Fixed bug where iterating a :class:`.Result` from a :class:`_orm.Session`
after that :class:`_orm.Session` were closed would partially attach objects
to that session in an essentially invalid state.   It now raises an exception
with a link to new documentation if an **un-buffered** result is iterated
from a :class:`_orm.Session` that was closed or otherwise had the
:meth:`_orm.Session.expunge_all` method called after that :class:`.Result`
was generated.  The "prebuffer_rows" execution option, as is used by the
asyncio extension, may be used to produce a :class:`.Result` where the ORM
objects are prebuffered, and in this case iterating the result will produce
a series of detached objects.

Fixes: #7128
Change-Id: I59f0ae32a83a64587937741b80f31ff825bbb574

An extra layer of warning messages has been added to the functionality
of :meth:`_orm.Query.join` and the ORM version of
:meth:`_sql.Select.join`, where a few places where "automatic aliasing"
continues to occur will now be called out as a pattern to avoid, mostly
specific to the area of joined table inheritance where classes that share
common base tables are being joined together without using explicit aliases.
One case emits a legacy warning for a pattern that's not recommended,
the other case is fully deprecated.

The automatic aliasing within ORM join() which occurs for overlapping
mapped tables does not work consistently with all APIs such as
``contains_eager()``, and rather than continue to try to make these use
cases work everywhere, replacing with a more user-explicit pattern
is clearer, less prone to bugs and simplifies SQLAlchemy's internals
further.

The warnings include links to the errors.rst page where each pattern is
demonstrated along with the recommended pattern to fix.

* Improved the exception message generated when configuring a mapping with
  joined table inheritance where the two tables either have no foreign key
  relationships set up, or where they have multiple foreign key relationships
  set up. The message is now ORM specific and includes context that the
  :paramref:`_orm.Mapper.inherit_condition` parameter may be needed
  particularly for the ambiguous foreign keys case.

* Add explicit support in the _expect_warnings() assertion for nested
  _expect_warnings calls

* generalize the NoCache fixture, which we also need to catch warnings
  during compilation consistently

* generalize the __str__() method for the HasCode mixin so all warnings
  and errors include the code link in their string

Fixes: #6974
Change-Id: I84ed79ba2112c39eaab7973b6d6f46de7fa80842

Provide better error message when trying to insepct and async engine
or asnyc connection.

Change-Id: I907f3a22c6b76fe43df9d40cb0e69c57f74a7982

(redo of 2999/I5609025feee8cfdecc09b55bfbf1bd13fa2e6602)

This PR is designed to bring more clarity within the docs by renaming object
instances that may be consfusingly similar to class, method, and attribute names.

For example, instances of the class `MetaData` are available on some  objects as
`.metadata` property, and had appeared within the docs as both `meta` and
`metadata` which has confused some users in the past. By this PR, the docs now
utilize the following naming convention:

* MetaData - SQLAlchemy class
* .metadata - SQLAlchemy API attributes
* metadata_obj - developer instantiated metadata objects or references

Detailed Changes:

* standardized `meta` and `metadata` instances to `metadata_obj`. note: the docs were evenly split between 'meta' and 'metadata'.
* standardized 'cursor' to 'cursor_obj' to avoid confusion with the method.
* standardized a 'scalar_subquery = ' to 'scalar_subq' to avoid confusion with the method.
* standardized a 'cte = ' to 'cte_obj' to avoid confusion with the method

Change-Id: I79c98aee16c5fc6649289b2dd7d6dfc368222fb4

The warning that's emitted for :func:`_orm.relationship` when multiple
relationships would overlap with each other as far as foreign key
attributes written towards, now includes the specific "overlaps" argument
to use for each warning in order to silence the warning without changing
the mapping.

Fixes: #6400
Change-Id: I43c04f8018dda4e0f83ae58b8c7fd95084d9e95d

This adds a new description to errors.rst and adds support
for any SQLAlchemy warning to refer to an errors.rst code.

Fixes: #6148
Closes: #6250
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6250
Pull-request-sha: dbcaeb54e31517fe88f6f8c515f1024002675f13

Change-Id: I4303c62ac9b1f13f67a34f825687014f1771c98c

Fixes reference to relationship.back_populates in errors.rst.

Change-Id: I60ac97d5adcf453aa8363266210ceda4688baa6d

Scaled back the warning message added in :ticket:`5171` to not warn for
overlapping columns in an inheritance scenario where a particular
relationship is local to a subclass and therefore does not represent an
overlap.

Add errors documentation for the warning and also expand
``util.warn()`` to include a code parameter.

Fixes: #6171
Change-Id: Icb1f12d8d645d439ffd2bbb7371c6b00042b6ae3