vdm 0.2

Full Versioned Domain Model

To encapsulate changing multiple objects at once as well as to facilitate domain object traversal it is necessary to introduce an explicit ‘Revision’ object to represent a single changeset to the domain model.

One also needs to introduce the concept of ‘State’. This allows us to make (some) domain objects stateful, in particular those which are to be versioned (State is necessary to support delete/undelete functionality as well as to implement versioned many-to-many relationships).

For each original domain object that comes versioned we end up with 2 domain objects:

• The ‘continuity’: the original domain object.

• The ‘version/revision’ of that domain object.

Often a user will never need to be concerned (explicitly) with the version/revision object as they will just interact with the original domain object, which will, where necessary, ‘proxy’ requests down to the ‘version/revision’.

To give a flavour of all of this here is a pseudo-code example:

# we need a session of some kind to track which objects have been changed
# each session then has a single revision
rev1 = Revision(author='me')
session.revision = rev1
# Book and Author are domain objects which has been made versioned using this library
# typo!
b1 = Book(name='warandpeace', title='War and Peacee')
b2 = Book(name='annakarenina', title='Anna')
b3 = Book(name='warandpeace')
a1 = Author(name='tolstoy')
# this is just shorthand for ending this revision and saving all changes
# this may vary depending on the implementation
rev1.commit()
timestamp1 = rev1.timestamp

# some time later

rev2 = Revision(author='me')
session.revision = rev2
b1 = Book.get(name='warandpeace')
# correct typo
b1.title = 'War and Peace'
# add the author
a1 = Author.get(name='tolstoy')
b1.authors.append(a1)
# duplicate item so delete
b3.delete()
rev2.commit()

# some time even later
rev1 = Revision.get(timestamp=timestamp1)
b1 = Book.get(name='warandpeace')
b1 = b1.get_as_of(rev1)
assert b1.title == 'War and Peacee'
assert b1.authors == []
# etc

Code in Action

To see some real code in action take a look at:

vdm/sqlalchemy/demo.py
vdm/sqlalchemy/demo_test.py

vdm/sqlobject/demo.py
vdm/sqlobject/demo_test.py

The code for elixir, which is not yet fully functional, can be found in:

vdm/elixir/