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
Related
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.
I am using model inheritance in my models.py. This is my code:
class Email(models.Model):
stuff = models.CharField(max_length=40, blank=True,
null=True, default="")
class TypeMod(Email):
pass
When I run makemigrations, I get the following message although I have set the default value for all of my fields in the Email model:
You are trying to add a non-nullable field 'id' to typemod 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
What am I doing wrong?!
It doesn't necessarily have to do with vagrant. I used a native Linux machine and got the same message. My solution was to remove the existing migrations.
Well I figured it out! For anyone who has the same issue, I am using vagrant and this project is running on a vm.So the problem was that the parent model was not abstract before, so a table was made in the database for the parent model. So when I switched to abstract, the table was still there. The way I solved it was that I ran "vagrant distroy" and restarted the vm.
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.
Suppose I have this model which is associated with south:
class MyModel(models.Model):
field_a = models.CharField(max_length=30)
field_b = models.CharField(max_length=30)
Now later on I want to add one more field so I created that field between field_a and field_b now my model looks like:
class MyModel(models.Model):
field_a = models.CharField(max_length=30)
field_c = models.CharField(max_length=30)
field_b = models.CharField(max_length=30)
Then I migrated the changes, when checking the table structure in MySQL the field_c is created at the end of all fields. How can I tell south to maintain the fields order e.g. insert after field_a.
In MySQL we can insert the new field before or after any existing field. Is it possible to do that in south?
I do not believe that it is not possible to force the order of field creation in MySQL when using Django (or South). New fields will always be appended to the end of the table definition.
However: The order of the columns should be irrelevant to you. Read on...
If you're writing SQL queries against your database, it is bad form to do select * and rely on the field ordering of the table. Instead you should specify which fields you want to select, and the order in which you want them reported. This makes the underlying tables' field-ordering unimportant.
Additionally, if anyone in your organisation is currently writing reports which use select *: by inserting new columns between others you could easily break reports that use field position (rather than field name), as the indexing of the columns will change.
As Steve Mayne said, there is no direct way to do it if you keep updating your models. There is a hack to do it though.
The first time you create your model, all definitions go one after another, as you have already observed.
If you want to be absolutely sure of the order of your models after you add a new column, log into the MySQL shell. This is usually done with mysql -u<user> -p. Tell it to use your database with USE db.
Then, manually insert the new column with ALTER TABLE table ADD field_c VARCHAR(30) AFTER field_a.
Now you can tell Django about it by editing your MyModel class.
class MyModel(models.Model):
field_a = models.CharField(max_length=30)
field_c = models.CharField(max_length=30)
field_b = models.CharField(max_length=30)
Obviously, this is really annoying if your model changes a lot. Hopefully that doesn't happen.
Here is a work-around that has worked for me:
1) Create your model in your model.py file as you would, but add only the first field.
2) Make the migration, but do not migrate yet.
3) Add each field one by one and make migration after each.
4) After having them all, migrate.
This has worked for me. If you have your model designed already, then you can:
0) Delete your model, make migration, migrate, and then undo deleting (you will lose your data, dump it if needed).
1) cut/delete each field from the bottom one by one but the first one.
2) make migration
3) undo to have the 2nd field back and make migration. Repeat for the others.
4) finally, migrate.