Do not break when after is greater than list_length (#999)

Author: bellini666

graphene_django/fields.py

@@ -144,7 +144,10 @@ def resolve_connection(cls, connection, args, iterable, max_limit=None):
                 min(max_limit, list_length) if max_limit is not None else list_length
             )
 
-        after = get_offset_with_default(args.get("after"), -1) + 1
+        # If after is higher than list_length, connection_from_list_slice
+        # would try to do a negative slicing which makes django throw an
+        # AssertionError
+        after = min(get_offset_with_default(args.get("after"), -1) + 1, list_length)
 
         if max_limit is not None and "first" not in args:
             args["first"] = max_limit

graphene_django/tests/test_query.py

@@ -1,4 +1,5 @@
 import datetime
+import base64
 
 import pytest
 from django.db import models
@@ -1084,6 +1085,42 @@ class Query(graphene.ObjectType):
     assert result.data == expected
 
 
+def test_connection_should_limit_after_to_list_length():
+    reporter_1 = Reporter.objects.create(
+        first_name="John", last_name="Doe", email="johndoe@example.com", a_choice=1
+    )
+    reporter_2 = Reporter.objects.create(
+        first_name="Some", last_name="Guy", email="someguy@cnn.com", a_choice=1
+    )
+
+    class ReporterType(DjangoObjectType):
+        class Meta:
+            model = Reporter
+            interfaces = (Node,)
+
+    class Query(graphene.ObjectType):
+        all_reporters = DjangoConnectionField(ReporterType)
+
+    schema = graphene.Schema(query=Query)
+    query = """
+        query ReporterPromiseConnectionQuery ($after: String) {
+            allReporters(first: 1 after: $after) {
+                edges {
+                    node {
+                        id
+                    }
+                }
+            }
+        }
+    """
+
+    after = base64.b64encode(b"arrayconnection:10").decode()
+    result = schema.execute(query, variable_values=dict(after=after))
+    expected = {"allReporters": {"edges": []}}
+    assert not result.errors
+    assert result.data == expected
+
+
 REPORTERS = [
     dict(
         first_name="First {}".format(i),