Single Table Inheritance

What is Single Table Inheritance ?

Single Table Inheritance is a mechanism through which ActiveRecord can share fields and behavior between different models. For eg. let’s say we need to create a model vehicle and car. Since vehicle and car will share their attributes and methods we can use Single Table Inheritance on them.

Let’s generate vehicles model and a cars model with vehicles as its parent:

rails generate model vehicle type:string color:string price:decimal{10.2}
rails generate model car --parent=Vehicle

The Car model will look something like this:

class Car < Vehicle
end

The Car class inherits Vehicle in Rails and thus all behaviors, associations and public methods from Vehicle are available to Car as well. While both the models will share a same table vehicles in the database. Let’s create a Car record:

Car.create(color: 'Red', price: 10000)

This executes the following query in the database:

INSERT INTO "vehicles" ("type", "color", "price") VALUES ('Car', 'Red', 10000)

We can see that the record is inserted in vehicles table even though we’re using the Car model. Additionally, a type column is also set in the table with the value Car. This column is used to identify which model does the record belongs to.

Here is how the record is stored in the table:

select * from vehicles;
 id | type | color |  price  |         created_at         |         updated_at
----+------+-------+---------+----------------------------+----------------------------
  1 | Car  | Red   | 10000.0 | 2024-04-14 09:01:12.180265 | 2024-04-14 09:01:12.180265
(1 row)

Similarly we can create a Bus model and insert a new record for Bus model in vehicles table:

rails generate model bus --parent=Vehicle
Bus.create(color: 'Red', price: 10000)

Let’s see how our table vehicles looks like:

select * from vehicles;
 id | type | color |  price  |         created_at         |         updated_at
----+------+-------+---------+----------------------------+----------------------------
  1 | Car  | Red   | 10000.0 | 2024-04-14 09:01:12.180265 | 2024-04-14 09:01:12.180265
  2 | Bus  | Red   | 10000.0 | 2024-04-14 09:09:52.739512 | 2024-04-14 09:09:52.739512

We can see that the new record has type set as Bus so it refers to the Bus model.

What is Single Table Inheritance ?#

What is Single Table Inheritance ?