I have a OnetoOne field with primary_key=True in a model.
Now I want to change that to a ForeignKey but cannot since there is no 'id'.
From this:
user = models.OneToOneField(User, primary_key=True, on_delete=models.CASCADE)
To this:
user1 = models.ForeignKey(User, related_name='questionnaire', on_delete=models.CASCADE)
Showing this while makemigrations:
You are trying to add a non-nullable field 'id' to historicaluserquestionnaire without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
2) Quit, and let me add a default in models.py
So how to do that?
Thanks!
just delete your previous migrations
e.g
migrations:
|
|- 001.initail.py
|- 002.examplefiles.py
you just have to delete you most recent migrations and
run the command
python3 manage.py makemigrations
python3 manage.py migrate
Django models have a default ID attribute. Since you changed the model, the previous data records has now conflicts with your new column attributes because when previous records were added, there were no ForeignKey. Remember, In your foreignkey, null values are not allowed. There are multiple fixes to your solution. Follow these step one-by-one and see what resolves your eror.
Delete your migrations file and run migrations again
Set NULL=True in your foreign key
OR
Delete previous records of the table to resolve the conflict, so that when you run migrations, it adds a default value of id itself
instead of deleting, you can also try and change your migrations files directly like this
https://stackoverflow.com/a/29074241/11979793
The problem is that your trying to remove the primary key, but Django is then going to add a new primary key called "id". This is non-nullable and unique, so you can't really provide a one-off default.
The easiest solution is to just create a new model and copy your table over in a SQL migration, using the old user_id to populate the id field. Be sure to reset your table sequence to avoid collisions.
Related
I have an existing model that looks somewhat like the following...
class Resource(models.Model):
id = models.AutoField(primary_key=True)
We have been using this for some time, and now have ~1M instances of these Resource objects (and associated ForeignKey/else usages) in our database.
I now have a need to track another ID on this model, one that I want to enforce is unique.
other_id = models.IntegerField(unique=True)
This other_id information is currently stored in some external CSVs, and I want to (at some point in the process) load this information in to all existing Resource instances.
After adding the above field, Django's makemigrations works just fine. However when I go to apply said migration against an existing database I get an error indicating that I need to provide a default to use for all existing Resource instances. I'm sure many of you have seen something similar.
What is the best approach to getting around this limitation? Some methods I have thought of...
Remove the unique=True requirement
apply the migration
externally load in the other_id value to all existing models (through some management command, or 1-off script)
add the unique=True back in and apply the migration
Dump all existing data to JSON
flush all tables
apply the migration (with unique=True)
write a script that loads the data back in, adding the correct other_id value
(unsure if this is possible) - Write some custom migration logic to automatically reference these external CSVs to load other_id values when I run manage.py migrate. This could hit issues if (at some point in the future) someone re-runs these migrations and this part fails (cannot find existing resource id in the CSVs to pull out other_id).
All of these feel complicated, but then again I guess what I am trying to do isn't the simplest thing either.
Any ideas? I have to imagine someone has had to work around a similar issue in the past.
Thanks!
Actually, the source or your issue is not the unique constraint by itself but the fact that your field doesn't allow nulls and has no default value - you'd have the very same error with a non-unique field.
The proper solution here is to allow the field to be null (null=True) and default it to None (which will translate to sql "null"). Since null values are excluded from unique constraints (at least if your db vendor respects SQL standard), this allow you to apply the schema change while still making sure you cannot have a duplicate for non-null values.
Then you may want a data migration to load the known "other_id" values, and eventually a third schema migration to disallow null values for this field - if and only if you know you have filled this field for all records.
Django has something called Data Migrations where you create a migration file that modifies/remove/add data to your database as you apply your migrations.
In this case you would create 3 different migrations:
Create a migration that allow null values with null=True.
Create a data migration that populate the data.
Create a migration that disallow null values by removing the null=True added in step 1.
As you then run python manage.py migrate it would apply all of the migrations in step 1-3 in the correct order.
