add constraint check for recursion

src/grung/db.py

@@ -1,5 +1,6 @@
 import inspect
 import re
+from collections.abc import Iterable
 from functools import reduce
 from operator import ior
 from pathlib import Path
@@ -9,7 +10,7 @@ from tinydb import Query, TinyDB, table
 from tinydb.storages import MemoryStorage
 from tinydb.table import Document
 
-from grung.exceptions import UniqueConstraintError
+from grung.exceptions import CircularReferenceError, UniqueConstraintError
 from grung.types import Record
 
 
@@ -66,8 +67,15 @@ class RecordTable(table.Table):
             super().remove(doc_ids=[document.doc_id])
 
     def _check_constraints(self, document) -> bool:
+        self._check_for_recursion(document)
         self._check_unique(document)
 
+    def _check_for_recursion(self, document) -> bool:
+        ref = document.reference
+        for field in document._metadata.fields.values():
+            if isinstance(field.default, Iterable) and ref in document[field.name]:
+                raise CircularReferenceError(ref, field)
+
     def _check_unique(self, document) -> bool:
         matches = []
         queries = reduce(

src/grung/exceptions.py

@@ -9,7 +9,7 @@ class UniqueConstraintError(Exception):
             f"  * Record:  {dict(document)}\n"
             f"  * Query:   {query}\n"
             f"  * Error:   Unique constraint failure\n"
-            "   * The record matches the following existing records:\n\n" + "\n".join(str(c) for c in collisions)
+            "  * The record matches the following existing records:\n\n" + "\n".join(str(c) for c in collisions)
         )
 
 
@@ -25,3 +25,18 @@ class PointerReferenceError(Exception):
             f"  * Error:     Invalid Pointer\n"
             "   * This collection member does not refer an existing record. Do you need to save it first?"
         )
+
+
+class CircularReferenceError(Exception):
+    """
+    Thrown when a record contains a reference to itself.
+    """
+
+    def __init__(self, reference, field):
+        super().__init__(
+            "\n"
+            f"  * Reference: {reference}\n"
+            f"  * Field:     {field.name}\n"
+            f"  * Error:     Circular Reference\n"
+            f"  * This record contains a reference to itself. This will lead to infinite recursion."
+        )

src/grung/types.py

@@ -257,8 +257,6 @@ class Pointer(Field):
                 raise PointerReferenceError("Value {value} does not look like a reference!")
             return value
         if value:
-            if not value.doc_id:
-                raise PointerReferenceError(value)
             return f"{value._metadata.table}::{value._metadata.primary_key}::{value[value._metadata.primary_key]}"
         return None
 

test/test_db.py

@@ -10,7 +10,7 @@ from tinydb.storages import MemoryStorage
 
 from grung import examples
 from grung.db import GrungDB
-from grung.exceptions import PointerReferenceError, UniqueConstraintError
+from grung.exceptions import CircularReferenceError, UniqueConstraintError
 
 
 @pytest.fixture
@@ -49,13 +49,7 @@ def test_crud(db):
 
 
 def test_pointers(db):
-    user = examples.User(name="john", email="john@foo")
-    players = examples.Group(name="players", members=[user])
-
-    with pytest.raises(PointerReferenceError):
-        players = db.save(players)
-
-    user = db.save(user)
+    user = db.save(examples.User(name="john", email="john@foo"))
     players = db.save(examples.Group(name="players", members=[user]))
 
     user = db.table("User").get(doc_id=user.doc_id)
@@ -86,6 +80,11 @@ def test_subgroups(db):
     kirk = db.table("User").get(doc_id=kirk.doc_id)
     assert kirk.reference in unique_users
 
+    # recursion!
+    with pytest.raises(CircularReferenceError):
+        tos.members = [tos]
+        db.save(tos)
+
 
 def test_unique(db):
     user1 = examples.User(name="john", email="john@foo")