Thanks Stavros Korokithakis for the report and Tim Graham for the
review.

Thanks Tim Graham for the patch series review.

Thanks Gwildor Sok for the report and Tim Graham for the review.

The method duplicates the functionality of Field.value_from_object()
and has the additional downside of being a privately named public
API method.

Thanks Tim Graham for the review.

Thanks to gfairchild for the report and Claude for the review.

As suggested by Anssi. This has the slightly strange side effect of
passing the expression to Expression.convert_value has the expression
passed back to it, but it allows more complex patterns of expressions.

Refactored compiler SELECT, GROUP BY and ORDER BY generation.
While there, also refactored select_related() implementation
(get_cached_row() and get_klass_info() are now gone!).

Made get_db_converters() method work on expressions instead of
internal_type. This allows the backend converters to target
specific expressions if need be.

Added query.context, this can be used to set per-query state.

Also changed the signature of database converters. They now accept
context as an argument.

This is a reliqua from the early days of the modeltests/regressiontests era.

Complete rework of translating data values from database

Deprecation of SubfieldBase, removal of resolve_columns and
convert_values in favour of a more general converter based approach and
public API Field.from_db_value(). Now works seamlessly with aggregation,
.values() and raw queries.

Thanks to akaariai in particular for extensive advice and inspiration,
also to shaib, manfre and timograham for their reviews.

Thanks Rahul Priyadarshi.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

* override_settings may now be imported from django.test
* removed Approximate from django.test
* updated documentation for things importable from django.test

Thanks akaariai for the suggestion.