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, 10 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index 336b7d9aa..1ca65c733 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -20,6 +20,7 @@ database to return iterable result sets.
 """
 import itertools
 import operator
+import types
 
 from . import attributes
 from . import exc as orm_exc
@@ -2229,7 +2230,8 @@ class Query(
             # non legacy argument form
             _props = [(target,)]
         elif not legacy and isinstance(
-            target, (expression.Selectable, type, AliasedClass,)
+            target,
+            (expression.Selectable, type, AliasedClass, types.FunctionType),
         ):
             # non legacy argument form
             _props = [(target, onclause)]
@@ -2284,7 +2286,13 @@ class Query(
                     legacy=True,
                     apply_propagate_attrs=self,
                 ),
-                prop[1] if len(prop) == 2 else None,
+                (
+                    coercions.expect(roles.OnClauseRole, prop[1])
+                    if not isinstance(prop[1], str)
+                    else prop[1]
+                )
+                if len(prop) == 2
+                else None,
                 None,
                 {
                     "isouter": isouter,