Not sure on this one. I wanted to use it in the recipe I just posted at #9246. However it doesn't "work" anyway because I can't call super() on the mixin and get the DeclarativeBase initializer anyway (which seems weird? )

class InitializeRelationships:
    """initialize all relationships"""

    def __init__(self, **kw):
        mapper = inspect(self.__class__)
        for arg in mapper.relationships:
            if arg.key not in kw:
                kw.setdefault(
                    arg.key,
                    None if not arg.uselist else arg.collection_class(),
                )
        super().__init__(**kw)

class Model(InitializeRelationships, DeclarativeBase):
    pass

above, super().__init__() is going back down to object.__init__ again. I thought DeclarativeBase has __init__() on it also? but it really can't, because __init__() is part of the base.

If we want mixins on the base to do init we can of course use the init mapper event.

not sure how I want to do this pattern

we can improve the behavior like this:

diff --git a/lib/sqlalchemy/orm/decl_api.py b/lib/sqlalchemy/orm/decl_api.py
index e3e2611da1..5f2b3c6966 100644
--- a/lib/sqlalchemy/orm/decl_api.py
+++ b/lib/sqlalchemy/orm/decl_api.py
@@ -567,7 +567,7 @@ def _setup_declarative_base(cls: Type[Any]) -> None:
     if "metadata" not in cls.__dict__:
         cls.metadata = cls.registry.metadata  # type: ignore
 
-    if "__init__" not in cls.__dict__:
+    if getattr(cls, "__init__", object.__init__) is object.__init__:
         cls.__init__ = cls.registry.constructor

that way at least the super __init__ is called. The super __init__ itself can't call super() again but at least things are less mysterious

Mike Bayer has proposed a fix for this issue in the main branch:

check for superclasses of user defined init https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/4429