Why is this giving me an MRO error? - django

I'm using Django 1.8rc1. When I try to do a makemigrations with this models.py:
from django.db import models
from datetime import datetime
class TrackedModel(models.Model):
created_date = models.DateField()
modified_date = models.DateField()
class Meta:
abstract = True
class Project(models.Model):
name = models.CharField(max_length=12)
due_date = models.DateField()
complete_date = models.DateField(default=datetime.now)
I am getting:
TypeError: Cannot create a consistent method resolution
order (MRO) for bases Model, TrackedModel
I can't even see where it would be getting confused over methods with such a simple abstract model. In case you're wondering, the Project model is inheriting from models.Model in the example but that was just to troubleshoot -- ultimately I want Project to inherit from TrackedModel.
What am I missing?

Figured it out. The migration history got me again. I had to clear out the files in my app's migrations folder. Apparently it had stored a previous set of models that were not set up right, during a previous migration.

Related

Report building in Django

I am trying to build a system which will enable my users to create their own reports based on the model of their choosing without me having to code them every time they need updating.
In order to do this, I've come up with the following models:-
from django.db import models
from django.contrib.contenttypes.models import ContentType
class ReportField(models.Model):
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
data_method = models.CharField(max_length=100)
def get_value_for(self, object):
return getattr(object, self.data_method)
class Report(models.Model):
name = models.CharField(max_length=200)
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
data_fields = models.ManyToManyField(ReportField)
The idea is that users can create a Report based on the model they're interested in. They can then add any number of ReportFields to that report and, when the report runs, it will call the data_method (the name of a property) on each instance of the model in the db.
The bit I'm having trouble with is defining which properties the users can have access to. I need to have a way of creating a load of ReportFields with certain data_methods for each model. But I don't want to create them by hand - I want it to work in a similar way to the way Permissions work in Django, if that's possible, like this:-
class MyModel(models.Model):
class Meta:
data_methods = (
('property_name_1', 'Property Name 1'),
('property_name_2', 'Property Name 2'),
etc.
)
From reading the source code, Django seems to run a management command after every migration on that model to make sure the model permissions are created. Is that the only way to do this? Am I going in the right direction here, or is there a better way?

(fields.E300) Field defines a relation with model which is either not installed, or is abstract

I have 2 apps installed in my Django Project "aplikacja"
The first one named: "Godzina"
from django.db import models
class Godzina (models.Model):
GODZINA = (
('19', '19'),
('20', '20'),
('21', '21'),
)
godzina = models.CharField(max_length=6, choices=GODZINA, verbose_name='jezyk')
and the second named: "UserProfile"
from django.db import models
from django.contrib.auth.models import User
from godzina.models import Godzina
class UserProfile(models.Model):
czas = models.ForeignKey('Godzina')
user = models.OneToOneField(User)
I'm getting such error:
userprofile.UserProfile.czas: (fields.E300) Field defines a relation with model 'Godzina', which is either not installed, or is abstract.
What does it mean? I would like that User can only pick such time as an administrator put in the app "Godzina" For example I'm defining hours 19 pm, 20 pm and then user can choose those values in UserProfile app
Is it possible to fix this problem?
You should add the app name to the related model name in the FK definition:
czas = models.ForeignKey('firstapp.Godzina')
A generic information when using the models from one app to another app is
model_variable = models.ForeignKey('the_appname.the_model_class_name')
In this case,for the Django-project “aplikacja”, for the second app(UserProfile) it should be :
czas = models.ForeignKey(‘Godzina.Godzina')
After this I suggest you remove all the files in the migrations folder(under the apps you created: Godzina and UserProfile) except the init file. Also remove the SQLite file. Then run
Python manage.py makemigrations
and
python manage.py migrate.
These steps should most probably fix the problem.
Not related to OPs scenario but one potential mistake that will cause the same above error is specifying wrong app_label in Meta
class ABC(models.Model):
foo = models.TextField()
class Meta:
app_label = 'appp'
class XYZ(models.Model):
abc = models.ForeignKey('ABC', on_delete=models.CASCADE)
class Meta:
app_label = 'app'
Errors:
django.core.management.base.SystemCheckError: SystemCheckError: System check identified some issues:
ERRORS:
app.XYZ.abc: (fields.E300) Field defines a relation with model 'ABC', which is either not installed, or is abstract.
app.XYZ.abc: (fields.E307) The field app.XYZ.abc was declared with a lazy reference to 'app.abc', but app 'app' doesn't provide model 'abc'.
I had the same error, and it was an issue due to the similar names of the classes.
from django.db import models
from django.contrib.auth.models import User
from godzina.models import Godzina
class UserProfile(models.Model):
czas = models.ForeignKey('Godzina')
user = models.OneToOneField(User)
class Meta: #you have to add abstract class
abstract = True
More clarity for users that may encounter similar problem.
It may be confusing if the app name and the model name is the same, for example
the app name is "student", and in student/models, you have a class called
Student, which inherits models.Model
place holder implementation:
model_variable = models.ForeignKey('the_appname.the_model_class_name)
more precise example:
with an app name as student and model name also Student, and another app attendance, with an attribute "student_id"
student_id = models.ForeignKey('student.student)
If you arrived here by search, it might help you to have a reminder to check that you remembered to make your class a model (just in case, like me, you were blindly looking for more complicated reasons for a few minutes!)
If you didn't inherit from models.Model you'd get the error described by the OP:
class Godzina:
...
Make sure you've got:
class Godzina(models.Model):
...
run into the same issue in
django-simple-history==2.10.0,
django=2.2.13
to fix it I need to indicate model instead of using it name str 'Godzina'
class UserProfile(models.Model):
czas = models.ForeignKey(Godzina, on_delete=xxxxx)

