add before_insert hook

src/grung/db.py

@@ -22,6 +22,7 @@ class RecordTable(table.Table):
         super().__init__(db.storage, name, **kwargs)
 
     def insert(self, document):
+        document.before_insert()
         doc = document.serialize(self._db)
         self._check_constraints(doc)
 

src/grung/types.py

@@ -75,6 +75,9 @@ class Record(Dict[(str, Field)]):
             rec[name] = _field.deserialize(self[name], db)
         return self.__class__(rec, doc_id=self.doc_id)
 
+    def before_insert(self):
+        pass
+
     def __setattr__(self, key, value):
         if key in self:
             self[key] = value