Fix backreferences to the same table

src/grung/examples.py

@@ -21,4 +21,5 @@ class Group(Record):
             Field("name", unique=True),
             Collection("members", User),
             Collection("groups", Group),
+            BackReference("parent", Group),
         ]

src/grung/types.py

@@ -127,15 +127,19 @@ class Pointer(Field):
     value_type: type = Record
 
     def serialize(self, value: value_type) -> str:
+        return Pointer.reference(value)
+
+    def deserialize(self, value: str, db: TinyDB, recurse: bool = True) -> value_type:
+        return Pointer.dereference(value, db, recurse)
+
+    @classmethod
+    def reference(cls, value: Record):
         if value:
             if not value.doc_id:
                 raise PointerReferenceError(value)
             return f"{value._metadata.table}::{value.uid}"
         return None
 
-    def deserialize(self, value: str, db: TinyDB, recurse: bool = True) -> value_type:
-        return Pointer.dereference(value, db, recurse)
-
     @classmethod
     def dereference(cls, value: str, db: TinyDB, recurse: bool = True):
         if not value:
@@ -186,7 +190,7 @@ class Collection(Field):
             return
 
         for member in record[self.name]:
-            reference = Pointer.dereference(member, db=db)
-            for backref in reference._metadata.backrefs(type(record)):
-                reference[backref.name] = record
-                db.table(reference._metadata.table).upsert(reference)
+            target = Pointer.dereference(member, db=db)
+            for backref in target._metadata.backrefs(type(record)):
+                target[backref.name] = Pointer.reference(record)
+                db.table(target._metadata.table).update({backref.name: record}, where("uid") == target.uid)