track_on needs to be a fixed size, support sub-tuples

Fixed regression in ``selectinload`` loader strategy that would cause it to
cache its internal state incorrectly when handling relationships that join
across more than one column, such as when using a composite foreign key.
The invalid caching would then cause other loader operations to fail.

Fixes: #6410
Change-Id: I9f95ccca3553e7fd5794c619be4cf85c02b04626

1 files changed, 10 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/lambdas.py b/lib/sqlalchemy/sql/lambdas.py
index 06db8f95e..ddc4774db 100644
--- a/lib/sqlalchemy/sql/lambdas.py
+++ b/lib/sqlalchemy/sql/lambdas.py
@@ -772,7 +772,16 @@ class AnalyzedCode(object):
         from the "track_on" parameter passed to a :class:`.LambdaElement`.
 
         """
-        if isinstance(elem, traversals.HasCacheKey):
+
+        if isinstance(elem, tuple):
+            # tuple must contain hascachekey elements
+            def get(closure, opts, anon_map, bindparams):
+                return tuple(
+                    tup_elem._gen_cache_key(anon_map, bindparams)
+                    for tup_elem in opts.track_on[idx]
+                )
+
+        elif isinstance(elem, traversals.HasCacheKey):
 
             def get(closure, opts, anon_map, bindparams):
                 return opts.track_on[idx]._gen_cache_key(anon_map, bindparams)