When createing a history instance of the updated object `create_history`
sets attributes using `setattr`. Since SQLAlchemy model instances are
Python objects they don't prevent new attributes being created by setattr,
which means that if history models are missing some of the columns the
attributes will still be assigned, but their values will not be persisted
by SQLAlchemy since database columns for them do not exist.
To avoid this, we check that the attribute is defined on the `history_cls`
and raise an error if it isn't.
history-meta's dynamic magic is insufficient for templates, where we
need to be able to refer to the specific history table to take
advantage of sqlalchemy's relationship management (2/3rds of an ORM).
So replace it with a custom made version table.
Had to change the version decorator slightly for this
* can now handle nullable foreign keys - would previously raise
AttributeError
* can now handle default values - we would previously not insert
default values that hadn't been generated yet, which if the
field is not nullable will result in IntegrityErrors. We were
deliberately removing 'default' attributes from columns. Only
remove them if nullable is set to true now.
adapted to recording inserts and updates.
This removes need to manually create history tables.
Our code still remains in control of when history records are
created.
