added unique constraint on RecipeBookEntry table

Signed-off-by: Tobias Lindenberg <tobias@lindenberg.pm>

cookbook/migrations/0096_auto_20210109_2044.py

@@ -0,0 +1,31 @@
+# Generated by Django 3.1.5 on 2021-01-09 19:44
+
+from django.db import migrations
+
+
+def delete_duplicate_bookmarks(apps, schema_editor):
+    """
+    In this migration, a unique constraint is set on the fields `recipe` and `book`.
+    If there are already duplicate entries, the migration will fail.
+    Therefore all duplicate entries are deleted beforehand.
+    """
+    RecipeBookEntry = apps.get_model('cookbook', 'RecipeBookEntry')
+
+    for row in RecipeBookEntry.objects.all():
+        if RecipeBookEntry.objects.filter(recipe=row.recipe, book=row.book).count() > 1:
+            row.delete()
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('cookbook', '0095_auto_20210107_1804'),
+    ]
+
+    operations = [
+        # run function to delete duplicated bookmarks
+        migrations.RunPython(delete_duplicate_bookmarks),
+        migrations.AlterUniqueTogether(
+            name='recipebookentry',
+            unique_together={('recipe', 'book')},
+        ),
+    ]