add eager_defaults="auto" for inserts

Added a new default value for the :paramref:`.Mapper.eager_defaults`
parameter "auto", which will automatically fetch table default values
during a unit of work flush, if the dialect supports RETURNING for the
INSERT being run, as well as
:ref:`insertmanyvalues <engine_insertmanyvalues>` available. Eager fetches
for server-side UPDATE defaults, which are very uncommon, continue to only
take place if :paramref:`.Mapper.eager_defaults` is set to ``True``, as
there is no batch-RETURNING form for UPDATE statements.

Fixes: #8889
Change-Id: I84b91092a37c4cd216e060513acde3eb0298abe9

2 files changed, 39 insertions, 15 deletions

diff --git a/lib/sqlalchemy/testing/__init__.py b/lib/sqlalchemy/testing/__init__.py
index 993fc4954..6454750f5 100644
--- a/lib/sqlalchemy/testing/__init__.py
+++ b/lib/sqlalchemy/testing/__init__.py
@@ -56,6 +56,7 @@ from .config import requirements as requires
 from .config import skip_test
 from .config import Variation
 from .config import variation
+from .config import variation_fixture
 from .exclusions import _is_excluded
 from .exclusions import _server_version
 from .exclusions import against as _against
diff --git a/lib/sqlalchemy/testing/config.py b/lib/sqlalchemy/testing/config.py
index 6adcf5b64..b444eb39f 100644
--- a/lib/sqlalchemy/testing/config.py
+++ b/lib/sqlalchemy/testing/config.py
@@ -157,9 +157,33 @@ class Variation:
     def __str__(self):
         return f"{self._argname}={self._name!r}"
 
+    def __repr__(self):
+        return str(self)
+
     def fail(self) -> NoReturn:
         fail(f"Unknown {self}")
 
+    @classmethod
+    def idfn(cls, variation):
+        return variation.name
+
+    @classmethod
+    def generate_cases(cls, argname, cases):
+        case_names = [
+            argname if c is True else "not_" + argname if c is False else c
+            for c in cases
+        ]
+
+        typ = type(
+            argname,
+            (Variation,),
+            {
+                "__slots__": tuple(case_names),
+            },
+        )
+
+        return [typ(casename, argname, case_names) for casename in case_names]
+
 
 def variation(argname, cases):
     """a helper around testing.combinations that provides a single namespace
@@ -203,26 +227,17 @@ def variation(argname, cases):
         else (entry, None)
         for entry in cases
     ]
-    case_names = [
-        argname if c is True else "not_" + argname if c is False else c
-        for c, l in cases_plus_limitations
-    ]
 
-    typ = type(
-        argname,
-        (Variation,),
-        {
-            "__slots__": tuple(case_names),
-        },
+    variations = Variation.generate_cases(
+        argname, [c for c, l in cases_plus_limitations]
     )
-
     return combinations(
         *[
-            (casename, typ(casename, argname, case_names), limitation)
+            (variation._name, variation, limitation)
             if limitation is not None
-            else (casename, typ(casename, argname, case_names))
-            for casename, (case, limitation) in zip(
-                case_names, cases_plus_limitations
+            else (variation._name, variation)
+            for variation, (case, limitation) in zip(
+                variations, cases_plus_limitations
             )
         ],
         id_="ia",
@@ -230,6 +245,14 @@ def variation(argname, cases):
     )
 
 
+def variation_fixture(argname, cases, scope="function"):
+    return fixture(
+        params=Variation.generate_cases(argname, cases),
+        ids=Variation.idfn,
+        scope=scope,
+    )
+
+
 def fixture(*arg: Any, **kw: Any) -> Any:
     return _fixture_functions.fixture(*arg, **kw)