Remove user_id from PromotionRules

[jhawthorn]

Both the belongs_to and the column have been around since promo was merged into spree_core, but I don't think they have ever been used.

[jasonfb]

I don't think we are using this. I checked our DB and there are no values in the user_id column, and there existing (non-scalable) "By User" rule should be using the join table spree_promotion_rules_users, so I this seems 👍 to me

[gmacdougall]

Should we consider deprecating this instead and removing the column in the next major version?

[jrochkind]

I think it's probably just okay to remove, I can't think of any use case for it (when I saw it there myself a month or so ago, I couldn't figure it out either, heh). In what cases would a promotion be associated with exactly one and only one user? A single-user-specific promotion seems unlikely.

If you want to be extra safe, you could have the migration that removes it written to make sure the column is nil in the database, and refrain from removing it if it has any values, emitting some scary warning that you might want to look into it and remove it yourself.

[mamhoff]

👍

[jhawthorn]

Rebased and added a note to CHANGELOG