fix(apis_entities): only set collection if the attribute exists

`collection` is a leftover of the TempEntityClass. some projects still
use that, so we set it if it exists, but we check first if the object we
are working with even has such an attribute.

Closes: #313

apis_core/apis_entities/forms.py

@@ -225,12 +225,18 @@ def sort_fields_list(field_names, entity_name):
 
         def save(self, *args, **kwargs):
             obj = super(GenericEntitiesForm, self).save(*args, **kwargs)
-            if obj.collection.all().count() == 0:
-                col_name = getattr(
-                    settings, "APIS_DEFAULT_COLLECTION", "manually created entity"
-                )
-                col, created = Collection.objects.get_or_create(name=col_name)
-                obj.collection.add(col)
+
+            # backwards compatibility
+            # collection is a field of TempEntityClass - this block
+            # can be removed when TempEntityClass is gone from apis_entities
+            # for now we at least check if the attribute exist
+            if hasattr(obj, "collection"):
+                if obj.collection.all().count() == 0:
+                    col_name = getattr(
+                        settings, "APIS_DEFAULT_COLLECTION", "manually created entity"
+                    )
+                    col, created = Collection.objects.get_or_create(name=col_name)
+                    obj.collection.add(col)
             return obj
 
     return GenericEntitiesForm