The application I'm creating deals with maintenance issues allowing a user to report a defect and notify the people responsible for looking at the problem. Think of the application as performing maintenance on 'parts' of a car, but those parts requiring different 'trades' to fix or manufacture them. ie. The auto electrician for the electrics of the car, but also the manufacturer of the electric components.
I'm trying to find an intuitive way of letting the user know that once they have selected the 'part' they then also need to select the 'trade or trades' associated with that part also.
Here is the mockup for reference
The 'parts' list is huge, comparitive to all the parts contained within all cars (so hundreds of thousands potentially). So I have used a miller column style (not tree view)to allow for the size - this is not the issue (unless you have a better suggestion feel free!) then I have used an add button that adds the selection to a collection box, that you can then again select the item and add a trade. The trades list is small, only about 15 maximum, depending on the part selected.
I'm having trouble displaying to the user that they have to perform this 2 step process in order to make the selection complete as a part with trades. At the moment in my mock up it is too arbitrary and I'd like it to be more obvious.
Also a trade can be assigned at any level, meaning it can be assigned to the whole engine (Parent), or just the spark plug within the engine(3rd child down).
Any suggestions, mock ups or useful information would be much appreciated.