Django migration error :you cannot alter to or from M2M fields, or add or remove through= on M2M fields

I'm trying to modify a M2M field to a ForeignKey field. The command validate shows me no issues and when I run syncdb :
ValueError: Cannot alter field xxx into yyy they are not compatible types (you cannot alter to or from M2M fields, or add or remove through= on M2M fields)
So I can't make the migration.
class InstituteStaff(Person):
user = models.OneToOneField(User, blank=True, null=True)
investigation_area = models.ManyToManyField(InvestigationArea, blank=True,)
investigation_group = models.ManyToManyField(InvestigationGroup, blank=True)
council_group = models.ForeignKey(CouncilGroup, null=True, blank=True)
#profiles = models.ManyToManyField(Profiles, null = True, blank = True)
profiles = models.ForeignKey(Profiles, null = True, blank = True)
Any suggestions?
I stumbled upon this and although I didn't care about my data much, I still didn't want to delete the whole DB. So I opened the migration file and changed the AlterField() command to a RemoveField() and an AddField() command that worked well. I lost my data on the specific field, but nothing else.
I.e.
migrations.AlterField(
model_name='player',
name='teams',
field=models.ManyToManyField(related_name='players', through='players.TeamPlayer', to='players.Team'),
),
to
migrations.RemoveField(
model_name='player',
name='teams',
),
migrations.AddField(
model_name='player',
name='teams',
field=models.ManyToManyField(related_name='players', through='players.TeamPlayer', to='players.Team'),
),
NO DATA LOSS EXAMPLE
I would say: If machine cannot do something for us, then let's help it!
Because the problem that OP put here can have multiple mutations, I will try to explain how to struggle with that kind of problem in a simple way.
Let's assume we have a model (in the app called users) like this:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person)
def __str__(self):
return self.name
but after some while we need to add a date of a member join. So we want this:
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership') # <-- through model
def __str__(self):
return self.name
# and through Model itself
class Membership(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
Now, normally you will hit the same problem as OP wrote. To solve it, follow these steps:
start from this point:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person)
def __str__(self):
return self.name
create through model and run python manage.py makemigrations (but don't put through property in the Group.members field yet):
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person) # <-- no through property yet!
def __str__(self):
return self.name
class Membership(models.Model): # <--- through model
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
create an empty migration using python manage.py makemigrations users --empty command and create conversion script in python (more about the python migrations here) which creates new relations (Membership) for an old field (Group.members). It could look like this:
# Generated by Django A.B on YYYY-MM-DD HH:MM
import datetime
from django.db import migrations
def create_through_relations(apps, schema_editor):
Group = apps.get_model('users', 'Group')
Membership = apps.get_model('users', 'Membership')
for group in Group.objects.all():
for member in group.members.all():
Membership(
person=member,
group=group,
date_joined=datetime.date.today()
).save()
class Migration(migrations.Migration):
dependencies = [
('myapp', '0005_create_models'),
]
operations = [
migrations.RunPython(create_through_relations, reverse_code=migrations.RunPython.noop),
]
remove members field in the Group model and run python manage.py makemigrations, so our Group will look like this:
class Group(models.Model):
name = models.CharField(max_length=128)
add members field the the Group model, but now with through property and run python manage.py makemigrations:
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
and that's it!
Now you need to change creation of members in a new way in your code - by through model. More about here.
You can also optionally tidy it up, by squashing these migrations.
Potential workarounds:
Create a new field with the ForeignKey relationship called profiles1 and DO NOT modify profiles. Make and run the migration. You might need a related_name parameter to prevent conflicts. Do a subsequent migration that drops the original field. Then do another migration that renames profiles1 back to profiles. Obviously, you won't have data in the new ForeignKey field.
Write a custom migration: https://docs.djangoproject.com/en/1.7/ref/migration-operations/
You might want to use makemigration and migration rather than syncdb.
Does your InstituteStaff have data that you want to retain?
If you're still developing the application, and don't need to preserve your existing data, you can get around this issue by doing the following:
Delete and re-create the db.
go to your project/app/migrations folder
Delete everything in that folder with the exception of the init.py file. Make sure you also delete the pycache dir.
Run syncdb, makemigrations, and migrate.
Another approach that worked for me:
Delete the existing M2M field and run migrations.
Add the FK field and run migrations again.
FK field added in this case has no relation to the previously used M2M field and hence should not create any problems.
This link helps you resolve all problems related to this
The one which worked for me is python3 backend/manage.py migrate --fake "app_name"
I literally had the same error for days and i had tried everything i saw here but still didn'y work.
This is what worked for me:
I deleted all the files in migrations folder exceps init.py
I also deleted my database in my case it was the preinstalled db.sqlite3
After this, i wrote my models from the scratch, although i didn't change anything but i did write it again.
Apply migrations then on the models and this time it worked and no errors.
This worked for Me as well
Delete last migrations
run command python manage.py migrate --fake <application name>
run command 'python manage.py makemigrations '
run command 'python manage.py migrate'
Hope this will solve your problem with deleting database/migrations
First delete the migrations in your app (the folders/ files under 'migrations'
folder)
Showing the 'migrations' folder
Then delete the 'db.sqlite3' file
Showing the 'db.sqlite3' file
And run python manage.py makemigrations name_of_app
Finally run python manage.py migrate
I had the same problem and found this How to Migrate a ‘through’ to a many to many relation in Django article which is really really helped me to solve this problem. Please have a look. I'll summarize his answer here,
There is three model and one(CollectionProduct) is going to connect as many-to-many relationship.
This is the final output,
class Product(models.Model):
pass
class Collection(models.Model):
products = models.ManyToManyField(
Product,
blank=True,
related_name="collections",
through="CollectionProduct",
through_fields=["collection", "product"],
)
class CollectionProduct(models.Model):
collection = models.ForeignKey(Collection, on_delete=models.CASCADE)
product = models.ForeignKey(Product, on_delete=models.CASCADE)
class Meta:
db_table = "product_collection_products"
and here is the solution,
The solution
Take your app label (the package name, e.g. ‘product’) and your M2M field name, and combine them together with and underscore:
APPLABEL + _ + M2M TABLE NAME + _ + M2M FIELD NAME
For example in our case, it’s this:
product_collection_products
This is your M2M’s through database table name. Now you need to edit your M2M’s through model to this:
Also found another solution in In Django you cannot add or remove through= on M2M fields article which is going to edit migration files. I didn't try this, but have a look if you don't have any other solution.
this happens when adding 'through' attribute to an existing M2M field:
as M2M fields are by default handled by model they are defined in (if through is set).
although when through is set to new model the M2M field is handled by that new model, hence the error in alter
solutions:-
you can reset db or
remove those m2m fields and run migration as explained above then create them again
*IF YOU ARE IN THE INITIAL STAGES OF DEVELOPMENT AND CAN AFFORD TO LOOSE DATA :)
delete all the migration files except init.py
then apply the migrations.
python manage.py makemigrations
python manage.py migrate
this will create new tables.

