A library of standard cubes are available from CubicWeb Forge Cubes provide entities and views.
The available application entities in standard cubes are:
To import a cube in your instance just change the line in the __pkginfo__.py file and verify that the cube you are planning to use is listed by the command cubicweb-ctl list. For example:
__use__ = ('comment',)
will make the Comment entity available in your BlogDemo cube.
Change the schema to add a relationship between BlogEntry and Comment and you are done. Since the comment cube defines the comments relationship, adding the line:
comments = ObjectRelation('Comment', cardinality='1*', composite='object')
to the definition of a BlogEntry will be enough.
Once you modified your data model, you need to synchronize the database with your model. For this purpose, CubicWeb provides a very useful command cubicweb-ctl shell blogdemo which launches an interactive shell where you can enter migration commands (see cubicweb-ctl tool for more details)). As you added the cube named comment, you need to run:
add_cube('comment')
You can now start your instance and comment your blog entries.