Collection: also save the collection UID on the model itself.

This enables us to have db-constraints for making sure that UIDs are
unique, as well as being more efficient for lookups (which are very
common).

The UID should always be the same as the main_item.uid, though that's
easily enforced as neither of them is allowed to change.

django_etebase/migrations/0033_collection_uid.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.1.1 on 2020-12-14 11:21
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_etebase', '0032_auto_20201013_1409'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='collection',
+            name='uid',
+            field=models.CharField(db_index=True, max_length=43, null=True, validators=[django.core.validators.RegexValidator(message='Not a valid UID', regex='^[a-zA-Z0-9\\-_]{20,}$')]),
+        ),
+    ]

django_etebase/migrations/0034_auto_20201214_1124.py

@@ -0,0 +1,22 @@
+# Generated by Django 3.1.1 on 2020-12-14 11:24
+
+from django.db import migrations
+
+
+def update_collection_uid(apps, schema_editor):
+    Collection = apps.get_model("django_etebase", "Collection")
+
+    for collection in Collection.objects.all():
+        collection.uid = collection.main_item.uid
+        collection.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("django_etebase", "0033_collection_uid"),
+    ]
+
+    operations = [
+        migrations.RunPython(update_collection_uid),
+    ]

django_etebase/migrations/0035_auto_20201214_1126.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.1.1 on 2020-12-14 11:26
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_etebase', '0034_auto_20201214_1124'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='collection',
+            name='uid',
+            field=models.CharField(db_index=True, max_length=43, validators=[django.core.validators.RegexValidator(message='Not a valid UID', regex='^[a-zA-Z0-9\\-_]{20,}$')]),
+        ),
+    ]

django_etebase/migrations/0036_auto_20201214_1128.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.1.1 on 2020-12-14 11:28
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_etebase', '0035_auto_20201214_1126'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='collection',
+            name='uid',
+            field=models.CharField(db_index=True, max_length=43, unique=True, validators=[django.core.validators.RegexValidator(message='Not a valid UID', regex='^[a-zA-Z0-9\\-_]{20,}$')]),
+        ),
+    ]