add django backend in /core

ported from laravel in c3lf/lfbackend repo

core/inventory/models.py

@@ -0,0 +1,48 @@
+from django.core.files.base import ContentFile
+from django.db import models, IntegrityError
+from django_softdelete.models import SoftDeleteModel
+
+
+class ItemManager(models.Manager):
+
+    def create(self, **kwargs):
+        if 'uid' in kwargs:
+            raise ValueError('uid must not be set manually')
+        uid = Item.objects.filter(event=kwargs['event']).count() + 1
+        kwargs['uid'] = uid
+        return super().create(**kwargs)
+
+
+class Item(models.Model):
+    iid = models.AutoField(primary_key=True)
+    uid = models.IntegerField()
+    description = models.TextField()
+    event = models.ForeignKey('Event', models.CASCADE, db_column='eid')
+    container = models.ForeignKey('Container', models.CASCADE, db_column='cid')
+    returned_at = models.DateTimeField(blank=True, null=True)
+    created_at = models.DateTimeField(blank=True, null=True)
+    updated_at = models.DateTimeField(blank=True, null=True)
+
+    objects = ItemManager()
+
+    class Meta:
+        unique_together = (('uid', 'event'),)
+
+
+class Container(models.Model):
+    cid = models.AutoField(primary_key=True)
+    name = models.CharField(max_length=255)
+    created_at = models.DateTimeField(blank=True, null=True)
+    updated_at = models.DateTimeField(blank=True, null=True)
+
+
+class Event(models.Model):
+    eid = models.AutoField(primary_key=True)
+    name = models.CharField(max_length=255)
+    slug = models.CharField(max_length=255, unique=True)
+    start = models.DateTimeField(blank=True, null=True)
+    end = models.DateTimeField(blank=True, null=True)
+    pre_start = models.DateTimeField(blank=True, null=True)
+    post_end = models.DateTimeField(blank=True, null=True)
+    created_at = models.DateTimeField(blank=True, null=True)
+    updated_at = models.DateTimeField(blank=True, null=True)