introduce deferred lambdas

The coercions system allows us to add in lambdas as arguments
to Core and ORM elements without changing them at all.   By allowing
the lambda to produce a deterministic cache key where we can also
cheat and yank out literal parameters means we can move towards
having 90% of "baked" functionality in a clearer way right in
Core / ORM.

As a second step, we can have whole statements inside the lambda,
and can then add generation with __add__(), so then we have
100% of "baked" functionality with full support of ad-hoc
literal values.

Adds some more short_selects tests for the moment for comparison.

Other tweaks inside cache key generation as we're trying to
approach a certain level of performance such that we can
remove the use of "baked" from the loader strategies.

As we have not yet closed #4639, however the caching feature
has been fully integrated as of
b0cfa7379cf8513a821a3dbe3028c4965d9f85bd, we will also
add complete caching documentation here and close that issue
as well.

Closes: #4639
Fixes: #5380
Change-Id: If91f61527236fd4d7ae3cad1f24c38be921c90ba

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/sqlalchemy/future/selectable.py b/lib/sqlalchemy/future/selectable.py
index 473242bf8..9d0ae7c89 100644
--- a/lib/sqlalchemy/future/selectable.py
+++ b/lib/sqlalchemy/future/selectable.py
@@ -124,6 +124,8 @@ class Select(_LegacySelect):
         target = coercions.expect(
             roles.JoinTargetRole, target, apply_propagate_attrs=self
         )
+        if onclause is not None:
+            onclause = coercions.expect(roles.OnClauseRole, onclause)
         self._setup_joins += (
             (target, onclause, None, {"isouter": isouter, "full": full}),
         )