fix private attributes initialization

tests/test_model_definition/test_pydantic_private_attributes.py

@@ -0,0 +1,34 @@
+from typing import List
+
+import databases
+import sqlalchemy
+from pydantic import PrivateAttr
+
+import ormar
+from tests.settings import DATABASE_URL
+
+database = databases.Database(DATABASE_URL, force_rollback=True)
+metadata = sqlalchemy.MetaData()
+
+
+class BaseMeta(ormar.ModelMeta):
+    metadata = metadata
+    database = database
+
+
+class Subscription(ormar.Model):
+    class Meta(BaseMeta):
+        tablename = "subscriptions"
+
+    id: int = ormar.Integer(primary_key=True)
+    stripe_subscription_id: str = ormar.String(nullable=False, max_length=256)
+
+    _add_payments: List[str] = PrivateAttr(default_factory=list)
+
+    def add_payment(self, payment: str):
+        self._add_payments.append(payment)
+
+
+def test_private_attribute():
+    sub = Subscription(stripe_subscription_id="2312312sad231")
+    sub.add_payment("test")