Hello,
I would like to populate dropdown list with values from two different models. My first idea was to create a polymorphic association. But then I got stuck with the code for a dropdown list. How to make it return object_id and object_type?
Here are my models:
card.rb ----------------------------------------------- class Card < ActiveRecord::Base belongs_to :vehicle, :polymorphic => true end
No, wouldn't work, since a car and a boat can have the same id.
You can though use something like name and car_id or boat_id as identifier (like car_123 and boat_123) Then in you controller params[:id] (or whatever) would contain that and you could use some string functions to split at the _ the type and id
This is a good example of one of the problems with the polymorphic model as it is commonly implemented (not only in Rails solutions). String functions to split the type and id? Frankly, that is a kludge that is going to lead to interesting issues as you try to do more with your data. I'd suggest you consider that your underlying logical model is not correct for your requirements. Try modeling Car and Boat as subtypes of Vehicle, which would be the supertype. A Card would then have vehicle_id as a foreign key (unless cards can belong to more than one vehicle, in which case you need a join model between Vehicle and Card). At the implementation level, Vehicle could have a database table (and model) of its own. The Cars and Boats tables would then each have vehicle_id as a foreign key. You would also be able to use foreign key constraints in Cars and Boats to help ensure the integrity of your data - the polymorphic association method Rails uses does not allow for these constraints, which is a serious problem unless you don't need to worry about the reliability of the data in your database (but if you can't rely on the integrity of the data, what use is your database?). For more information, see "Case*Method: Entity Relationship Modelling" by Richard Barker:
STI better suits what you're asking about doing:
card.rb
What "better suits" can only be determined by a proper analysis of the business requirements. It might appear to solve the one issue that has been presented here, but is it the appropriate solution in the wider context of the whole system? The supertype-subtype model I suggested as a *possibility* is a well-known pattern that can be used in situations like this.
Single-Table Inheritance will result in a table with lots of NULLs in it. You then have the issue of ensuring that the columns for one model are filled, while columns for other models have NULL in them. On the subject of NULLs, see: NULL values in a database
If you decide STI is appropriate for your needs, this might help get you started: Rookie on Rails: Single Table Inheritance in Rails Or see the Agile Web Development with Rails book, of course.