Add raise/raiseload relationship loading strategypr/193

- available via `lazy='raise'` or by setting the `raiseload` strategy
  via `options()`
- behaves almost like `lazy='noload'`, but instead of returning `None`
  it raises `InvalidRequestError`
- based on code from Mike Bayer that was posted to the sqlalchemy
  mailing list: https://groups.google.com/forum/#!topic/sqlalchemy/X_wA8K97smE

1 files changed, 23 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/strategy_options.py b/lib/sqlalchemy/orm/strategy_options.py
index cb7a5fef7..5b2a190b9 100644
--- a/lib/sqlalchemy/orm/strategy_options.py
+++ b/lib/sqlalchemy/orm/strategy_options.py
@@ -854,6 +854,29 @@ def noload(*keys):
 
 
 @loader_option()
+def raiseload(loadopt, attr):
+    """Indicate that the given relationship attribute should remain unloaded.
+
+    Accessing the relationship attribute anyway raises an
+    :exc:`~sqlalchemy.exc.InvalidRequestError`.
+
+    This function is part of the :class:`.Load` interface and supports
+    both method-chained and standalone operation.
+
+    :func:`.orm.raiseload` applies to :func:`.relationship` attributes only.
+
+    .. versionadded:: 1.1.0
+    """
+
+    return loadopt.set_relationship_strategy(attr, {"lazy": "raise"})
+
+
+@raiseload._add_unbound_fn
+def raiseload(*keys):
+    return _UnboundLoad._from_keys(_UnboundLoad.raiseload, keys, False, {})
+
+
+@loader_option()
 def defaultload(loadopt, attr):
     """Indicate an attribute should load using its default loader style.