versioning

{ Repost from my personal blog @ https://blog.codezero.xyz/implementing-revisioning-feature-in-open-event } As I said in my previous blog post about Adding revisioning to SQLAlchemy Models, In an application like Open Event, where a single piece of information can be edited by multiple users, it's always good to know who changed what. One should also be able to revert to a previous version if needed. Let's have a quick run through on how we can enable SQLAlchemy-Continuum on our project. Install the library SQLAlchemy-Continuum with pip Add __versioned__ = {} to all the models that need to be versioned. Call make_versioned() before the models are defined Call configure_mappers from SQLAlchemy after declaring all the models. Example: import sqlalchemy as sa from sqlalchemy_continuum import make_versioned # Must be called before defining all the models make_versioned() class Event(Base): __tablename__ = 'events' __versioned__ = {} # Must be added to all models that are to be versioned id = sa.Column(sa.Integer, primary_key=True, autoincrement=True) name = sa.Column(sa.String) start_time = sa.Column(db.DateTime, nullable=False) end_time = sa.Column(db.DateTime, nullable=False) description = db.Column(db.Text) schedule_published_on = db.Column(db.DateTime) # Must be called after defining all the models sa.orm.configure_mappers() We have SQLAlchemy-Continuum enabled now. You can do all the read/write operations as usual. (No change there). Now, for the part where we give the users an option to view/restore revisions. The inspiration for this, comes from wordpress's wonderful revisioning functionality. The layout is well designed. The differences are shown in an easy-to-read form. The slider on top makes it intuitive to move b/w revisions. We have a Restore this Revision button on the top-right to switch to that revision. A similar layout is what we would like to achieve in Open Event. A slider to switch b/w sessions A pop-over infobox on the slider to show who made that change A button to switch to that selected revision. The colored-differences shown in side-by-side manner. To make all this a bit easier, SQLAlchemy-Continuum provides us with some nifty methods. count_versions is a method that allows us to know the number of revisions a record has. event = session.query(Event).get(1) count = count_versions(event) # number of versions of that event Next one is pretty cool. All the version objects have a property called as changeset which holds a dict of changed fields in that version. event = Event(name=u'FOSSASIA 2016', description=u'FOSS Conference in Asia') session.add(article) session.commit(article) version = event.versions[0] # first version version.changeset # { # 'id': [None, 1], # 'name': [None, u'FOSSASIA 2016'], # 'description': [None, u'FOSS Conference in Asia'] # } event.name = u'FOSSASIA 2017' session.commit() version = article.versions[1] # second version version.changeset # { # 'name': [u'FOSSASIA 2016', u'FOSSASIA 2017'], # } As you can see, dict holds the fields that changed and the content the changed (before and after). And this is what we'll be using for generating those pretty diffs that the guys and girls over at wordpress.com have done. And for this we'll be using two things. A library named diff-match-patch. It is a library from Google which offers robust algorithms to perform the…