Active Record Associations and papermill

I have a model "Chapter"
I figured out how to add assets to chapter through papermill plugin.
Now the question:
how can i add additional description to each asset?

It can look like:
chapter has_many items
item belongs_to chapter
item has_one papermill_asset
papermill_asset belongs_to item

But i don't know how to build models and view for described scheme.

How can I add new line in "intem" table then papermill creats new one
in papermill_assets?

Or may be you know the true way to solve this problem?

Any help?

Thx.