Mr_Monkey What approach do you think for setting the position value for an entity in the front-end. In my proposal I have described it using a input field. MusicBrainz uses Up and Down arrow icon to set the order of the entities.
Also we will need to add a number attribute to the series_order table. This number will be displayed in the # column of the entity table.
using arrows to arrange entity order sometimes becomes clumsy when there are many entities in the series. using input field will a good choice but we will need to set 2 validation for the position input field.
(i) position value. should be assigned for all the entities when the ordering type is manual.
(ii) No two entity can have same position value.
Violating the above two will pop a warning message so the that user can fix that.
Quoting your reply
> Instead of the position attribute on series, I’d like to see a more generic relationship attributes system that could be used for other cases.
For example, that position would only be relevant to "series contains X” relationships, while for example we will want a way to describe that a relationship started and ended at a certain date (e.g. "Author_A was married to Author_B from start_date to end_date”).
Mr_Monkey If I am not wrong, currently we can't set this kind of relationship in BookBrainz: `Author_A was married to Author_B from start_date to end_date.