Bonjour tout le monde !

Débutant sur Django, j'essaye actuellement de comprendre le framework en réalisant un blog basique. Je cherche à ajouter une valeur par défaut sur une relation entre modèles. Je m'explique j'ai une classe "Post" et une classe "Category". Les deux sont liées via une foreignkey ajoutée à la classe "Post". J'aimerai qu'à la création d'un nouvel article, la valeur "Unknown" soit automatiquement assignée à la catégorie de l'article.

Avant toute chose voici ma configuration :
 
système -> Windows
jeu d'instructions -> 64 bits
distribution -> 6.3.9600
version python -> 3.8.2
version django -> 3.1.2

Voici le modèle de la classe "Post" :

class Post(models.Model):
    title = models.CharField(max_length=255)
    author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    text = models.TextField()
    published_date = models.DateField(auto_now_add=True)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)

    def __str__(self):
        return self.title + " | " + str(self.author)

    def get_absolute_url(self):
        return reverse("home")

et celui de la classe "Category" :

class Category(models.Model):
    name = models.CharField(max_length=40, unique=True, default="Unknown")

    def __str__(self):
        return self.name

A noter que des articles étaient déjà existant avant l'ajout de la catégorie.
Lorsque j'effectue le makemigrations, la console m'affiche ceci :

(venv) C:\geomaps>python manage.py makemigrations
You are trying to add a non-nullable field 'category' to post 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: 1
Please enter the default value now, as valid Python
The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now
Type 'exit' to exit this prompt
>>> 'Unknown'
Migrations for 'blog':
  blog\migrations\0002_auto_20201016_2354.py
    - Create model Category
    - Add field category to post

et lorsque je fais le migrate :

(venv) C:\geomaps>python manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, blog, contenttypes, sessions
Running migrations:
  Applying blog.0002_auto_20201016_2354...Traceback (most recent call last):
  File "C:\geomaps\venv\lib\site-packages\django\db\models\fields\__init__.py", line 1774, in get_prep_value
    return int(value)
ValueError: invalid literal for int() with base 10: 'Unknown'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    main()
  File "manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "C:\geomaps\venv\lib\site-packages\django\core\management\__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\base.py", line 85, in wrapped
    res = handle_func(*args, **kwargs)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\commands\migrate.py", line 243, in handle
    post_migrate_state = executor.migrate(
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\operations\fields.py", line 104, in database_forwards
    schema_editor.add_field(
  File "C:\geomaps\venv\lib\site-packages\django\db\backends\sqlite3\schema.py", line 328, in add_field
    self._remake_table(model, create_field=field)
  File "C:\geomaps\venv\lib\site-packages\django\db\backends\sqlite3\schema.py", line 189, in _remake_table
    self.effective_default(create_field)
  File "C:\geomaps\venv\lib\site-packages\django\db\backends\base\schema.py", line 303, in effective_default
    return field.get_db_prep_save(self._effective_default(field), self.connection)
  File "C:\geomaps\venv\lib\site-packages\django\db\models\fields\related.py", line 971, in get_db_prep_save
    return self.target_field.get_db_prep_save(value, connection=connection)
  File "C:\geomaps\venv\lib\site-packages\django\db\models\fields\__init__.py", line 823, in get_db_prep_save
    return self.get_db_prep_value(value, connection=connection, prepared=False)
  File "C:\geomaps\venv\lib\site-packages\django\db\models\fields\__init__.py", line 2388, in get_db_prep_value
    value = self.get_prep_value(value)
  File "C:\geomaps\venv\lib\site-packages\django\db\models\fields\__init__.py", line 1776, in get_prep_value
    raise e.__class__(
ValueError: Field 'id' expected a number but got 'Unknown'.

j'ai donc supprimé le fichier de migration et refait un makemigrations en rentrant 1 à la place de 'Unknown':

(venv) C:\geomaps>python manage.py makemigrations
You are trying to add a non-nullable field 'category' to post 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: 1
Please enter the default value now, as valid Python
The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now
Type 'exit' to exit this prompt
>>> 1
Migrations for 'blog':
  blog\migrations\0002_auto_20201016_2356.py
    - Create model Category
    - Add field category to post

et voici le message d'erreur du migrate :

(venv) C:\geomaps>python manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, blog, contenttypes, sessions
Running migrations:
  Applying blog.0002_auto_20201016_2356...Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    main()
  File "manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "C:\geomaps\venv\lib\site-packages\django\core\management\__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\base.py", line 85, in wrapped
    res = handle_func(*args, **kwargs)
  File "C:\geomaps\venv\lib\site-packages\django\core\management\commands\migrate.py", line 243, in handle
    post_migrate_state = executor.migrate(
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "C:\geomaps\venv\lib\site-packages\django\db\migrations\executor.py", line 229, in apply_migration
    migration_recorded = True
  File "C:\geomaps\venv\lib\site-packages\django\db\backends\sqlite3\schema.py", line 35, in __exit__
    self.connection.check_constraints()
  File "C:\geomaps\venv\lib\site-packages\django\db\backends\sqlite3\base.py", line 343, in check_constraints
    raise IntegrityError(
django.db.utils.IntegrityError: The row in table 'blog_post' with primary key '1' has an invalid foreign key: blog_post.category_id contains a value '1' that does not have a corresponding value i
n blog_category.id.

J'ai essayé de trouver des éléments de réponse ici :

• Stackoverflow
• https://stackoverflow.com/questions/9311996/setting-default-value-for-foreign-key-attribute
  https://docs.djangoproject.com/fr/3.1/ref/models/fields/

Auriez-vous des idées pour résoudre mon problème ? Merci d'avance et belle journée !

-
Edité par guismoche 19 octobre 2020 à 8:49:30

Le problème vient, je pense, en partie, de ta classe category: tu lui dis que c'est un champ unique avec en valeur par défaut Unknown (je pense que par principe, un champ unique ne peut pas avoir de valeur par défaut)

Pour pouvoir affecter la valeur Unknown par défaut à ta clé, il faut que cette valeur soit présente dans la table Category; et il faut que le choix de valeur par défaut soit défini dans la table Post au niveau de la clé étrangère (on doit lui affecté l'id de la Category Unknown) et non pas dans la table Category.