Determine type in polymorphic relation

Rails
1

Hey,

I’ve got a polymorphic relation and would like to create an object. How
is it possible to determine whether its a ‘car’ or a ‘truck’?

A person can owe cars and trucks. I could pass params like the car/truck
id to the action but that doesn’t really help me.

For now I do it like this but I don’t like it at all:
person.cars = Car.find(params[:id]) if params[:type] == “Car”
person.truck = Truck.find(params[:id]) if params[:type] == “Truck”

This is really not how I want it. Does anyone knows how to solve that
kind of problem?

Thanks

2

I think that you should create some kind of an interface, for example
Vehicle, like this:

class Vehicle < ActiveRecord::Base
belongs_to :person, :polymorphic => true
end

After that you should make the inherited models Car and Truck:

class Car < Vehicle
end

class Truck < Vehicle
end

and finally define the person model:

class Person < ActiveRecord::Base
has_many :vehicles
end

Keep in mind that the Vehicle table should have a string column :type,
where the class of the record is stored.

With this models you can do this:

p = Person.create(:name => “John”)
p.vehicles << Car.create(:model => “Fiat”)
p.vehicles << Truck.create(:model => “Mercedes”)

For more information:

Does that help answer your question?

–Valentin

On Sep 22, 5:51 pm, Heinz S. [email protected]

3

I’m not sure of what you need to do but I’ll assume that you just want
to create an association between a person and a vehicle of type car or
truck (or whatever).

I am assuming that you have STI built into your vehicles table, with a
column named ‘type’ that stores the type of vehicle
(‘Car’/‘Truck’/‘Whatever’). If you just want to assign a vehicle to
the person I don’t see a reason why you should know the type of
vehicle it is.

Based on your code you know the id of the vehicle. Why not do
something simpler?:

person.car = Vehicle.find(params[:id])

You might not know the type of vehicle it is but does it matter?

And going even further, do you really need to retrieve the vehicle
row? I guess that if you can’t trust the id beeing an honest value you
should but if you could trust it, couldn’t you just do?:

person.vehicle_id = params[:id]

Maybe I am not understanding your need?

On Sep 22, 10:51 am, Heinz S. [email protected]

4

Heinz S. wrote:

First of all thank you guys! I think what Valentin wrote is just what I
was looking for. Person, Car, Truck was just an example. It’s a lot more
complexe in my application and you can’t just merge these two different
models into one and add a :type.

I’ll do it like Valentin suggested. Thanks again! :slight_smile:

It should be noted that Valentin suggested a STI solution (which is what
I would have recommended) and your question resolved around polymorphic
associations which is a different concept entirely. Polymorphic
associations deal with association polymorphism where as STI deals
primarily wih model polymorphism.

hth

ilan

5

First of all thank you guys! I think what Valentin wrote is just what I
was looking for. Person, Car, Truck was just an example. It’s a lot more
complexe in my application and you can’t just merge these two different
models into one and add a :type.

I’ll do it like Valentin suggested. Thanks again! :slight_smile: