error_on_load.md

error_on_load column loader strategy for marking column properties that should not be loaded.

It is similar to the deferred loader strategy, but causes an exception to be raised if a column would be loaded by a subsequent (deferred) query. Use error_on_load(k) instead of defer(k) to construct a query option that will cause an exception if an attempt is made to load column k.

This is handy when constructing a query with deferred columns (to optimize load time). It will cause an exception to be raised in your program instead of causing dramatic slowdown due to many deferred-load queries when you accidentally marked a column as deferred that should be loaded eagerly. Fail early.

erroronload.py

class NotSelected(Exception): pass

class NotSelectedColumnLoader(sa.orm.interfaces.LoaderStrategy):
    """Provide loading behavior for a error-on-load :class:`.ColumnProperty`."""

    def init(self):
        self.columns = self.parent_property.columns

    def create_row_processor(self, context, path, reduced_path, mapper, row, adapter):
        key = self.key
        def set_for_local_state(state, dict_, row):
            def raise_error(state):
                raise NotSelected(key)
            state.set_callable(dict_, key, raise_error)
        return set_for_local_state, None, None

class NotSelectedOption(sa.orm.interfaces.StrategizedOption):
    propagate_to_loaders = True

    def get_strategy_class(self):
        return NotSelectedColumnLoader

def error_on_load(*key):
    """Return a :class:`.MapperOption` that will raise
    :class:`NotSelected` with the given name when its column property
    is loaded.
    """
    return NotSelectedOption(key)