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Ruby ActiveRecord Basics Migrations and Relationships Single Table Inheritance

Posted by Sebastian Velandia
Sebastian Velandia
24,676 Points

How to add different fields beetween Employees and customers?

In the video Hampton explained how to add inheritance for employees and Customers, both inherit from Account and both have the same fields

How can I set up different fields for Employees and Customers?

let's say I want to add the field "preferences" for customers but not for employees and "Hour rate" for employees but not for customers

I do not know how to do that. Thanks

1 Answer

Joshua Shroy
Joshua Shroy
9,943 Points

Hey Sebastian,

If you need to add more fields (aka columns) to the database, you will first want to add them to the base model Account. To do this, use can use the following command:

rails g migration add_preferences_and_hour_rate_to_account preference:string hour_rate:integer

Then you can create validations on each model to specify exactly which data will be used from the base model.

For example..

class Account < ActiveRecord::Base
end

class Employee < Account
  validates_inclusion_of :hour_rate
  validates_exclusion_of :preferences

end

class Customer < Account
  validates_inclusion_of :preferences
  validates_exclusion_of :hour_rate
end
Sebastian Velandia
Sebastian Velandia
24,676 Points

Ho, Ok let me know if I understood, from the database point of view, I will have a table accounts with the columns for customers and employees? so when I create a new employee the columns for customers are going to be null in the data base? is that a good design pattern? is there another way to do it?

Joshua Shroy
Joshua Shroy
9,943 Points

You would actually have a table accounts with a column called type which would store the class/model it's associated with. I'm sorry I forgot to mention in the post above that you would need to create the column type as a string in accounts:

rails g migration add_type_to_accounts type:string

Let's say you created an employee with Employee.create(name: "Charles"), this record would be saved in the accounts table with :type = Employee. This would allow you to fetch this record later on with something like Account.where(name: 'Charles').first which will return a Employee object.

It's a great design pattern when you have models in need of near identical fields such as the one above. If you feel like you will need more customized fields than you might want to start thinking about creating separate tables for Employees and Customers.

Hope that helps!

Sebastian Velandia
Sebastian Velandia
24,676 Points

Thank you!, I think it's clearer to me now.. Thanks again =)

Joshua Shroy
Joshua Shroy
9,943 Points

No problem :)