mapper is implicitly combining one of its column-based attributes
with that of the parent, where those columns normally don't necessarily
share the same value.  This is an extension of an existing check that
was added via :ticket:`1892`; however this new check emits only a
warning, instead of an exception, to allow for applications that may
be relying upon the existing behavior.
fixes #3042

columns are always added to the list of columns to be "undeferred";
otherwise, the ORM can't load the row's identity.   Apparently,
one can defer the mapped primary keys and the ORM will fail, that
hasn't been changed.  But as load_only is essentially saying
"defer all but X", it's more critical that PK cols not be part of this
deferral. fixes #3080

of work such that loading for related many-to-one objects is slightly
more aggressive, in the case of a graph of self-referential objects
that are to be deleted; the load of related objects is to help
determine the correct order for deletion if passive_deletes is
not set.
- revert the changes to test_delete_unloaded_m2o, these deletes do in fact
need to occur in the order of the two child objects first.

implicitly initialized to None via first access; this action,
which has always resulted in a population of the attribute,
now emits an attribute event just like any other attribute set
operation and generates the same kind of history as one.  Additionally,
many mapper internal operations will no longer implicitly generate
these "None" values when various never-set attributes are checked.
These are subtle behavioral fixes to attribute mechanics which provide
a better solution to the problem of :ticket:`3060`, which also
involves recognition of attributes explicitly set to ``None``
vs. attributes that were never set.
fixes #3061

scenario, where an INSERT/DELETE can be turned into an UPDATE.
In this situation, a many-to-one relationship set to None, or
in some cases a scalar attribute set to None, may not be detected
as a net change in value, and therefore the UPDATE would not reset
what was on the previous row.   This is due to some as-yet
unresovled side effects of the way attribute history works in terms
of implicitly assuming None isn't really a "change" for a previously
un-set attribute.  See also :ticket:`3061`. fixes #3060

eager loads across a polymorphic-subclass boundary in conjunction
with polymorphic loading would fail to locate the subclass-link in the
chain, erroring out with a missing property name on an
:class:`.AliasedClass`. fixes #3055

- fix a bad comparison in MySQL w/ limit/offset thing

not really clear which approach is better in the long run.

- restore __len__ to identity map