From 9a8b3e3e25004d138d3ca33e472ae04226c72daf Mon Sep 17 00:00:00 2001
From: Thomas Steinacher <tom@eggdrop.ch>
Date: Fri, 19 Jun 2020 14:40:16 -0400
Subject: [PATCH] Meaningful reference errors

---
 mongoengine/base/fields.py |  4 ++++
 mongoengine/base/proxy.py  | 26 ++++++++++++++++++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/mongoengine/base/fields.py b/mongoengine/base/fields.py
index c1ad6c0ad..2a15ffcc4 100644
--- a/mongoengine/base/fields.py
+++ b/mongoengine/base/fields.py
@@ -10,6 +10,7 @@
 
 from mongoengine.base.common import ALLOW_INHERITANCE
 from mongoengine.base.datastructures import BaseDict, BaseList
+from mongoengine.base.proxy import DocumentProxy
 
 __all__ = ("BaseField", "ComplexBaseField", "ObjectIdField", "GeoJsonBaseField")
 
@@ -103,9 +104,12 @@ def __get__(self, instance, owner):
                     value = self.default() if callable(self.default) else self.default
                 else:
                     value = self.to_python(db_value)
+                    if isinstance(value, DocumentProxy):
+                        value._set_parent_ref(instance, name)
 
                 if hasattr(self, 'value_for_instance'):
                     value = self.value_for_instance(value, instance)
+
                 data[name] = value
 
             return data[name]
diff --git a/mongoengine/base/proxy.py b/mongoengine/base/proxy.py
index 01d206fac..498de3ba0 100644
--- a/mongoengine/base/proxy.py
+++ b/mongoengine/base/proxy.py
@@ -124,7 +124,19 @@ def __delslice__(self, i, j):
 
 
 class DocumentProxy(LocalProxy):
-    __slots__ = ('__document_type', '__document', '__pk')
+    __slots__ = (
+        '__document_type',
+        '__document',
+        '__pk',
+        '__parent_ref_instance',
+        '__parent_ref_field_name',
+    )
+
+    def _set_parent_ref(self, instance, field_name):
+        object.__setattr__(self, '_DocumentProxy__parent_ref_instance',
+                instance)
+        object.__setattr__(self, '_DocumentProxy__parent_ref_field_name',
+                field_name)
 
     def __init__(self, document_type, pk):
         object.__setattr__(self, '_DocumentProxy__document_type', document_type)
@@ -186,7 +198,17 @@ def _get_current_object(self):
             collection = self.__document_type._get_collection()
             son = collection.find_one({'_id': self.__pk})
             if son is None:
-                raise DoesNotExist('Document has been deleted.')
+                error =  'Document ({} {}'.format(
+                    collection.name, self.__pk
+                )
+                if self.__parent_ref_instance:
+                    error += ', referenced by {} {} in field {}'.format(
+                        self.__parent_ref_instance.__class__.__name__,
+                        self.__parent_ref_instance.pk,
+                        self.__parent_ref_field_name,
+                    )
+                error += ') has been deleted.'
+                raise DoesNotExist(error)
             document = self.__document_type._from_son(son)
             object.__setattr__(self, '_DocumentProxy__document', document)
         return self.__document