Fix buggy rewinding during QuerySetNoCache.__repr__

docs/changelog.rst

@@ -41,6 +41,7 @@ Changes in 1.0.0
 - Fixed stacklevel of many warnings (to point places emitting the warning more accurately)
 - Add support for collation/hint/comment to delete/update and aggregate #2842
 - BREAKING CHANGE: Remove LongField as it's equivalent to IntField since we drop support to Python2 long time ago (User should simply switch to IntField) #2309
+- BugFix - Prevent ``repr()`` from rewinding an actively iterated ``QuerySetNoCache`` #2870
 - Replace MongoEngine-created ``bson.SON`` objects with built-in dictionaries, SON providing no advantages since Python 3.7 as native dict preserved insertion order. #2898
 - BREAKING CHANGE: The obsolete ``slaves`` and ``is_slave`` connection options were silently ignored since 2014 and will now raise ``ConnectionFailure`` if provided #2920.
 - BugFix - Calling .clear on a ListField wasn't being marked as changed (and flushed to db upon .save()) #2858

mongoengine/queryset/queryset.py

@@ -186,4 +186,5 @@ def __iter__(self):
         if queryset._iter:
             queryset = self.clone()
         queryset.rewind()
+        queryset._iter = True
         return queryset

tests/fields/test_decimal128_field.py

@@ -1,5 +1,4 @@
 import json
-import random
 from decimal import Decimal
 
 import pytest
@@ -142,8 +141,8 @@ def test_storage(self):
 
     def test_json(self):
         Decimal128Document.drop_collection()
-        f = str(random.random())
-        Decimal128Document(dec128_fld=f).save()
+        value = "9.549665900909776e-05"
+        Decimal128Document(dec128_fld=value).save()
         json_str = Decimal128Document.objects.to_json()
         array = json.loads(json_str)
-        assert array[0]["dec128_fld"] == {"$numberDecimal": str(f)}
+        assert array[0]["dec128_fld"] == {"$numberDecimal": "0.00009549665900909776"}

tests/queryset/test_queryset.py

@@ -5590,6 +5590,37 @@ class Person(Document):
         qs = Person.objects.no_cache()
         assert repr(qs) == "[]"
 
+    def test_queryset_repr__not_iterated__lists_documents(self):
+        class Person(Document):
+            name = StringField()
+
+            def __repr__(self):
+                return f"<Person: {self.name}>"
+
+        Person.drop_collection()
+        Person.objects.insert([Person(name="a"), Person(name="b")])
+
+        querysets = (
+            Person.objects.order_by("name"),
+            Person.objects.order_by("name").no_cache(),
+        )
+        for queryset in querysets:
+            with self.subTest(queryset_type=type(queryset).__name__):
+                assert repr(queryset) == "[<Person: a>, <Person: b>]"
+
+    def test_no_cached_queryset_repr__during_iteration__reports_without_rewinding(self):
+        class Person(Document):
+            name = StringField()
+
+        Person.drop_collection()
+        Person.objects.insert([Person(name="a"), Person(name="b")])
+        qs = Person.objects.order_by("name").no_cache()
+        iterator = iter(qs)
+
+        assert next(iterator).name == "a"
+        assert repr(qs) == ".. queryset mid-iteration .."
+        assert next(iterator).name == "b"
+
     def test_no_cached_on_a_cached_queryset_raise_error(self):
         class Person(Document):
             name = StringField()