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, 0 deletions

diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 5f039aff7..53cc99ccd 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -1524,6 +1524,16 @@ class SubqueryLoader(PostLoader):
         # orig_compile_state = compile_state_cls.create_for_statement(
         # orig_query, None)
 
+        if orig_query._is_lambda_element:
+            util.warn(
+                'subqueryloader for "%s" must invoke lambda callable at %r in '
+                "order to produce a new query, decreasing the efficiency "
+                "of caching for this statement.  Consider using "
+                "selectinload() for more effective full-lambda caching"
+                % (self, orig_query)
+            )
+            orig_query = orig_query._resolved
+
         # this is the more "quick" version, however it's not clear how
         # much of this we need.    in particular I can't get a test to
         # fail if the "set_base_alias" is missing and not sure why that is.