I am design drop down list for a business application that has about 15 items and of those 15 items about 4 of them have 1 to many relationship, meaning those 4 can all be selected at once. However, the other 11 items are a 1 to 1 relationship, meaning you can only select one at a time.
My first instinct was to just have checkboxes next to the items that have a one to many relationship and if 2 items are select I would disable any items that do not fit into the relationship.
Here is a screenshot of what I am talking about. As you can see with Van selected Step/Drop Deck is greyed out because there is no relationship between Van and Step/Drop Deck. However, Van does have a relationship with Flatbed and Reefer so those remain enabled.
So far in my experience I have not seen this UI done anywhere and I was wondering if someone has experienced a similar problem and how you solved it, or if you haven't seen this problem how would you solve it?