Your data migration would look something like this:
from django.db import migrations
def populate_reference(apps, schema_editor):
MyModel = apps.get_model('yourappname', 'MyModel')
for obj in MyModel.objects.all():
obj.other_id = random_id_generator()
obj.save()
class Migration(migrations.Migration):
dependencies = [
('yourappname', '0001_initial'),
]
operations = [
migrations.RunPython(populate_reference),
]
You can create an empty migration file using the ./manage.py makemigrations --empty yourappname command.
I am new to django and I am facing some problem.
This was my previous model.
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
I added some data to this model.
After that I added one more field to it.
Now my model looks like this..
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
age = models.IntegerField()
When I ran the command python manage.py makemigrations I got the following error which is obvious.
You are trying to add a non-nullable field 'age' to blog without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
2) Quit, and let me add a default in models.py
Select an option:
I don't want to add default so I deleted the table contents by accessing db.sqlite3
But again after running python manage.py makemigrations I am getting the same error.why?
Even though you have deleted the table, any changes to your Django model will always refer to the previous migration of that model, hence it still requires you to set a default value. If you're still in development, then you could delete the migrations for that model and then makemigrations again. But, this is terrible practice and you should never do this unless you are consciously squashing your models, the better way is to do what Django is telling you and set a default. This'll help in the long run anyways with error handling.
The other answers address how to set defaults quite well.
When you add a new field (i.e., column) to a database table, that field applies to all rows in the table...the existing ones and any new ones. If the field cannot be null, then some value must be specified for all of the existing rows.
By default, IntegerField() is non-nullable. This explains why you are getting the warning. You have a few options:
You can choose a default "one-off" initial value like Django suggests. All existing rows will use this value for the new field.
Assign your own values to each existing row for the new field.
Use the blank and null arguments in your model field to allow the field to accept null values.
For the last option, you'd do this like so:
models.IntegerField(blank=True, null=True)
But that might not be the best choice depending on your use case. If nullable values don't make sense for your field, I would avoid making the field support nulls.
age = models.IntegerField(null=True)
Django models fields are null set to true by default.
The way you can solve this:
Delete the Blog table from the database.
Delete the migrations folder from the app that contains Blog model.
Remove all data from django_migrations table.
Now run makemigrations and then migrate command.
Hope this will solve your problem.
Do what Django is telling you, then you can remove the default attribute if you don't need it. But it's a good practice to keep it anyway
Using Django 1.4 in my app I defined a model called Member and another called Data.Every member has basic like ID and it is related to a Data object that contains additional variables describing the "member".
I had initially created the member model without specifying that the dob variable could be NULL. I have since then changed this to allow for blank or null but I was still getting the members_data.dob may not be NULL error.
I thought it was because I needed to run a syncdb command, which I did, however this did not fix the problem.
dob = models.CharField(max_length=200, blank=True, null=True)
Any ideas? Thanks
ps. If you want to get an overall picture of what I am trying to implement please refer to: Can I use JSON data to add new objects in Django?
Thanks so much.
The syncdb command only creates tables if they do not exist. It does not handle migrations for you. You have a few options:
If there is no important data in the table, drop the table and run syncdb again to recreate it.
Update the column to allow null in a db shell. The correct command depends on which database you are using.
Use a migration tool, like South.
To drop the table in sqlite:
Open a dbshell
./manage.py dbshell
Drop the table
drop table <tablename>
I have no idea what Django is trying to tell me. I have a model, WeekTwo, which inherits from Week, which inherits from modelsModel. I have another model, UserProfile. I want to use WeekTwo as a OneToOne Key in UserProfile, so I inserted the following line of code:
weekTwo = models.OneToOneField(WeekTwo)
However, when I try to migrate my database using python manage.py schemamigration my_app --auto, I get the following error:
The field 'UserProfile.weekTwo' does not have a default specified, yet is NOT NULL.
I tried adding default=0 to my weekTwo declaration, but now I'm getting this error when I try the schema migration:
IntegrityError: column weekTwo_id is not unique
Moreover, south is now telling me that I am in an interim state between migrations and that I might be able to recover. I literally have no idea what any of this means.
Be aware before going further, be sure that if South already did any migration that has been failed, It's better to redo|recover to the last migration that was working.
You have two options here, at first you can do Data Migration. Look at ref also.
