BookInstance.book needs default value (mdn#89)

* Add default null value for BookInstance.book

* Add migration for default BookInstance.book value

Author: hamishwillee

catalog/migrations/0024_auto_20210302_0630.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.1.2 on 2021-03-02 06:30
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('catalog', '0023_auto_20201201_0238'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='bookinstance',
+            name='book',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.RESTRICT, to='catalog.book'),
+        ),
+    ]

catalog/models.py

@@ -71,7 +71,7 @@ class BookInstance(models.Model):
     """Model representing a specific copy of a book (i.e. that can be borrowed from the library)."""
     id = models.UUIDField(primary_key=True, default=uuid.uuid4,
                           help_text="Unique ID for this particular book across whole library")
-    book = models.ForeignKey('Book', on_delete=models.RESTRICT)
+    book = models.ForeignKey('Book', on_delete=models.RESTRICT, null=True)
     imprint = models.CharField(max_length=200)
     due_back = models.DateField(null=True, blank=True)
     borrower = models.ForeignKey(User, on_delete=models.SET_NULL, null=True, blank=True)