Rails的“ has_many”协会

Question

I'm new to learning Rails and I'm a bit confused about associations.

Say for example, I have a Car which can belong to either a Owner , a Renter or a Company and can only belong to one of them and a Owner , Renter or Company can have many Cars .

How would you recommend I go about modelling this scenario? Should there be three foreign keys on the Car table for owner_id , render_id and company_id ? Or have some sort of join table for each of them which would result in something like:

| car_id | owner_id |
|--------|----------|
| 1      | 1        |
| 2      | 1        |
| 3      | 1        |

Or is there another way to achieve this? While considering that more dependents (more groups of renters, owners etc.) could be added.

Thanks in advance.

Answer 1

This is a classic example of where you would use a polymorphic association.

class Car
  belongs_to :possessor, polymorphic: true
end


class Owner
  has_many :cars, as: :possessor
end

class Renter
  has_many :cars, as: :possessor
end

class Company
  has_many :cars, as: :possessor
end

There are two new fields in the cars table, possessor_type and possessor_id and you can add them with a migration, and you can add other models that might possess a car and there's no need to add more columns to cars

Answer 2

One of possible ways: make Car to have foreign keys on Owner , Renter , Company .

Here is an example.

class Car < ApplicationRecord
    belongs_to :owner
    belongs_to :renter
    belongs_to :company
end

class Owner < ApplicationRecord
    has_many :cars
end

class Renter < ApplicationRecord
    has_many :cars
end

class Company < ApplicationRecord
    has_many :cars
end

Cars table
id| owner_id | renter_id | company_id |
- |----------|-----------|------------|
1 | 1        | 1         |2           |
2 | 1        | 1         |1           |
3 | 3        | 2         |1           |

The has_many Association