Django - "last_modified" or "auto_now_add" for an App (or more than one Model?)

I know Django has a feature of last_modified field (models.DateTimeField(auto_now_add=True)
)..
but let's say I have a certain App, and I want to know when was the last change for any of its Model (I don't really care which model was changed, I just want to know when was the latest change for this app..)
do I really have to write a last_modified field for each model (I have 9 of them for the moment...), and then check for each of them which is the latest?
any help will be appreciated :)
Thanks
You could create a base class that defines the last_modified field...
class YourBaseClassName(models.Model):
last_modified = models.DateTimeField(auto_now=True)
and then inherit from that
class AnotherClass(YourBaseClassName):
another_field = models.CharField(max_length=50)
In The End I made a table for constants for my app (actually I had it before for use of other things).
so the Table looks like this:
from django.db import models
from django.db.models.signals import post_save
class Constant(models.Model):
name = models.CharField(max_length=50)
value = models.CharField(max_length=50)
and added a consant named "version_date".
Than, I added this code to the bottom of my models.py, to track all changes in all the models in the app.
myapp = models.get_app('myapp')
models2track = models.get_models(myapp)
def update_version(sender, **kwargs):
for model in models2track:
post_save.disconnect(update_version, sender=model, dispatch_uid="some_uid"+model._meta.db_table)
version_date = Constant.objects.get_or_create(id=1,name="version date")[0]
version_date.value = str(int(time.time()))
version_date.save()
for model in models2track:
post_save.connect(update_version, sender=model, dispatch_uid="some_uid"+model._meta.db_table)
for model in models2track:
post_save.connect(update_version, sender=model, dispatch_uid="some_uid"+model._meta.db_table)
This way, I don't need to change my DB Schema.. only need to add the code mentioned.
thanks all

ForeignKey to multiple Models or Queryset

It is possible to make a ForeignKey to more than one model. I want to choose from different models like Parts and Machines Model.
I read this to combine multiple models into one list: How to combine 2 or more querysets in a Django view?
How can I get foreign key to that list somehow?
I know that you asked this over year ago, but I had a similar problem and I want to share a link to the solution for future readers.
Generally the contenttypes framework solves this problem, and I guess this is what Daniel Roseman was talking about.
How to use dynamic foreignkey in Django?
You need generic relations.
A generic relation allows you to dynamically the target model of the foreign key.
I'll provide a comprehensive answer for this question, I know its quite old, but it's still relevant.
We're gonna be using Generic Relations.
First, in settings.py make sure that django.contrib.contenttypes is included in the INSTALLED_APPS array.
Let's create a new model in models.py:
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
With content_type we can associate Image with any other model class, while object_id will hold the other model instance.
class Image(models.Model):
image = models.ImageField(
upload_to="imgs/products", blank=True)
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey()
To refer back to the Image model from a Company instance we need to make a reverse generic relation
class Company(models.Model):
name = models.CharField(max_length=100)
images = GenericRelation(Image)
In schema.py, we can create Images in a Company instance like:
company_instance = Company(name="Apple")
company_instance.save()
for img in imgs:
#Image(image=img, content_object=company_instance)
company_instance.images.create(image=img)
company_instance.images.all() # fetch all images
the company_instance.images field is just a GenericRelatedObjectManager (docs)
This is how the final Image table looks in the database:
The Django-polymorphic library provides a simple solution that should work well with the admin and forms too using formsets.
For example:
from polymorphic.models import PolymorphicModel
class BookOwner(PolymorphicModel):
book = models.ForeignKey(Book, on_delete=models.CASCADE)
class StaffBookOwner(BookOwner):
owner = models.ForeignKey(Staff, on_delete=models.CASCADE)
class StudentBookOwner(BookOwner):
owner = models.ForeignKey(Student, on_delete=models.CASCADE)
With this, you can use the parent model to set the owner to either a Staff or Student instance or use the child models directly.