Question

I have two model classes: Cars and Customers,

Model Cars:

class car < ActiveRecord::Base

  #car has attribute :town_code

  has_many :customers

end

Model Customers:

class customer < ActiveRecord::Base
  # customer has attribute :first_name, :last_name

  belongs_to :car
end

In my controller, I have the following code:

my_customer = Customer.find_all_by_first_name('John')
p my_customer.last_name
p my_customer.car_id

But I got no attribute 'car_id' error, I also got no attribute 'last_name' error.

—Question 1:—

I checked my database, I do have ‘car_id’ and ‘last_name’ columns on my customer table. Why I can not access them in the way how my controller code does?

—Question 2:—

but the code : my_customer.map(&:car_id) is working for accessing car_id, however, I do not quite understand the code .map(&:car_id), what does it do? Can anyone explains to me?

Answer 1

The reason you aren’t able to do my_customer.last_name is that my_customer is not a Customer here but an array of Customers, since you did find_all. That’s also why my_customer.map(&:car_id) works. What that bit of code means is: For each object in the array my_customer, call the method car_id and insert the results into a new array — and return that new array.

If customer belongs to car, you need a car_id in the customer table (which corresponds to an id column in the car table). Also, you shouldn’t have last_name in the car table, but rather in the customer table.

It sounds like you may need to step back and gain a better understanding of ActiveRecord associations. It’s not clear to me why a customer would belong_to a car, anyway.