Aggregates default to the database type instead of the field type

[Wolph]

First a little background information, I've got a couple of custom field types which get cast right after loading the model.
So instead of a Decimal I get a Euro object with added features like formatting and stuff like that.

However, it seems that the aggregate/annotate queries in Django do not respect the to_python() method but simply cast to the type specified by the database backend.
This can easily be fixed by modifying convert_values() so it returns this:

return field.to_python(connection.ops.convert_values(value, field))

instead of

return connection.ops.convert_values(value, field)

The convert_values() function: ​http://code.djangoproject.com/browser/django/trunk/django/db/models/sql/query.py#L298

Would this be the proper location to fix this? Either way, it has to be a bug that the max() of a value returns a different type.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Marc Tamlyn <marc.tamlyn@…>]

In e9103402c0fa873aea58a6a11dba510cd308cb84:

Fixed #18757, #14462, #21565 -- Reworked database-python type conversions

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.