What is Single Table Inheritance ?

Single Table Inheritance is a mechanism through which ActiveRecord can share fields and behavior between different models while using a single table.

For eg. let’s say we need to create a Vehicle model and Car model. Since Vehicle and Car will have common attributes and methods we can use Single Table Inheritance on them.

The schema of our example will look like this:

single-table-inheritance-example

1. Creating Vehicle and Car model for Single Table Inheritance

  • 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

2. Understanding relationship between Vehicle and Car

The Car class inherits Vehicle class which means all behaviors, associations and public methods from Vehicle class are available to Car class as well.

Because Single Table Inheritance both the models will share the same table vehicles which is the table created for the parent class.

Let’s create a Car record:

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

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)

3. Extending the concept to another model

Just like we did above, we can create a Bus model and set its parent as the Vehicle class. Our schema will look like this:

single-table-inheritance-example

Lets insert a new record for the Bus model in the 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 which means it refers to the Bus model.