Relational fields are now lazily constructed via properties instead of in the constructor

CHANGELOG.rst

@@ -6,6 +6,8 @@ Changelog
 0.13.5
 ------
 - Sample Starlette integration
+- Relational fields are now lazily constructed via properties instead of in the constructor,
+  this results in a significant overhead reduction for Model instantiation with many relationships.
 
 0.13.4
 ------
@@ -27,7 +29,7 @@ Changelog
 
 0.13.2
 ------
-* Security fixes for ``«model».save()`` & ``«model».dete()``:
+* Security fixes for ``«model».save()`` & ``«model».delete()``:
 
   This is now fully parametrized, and these operations are no longer susceptible to escaping issues.
 

tortoise/backends/base/executor.py

@@ -174,8 +174,7 @@ async def _prefetch_reverse_relation(
             self._field_to_db(instance._meta.pk, instance.pk, instance)
             for instance in instance_list
         }  # type: Set[Any]
-        backward_relation_manager = getattr(self.model, field)
-        relation_field = backward_relation_manager.relation_field
+        relation_field = self.model._meta.fields_map[field].relation_field
 
         related_object_list = await related_query.filter(
             **{"{}__in".format(relation_field): list(instance_id_set)}
@@ -301,9 +300,9 @@ def _make_prefetch_queries(self) -> None:
             self._prefetch_queries[field] = related_query
 
     async def _do_prefetch(self, instance_id_list: list, field: str, related_query) -> list:
-        if isinstance(getattr(self.model, field), fields.BackwardFKRelation):
+        if field in self.model._meta.backward_fk_fields:
             return await self._prefetch_reverse_relation(instance_id_list, field, related_query)
-        if isinstance(getattr(self.model, field), fields.ManyToManyField):
+        if field in self.model._meta.m2m_fields:
             return await self._prefetch_m2m_relation(instance_id_list, field, related_query)
         return await self._prefetch_direct_relation(instance_id_list, field, related_query)
 

tortoise/models.py

@@ -41,6 +41,22 @@ def _fk_getter(self, _key):
     return getattr(self, _key, None)
 
 
+def _rfk_getter(self, _key, ftype, frelfield):
+    val = getattr(self, _key, None)
+    if val is None:
+        val = RelationQueryContainer(ftype, frelfield, self)
+        setattr(self, _key, val)
+    return val
+
+
+def _m2m_getter(self, _key, field_object):
+    val = getattr(self, _key, None)
+    if val is None:
+        val = ManyToManyRelationManager(field_object.type, self, field_object)
+        setattr(self, _key, val)
+    return val
+
+
 class MetaInfo:
     __slots__ = (
         "abstract",
@@ -168,6 +184,32 @@ def finalise_fields(self) -> None:
                 ),
             )
 
+        # Create lazy reverse FK fields on model.
+        for key in self.backward_fk_fields:
+            _key = "_{}".format(key)
+            field_object = self.fields_map[key]  # type: fields.BackwardFKRelation  # type: ignore
+            setattr(
+                self._model,
+                key,
+                property(
+                    partial(
+                        _rfk_getter,
+                        _key=_key,
+                        ftype=field_object.type,
+                        frelfield=field_object.relation_field,
+                    )
+                ),
+            )
+
+        # Create lazy M2M fields on model.
+        for key in self.m2m_fields:
+            _key = "_{}".format(key)
+            setattr(
+                self._model,
+                key,
+                property(partial(_m2m_getter, _key=_key, field_object=self.fields_map[key])),
+            )
+
     def _generate_filters(self) -> None:
         get_overridden_filter_func = self.db.executor_class.get_overridden_filter_func
         for key, filter_info in self._filters.items():
@@ -311,7 +353,6 @@ def __init__(self, *args, **kwargs) -> None:
         # self._meta is a very common attribute lookup, lets cache it.
         meta = self._meta
         self._saved_in_db = meta.pk_attr in kwargs and meta.pk.generated
-        self._init_lazy_fkm2m()
 
         # Assign values and do type conversions
         passed_fields = {*kwargs.keys()}
@@ -330,7 +371,6 @@ def __init__(self, *args, **kwargs) -> None:
     def _init_from_db(cls, **kwargs) -> MODEL_TYPE:
         self = cls.__new__(cls)
         self._saved_in_db = True
-        self._init_lazy_fkm2m()
 
         meta = self._meta
 
@@ -341,27 +381,6 @@ def _init_from_db(cls, **kwargs) -> MODEL_TYPE:
 
         return self
 
-    def _init_lazy_fkm2m(self) -> None:
-        meta = self._meta
-        # Create lazy fk/m2m objects
-        for key in meta.backward_fk_fields:
-            field_object = meta.fields_map[key]
-            setattr(
-                self,
-                key,
-                RelationQueryContainer(
-                    field_object.type, field_object.relation_field, self  # type: ignore
-                ),
-            )
-
-        for key in meta.m2m_fields:
-            field_object = meta.fields_map[key]
-            setattr(
-                self,
-                key,
-                ManyToManyRelationManager(field_object.type, self, field_object),  # type: ignore
-            )
-
     def _set_field_values(self, values_map: Dict[str, Any]) -> Set[str]:
         """
         Sets values for fields honoring type transformations and