Added a new style of mapper-level inheritance loading
"polymorphic selectin".  This style of loading
emits queries for each subclass in an inheritance
hierarchy subsequent to the load of the base
object type, using IN to specify the desired
primary key values.

Fixes: #3948

Change-Id: I59e071c6142354a3f95730046e3dcdfc0e2c4de5

Fixed bug where :meth:`.Query.with_parent` would not work if the
:class:`.Query` were against an :func:`.aliased` construct rather than
a regular mapped class.  Also adds a new parameter
:paramref:`.util.with_parent.from_entity` to the standalone
:func:`.util.with_parent` function as well as
:meth:`.Query.with_parent`.

Change-Id: Ic684dd63cc90b582c7580c9bba3c92fa3f286da7
Fixes: #3607

a with_polymorphic, regardless of inheritance type, represents
multiple classes.  A subclass that wants to joinedload with innerjoin=True
needs to be demoted to an outerjoin because the parent entity rows
might not be of that type.   Looks more intuitive with a joined
inheritance load, but applies just as well to single or concrete.

Change-Id: I4d3d76106ae20032269f8848aad70a8e2f9422f9
Fixes: #3988

The inheritance documentation is confused, disorganized, and out
of date.  Reorganize and clarify, in particular in preparation
for new inheritance features.

Change-Id: Ibe6a35b4b0432bad866a6afb0598b2cf1b509f10

The ``lazy="select"`` loader strategy now makes used of the
:class:`.BakedQuery` query caching system in all cases.  This
removes most overhead of generating a :class:`.Query` object and
running it into a :func:`.select` and then string SQL statement from
the process of lazy-loading related collections and objects.  The
"baked" lazy loader has also been improved such that it can now
cache in most cases where query load options are used.

Change-Id: Ic96792fffaa045ae9aa0a4657d6d29235d3efb85
Fixes: #3954

Added new attribute event :meth:`.AttributeEvents.bulk_replace`.
This event is triggered when a collection is assigned to a
relationship, before the incoming collection is compared with the
existing one.  This early event allows for conversion of incoming
non-ORM objects as well.  The event is integrated with the
``@validates`` decorator.

The ``@validates`` decorator now allows the decorated method to receive
objects from a "bulk collection set" operation that have not yet
been compared to the existing collection.  This allows incoming values
to be converted to compatible ORM objects as is already allowed
from an "append" event.   Note that this means that the
``@validates`` method is called for **all** values during a collection
assignment, rather than just the ones that are new.

Change-Id: I27f59db008d9e521d31a3e30143d7cd997e4b7b3
Fixes: #3896

Addressed some long unattended performance concerns within the
joined eager loader query construction system.  The use of ad-hoc
:class:`.AliasedClass` objects per query, which produces lots of column
lookup overhead each time, has been replaced with a cached approach
that makes use of a small pool of :class:`.AliasedClass` objects
that are reused between invocations of joined eager loading.
Callcount reduction of SQL query generation for worst-case joined
loader scenarios (lots of joins, lots of columns) is reduced by
approximately 270%.

Change-Id: Ie43beecf37945b2bb7fff0aaa597a597293daa18
Fixes: #3915

Corrects some warnings and adds tox config.  Adds DeprecationWarning
to the error category.   Large sweep for string literals w/ backslashes
as this is common in docstrings

Co-authored-by: Andrii Soldatenko
Fixes: #3886
Change-Id: Ia7c838dfbbe70b262622ed0803d581edc736e085
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/337

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de