add collection migration & fix serializer according to the task

apps/collection/migrations/0027_auto_20191217_1852.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.7 on 2019-12-17 18:52
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('collection', '0026_merge_20191217_1151'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='collection',
+            name='slug',
+            field=models.SlugField(max_length=255, null=True, unique=True, verbose_name='Collection slug'),
+        ),
+    ]

apps/collection/serializers/back.py

@@ -43,6 +43,9 @@ class CollectionBackOfficeSerializer(CollectionBaseSerializer):
             'related_object_names',
             'rank',
         ]
+        extra_kwargs = {
+            'slug': {'read_only': True},
+        }
 
 
 class CollectionBindObjectSerializer(serializers.Serializer):