store what fields get persisted in trait metadata

rather than checking columns in the db

makes things more explicit

jupyterhub/orm.py

@@ -458,58 +458,6 @@ class Service(Base):
         """
         return db.query(cls).filter(cls.name == name).first()
 
-    def _check_data_only_column(self, column_name: str) -> bool:
-        """Check if a column is not a ForeignKey or in a
-        relationship.
-
-        Args:
-            column_name (str): ame of the column
-
-        Returns:
-            bool: returns `True` if the column is data-only,
-            returns `False` otherwise.
-        """
-        if hasattr(self, column_name):
-            column = getattr(Service, column_name)
-            if hasattr(column, 'foreign_keys'):
-                return len(column.foreign_keys) == 0
-            else:
-                return False
-        else:
-            return False
-
-    def update_column(self, column_name: str, value: any) -> bool:
-        """Update the value of a non-ForeignKey column.
-
-        Args:
-            column_name (str): Name of the column
-            value (any): New value
-
-        Returns:
-            bool: returns `True` if the column is updated,
-            returns `False` otherwise.
-        """
-        if self._check_data_only_column(column_name):
-            setattr(self, column_name, value)
-            return True
-        else:
-            return False
-
-    def get_column(self, column_name: str):
-        """Get non-ForeignKey, non-relationship column
-
-        Args:
-            column_name (str): Name of the column
-
-        Returns:
-            The value of requested column, None if it is
-            a foreign key or it does not exist.
-        """
-        if self._check_data_only_column(column_name):
-            return getattr(self, column_name)
-        else:
-            return None
-
 
 class Expiring:
     """Mixin for expiring entries