Improve ListSerializer

rest_framework/serializers.py

@@ -28,7 +28,7 @@ from django.utils.translation import gettext_lazy as _
 
 from rest_framework.compat import postgres_fields
 from rest_framework.exceptions import ErrorDetail, ValidationError
-from rest_framework.fields import get_error_detail
+from rest_framework.fields import get_attribute, get_error_detail
 from rest_framework.settings import api_settings
 from rest_framework.utils import html, model_meta, representation
 from rest_framework.utils.field_mapping import (
@@ -610,11 +610,6 @@ class ListSerializer(BaseSerializer):
         assert self.child is not None, '`child` is a required argument.'
         assert not inspect.isclass(self.child), '`child` has not been instantiated.'
 
-        instance = kwargs.get('instance', [])
-        data = kwargs.get('data', [])
-        if instance and data:
-            assert len(data) == len(instance), 'Data and instance should have same length'
-
         super().__init__(*args, **kwargs)
         self.child.bind(field_name='', parent=self)
 
@@ -700,13 +695,37 @@ class ListSerializer(BaseSerializer):
         ret = []
         errors = []
 
-        for idx, item in enumerate(data):
-            if (
-                hasattr(self, 'instance')
-                and self.instance
-                and len(self.instance) > idx
-            ):
-                self.child.instance = self.instance[idx]
+        if not self.instance and self.parent and self.parent.instance:
+            rel_mgr = get_attribute(self.parent.instance, self.source_attrs)
+            self.instance = rel_mgr.all() if rel_mgr else None
+
+        pk_field_name = next(
+            field_name
+            for field_name, field in self.child.fields.items()
+            if field.source == self.child.Meta.model._meta.pk.attname
+        )
+
+        item_instance_dict = {}
+        if self.instance:
+            item_pks = []
+
+            for item in data:
+                item_pk = item.get(pk_field_name)
+                if item_pk:
+                    item_pks.append(item_pk)
+
+            for item_instance in self.instance.filter(pk__in=item_pks):
+                item_instance_dict[item_instance.pk] = item_instance
+
+        for item in data:
+            self.child.initial_data = item
+
+            if self.instance:
+                item_pk = item.get(pk_field_name)
+                self.child.instance = (
+                    item_instance_dict.get(item_pk) if item_pk else None
+                )
+
             try:
                 validated = self.run_child_validation(item)
             except ValidationError as exc: