) as follows:
def save(self):
"""overriding save method so that we can save Null to database, instead of empty string (project requirement)"""
# get a list of all model fields (i.e. self._meta.fields)...
emptystringfields = [ field for field in self._meta.fields \
# ...that are of type CharField or Textfield...
if ((type(field) == django.db.models.fields.CharField) or (type(field) == django.db.models.fields.TextField)) \
# ...and that contain the empty string
and (getattr(self, field.name) == "") ]
# set each of these fields to None (which tells Django to save Null)
for field in emptystringfields:
setattr(self, field.name, None)
# call the super.save() method
super(MyModel, self).save()
If you have a model MyModel and want my_field to be Null or unique, you can override model's save method:
class MyModel(models.Model):
my_field = models.TextField(unique=True, default=None, null=True, blank=True)
def save(self, **kwargs):
self.my_field = self.my_field or None
super().save(**kwargs)
This way, the field cannot be blank will only be non-blank or null. nulls do not contradict uniqueness
For better or worse, Django considers NULL to be equivalent to NULL for purposes of uniqueness checks. There's really no way around it short of writing your own implementation of the uniqueness check which considers NULL to be unique no matter how many times it occurs in a table.
(and keep in mind that some DB solutions take the same view of NULL, so code relying on one DB's ideas about NULL may not be portable to others)