Improve QuerySet integer indexing error guidance

tests/test_queryset.py

@@ -129,6 +129,20 @@ def test_slicing_negative_values(db):
         _ = IntFields.all()[:-1]
 
 
+def test_integer_indexing_not_supported(db):
+    with pytest.raises(
+        ParamsError,
+        match=(
+            r"QuerySet indices must be slices, not integers\. "
+            r"QuerySets are lazy and do not support random access\. "
+            r"Use await queryset\.first\(\), await queryset\.offset\(n\)\.first\(\), "
+            r"or await queryset\.all\(\) and index the returned list\."
+        ),
+    ):
+        # Use an index way outside reasonable bounds for the dataset, so we don't accidentally support it.
+        _ = IntFields.all()[1000]
+
+
 def test_slicing_stop_before_start(db):
     with pytest.raises(
         ParamsError,

tortoise/queryset.py

@@ -554,6 +554,13 @@ def __getitem__(self, key: slice) -> QuerySet[MODEL]:
         or None.
         """
         if not isinstance(key, slice):
+            if isinstance(key, int):
+                raise ParamsError(
+                    "QuerySet indices must be slices, not integers. "
+                    "QuerySets are lazy and do not support random access. "
+                    "Use await queryset.first(), await queryset.offset(n).first(), "
+                    "or await queryset.all() and index the returned list."
+                )
             raise ParamsError("QuerySet indices must be slices.")
 
         if not (key.step is None or (isinstance(key.step, int) and key.step == 1)):