In Second way You can make weekTwo field null and blank first
weekTwo = models.OneToOneField(WeekTwo, null=True, blank=True)
Then let South generate a migration for you by
python manage.py schemamigration my_app --auto
I'm sure South won't complain about it now, Then
python manage.py migrate
If everything is okay now, You can now get back and change to weekTwo field to
weekTwo = models.OneToOneField(WeekTwo)
And generate migration then migrate them.
Anyway, When south find out your field in not NULL and doesn't have a default value, at the schemamigration step it will suggest you to provide a value for it, Again here your field is OneToOneField, because even south gives you chance to provide a default value for your existed record on model again uniqueness of weekTwo field will raise an error.
I think still you have to go with Data Migration if second way didn't work, Or gives it a shot and try the second way this time instead of make it null, blank change the whole Field type. Try it with;
weekTwo = models.ForeignKey(WeekTwo)
But keep in mind Data migration would be definitely smarter and standard way here.
Regarding Django & Python:
The Error:
Exception Type: DatabaseError
Exception Value:
column objects_thing.name_id does not exist
LINE 1: ...s_thing"."created", "objects_thing"."modified", "objects...
In my manage.py sql objects
CREATE TABLE "objects_thing" (
otherstuff,
otherstuff,
"name_id" integer NOT NULL REFERENCES "profiles_name" ("id"),
otherstuff,
);
So it clearly exists.
I've ran syncdb.
Why am I getting this error? And how do I go about fixing it? (I'm a newbie to all of this) Thank you in advance for the help.
EDIT:
Thing Model:
class Thing(models.Model):
created = models.DateTimeField(auto_now_add=True)
modified = models.DateTimeField(auto_now=True)
name = models.ForeignKey(Name)#The name for this thing
current_allocation = models.DecimalField(max_digits=13, decimal_places=2, null=True, blank=True)
target_allocation = models.DecimalField(max_digits=13, decimal_places=2, null=True, blank=True)
placeholder = models.ForeignKey(Entity, null=True, blank=True, on_delete=models.SET_NULL)
avatar = models.ImageField(upload_to='avatars/thing/', null=True, blank=True)
syncdb doesn't change existing tables in your database, so if you run that, and then change your model, your model is now out of sync with the table it represents. Running syncdb again will not fix that.
You either need to use something like south to do a migration, delete the table from your DB so that syncdb will recreate it, or manually run an ALTER TABLE on your DB.
EDIT (greater detail)
When you create a subclass of Model in models.py, it acts as a representation of a database table, but doesn't automatically have a database table. You get that by running python manage.py syncdb. Django, then, looks through all your models.py files, generates the SQL required to actually create a table like that and then runs it on your database. The end result is that you end up with actual database tables that are tied to your models.
However, syncdb only creates tables. It does not alter them. So, if you go and change one of your models (add a field, change the name of a field, etc.), nothing has happened at the database level. Running syncdb again will not help, either, because there's no new tables to create. You have to somehow get the table to match the model and vice versa, though, so that's where your options come in:
Use South (link above). South enables you to create migrations, so when you change something on your models you can run:
python manage.py schemamigration --auto yourapp
And it will generate code that will alter the table to match your model. You then need only apply the migration with:
python manage.py migrate yourapp
And you're done. The table now matches your model and all is right in the world again.
You can manually delete the table from your database. You wouldn't want to do this in production because all the data in that table will go along with it, but in development it shouldn't be a problem. After the table is gone, you can then run:
python manage.py syncdb
Because, the table no longer exists, Django will create it, but it will create it based on your current model's state. The net result is the same, your model and table match, so you're good to go.
You can manually alter the table. This requires that you figure out what SQL needs to be applied to change the table to match your model. You run this SQL on your database, and then the table is in parity with the model.
The point is that somehow, someway, you must update the table to reflect any changes you make to your models. The model isn't the table, it's just a programmatic representation of it.
The column might not necesarrily exist. The sql command just shows the sql used to create it, It is based on your current model. You could delete the table and re syncdb or manually add the column. https://docs.djangoproject.com/en/dev/ref/django-admin/#sql-appname-appname
I think there is a little confusion regarding the django Model and the actual database table
The django model is just some python code. It is a python object that is connected to a sql table. The database backend is specified in settings.py. The database contains the actual table. The error you are encountering is indicating that the python model is not the same as the actual database table.