add exclude method on QuerySet and fix missing default values on creation

ormar/fields/base.py

@@ -38,6 +38,11 @@ class BaseField:
             return Field(default=default)
         return None
 
+    @classmethod
+    def get_default(cls) -> Any:
+        if cls.has_default():
+            return cls.default if cls.default is not None else cls.server_default
+
     @classmethod
     def has_default(cls) -> bool:
         return cls.default is not None or cls.server_default is not None