Skip to main content

Entity Inheritance

Concrete Table Inheritance​

You can reduce duplication in your code by using entity inheritance patterns. The simplest and the most effective is concrete table inheritance.

For example, you have Photo, Question, Post entities:

@Entity()
export class Photo {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    description: string

    @Column()
    size: string
}
@Entity()
export class Question {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    description: string

    @Column()
    answersCount: number
}
@Entity()
export class Post {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    description: string

    @Column()
    viewCount: number
}

As you can see all those entities have common columns: id, title, description. To reduce duplication and produce a better abstraction we can create a base class called Content for them:

export abstract class Content {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    description: string
}
@Entity()
export class Photo extends Content {
    @Column()
    size: string
}
@Entity()
export class Question extends Content {
    @Column()
    answersCount: number
}
@Entity()
export class Post extends Content {
    @Column()
    viewCount: number
}

All columns (relations, embeds, etc.) from parent entities (parent can extend other entity as well) will be inherited and created in final entities.

This example will create 3 tables - photo, question and post.

Single Table Inheritance​

TypeORM also supports single table inheritance. Single table inheritance is a pattern when you have multiple classes with their own properties, but in the database they are stored in the same table.

@Entity()
@TableInheritance({ column: { type: "varchar", name: "type" } })
export class Content {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    title: string

    @Column()
    description: string
}
@ChildEntity()
export class Photo extends Content {
    @Column()
    size: string
}
@ChildEntity()
export class Question extends Content {
    @Column()
    answersCount: number
}
@ChildEntity()
export class Post extends Content {
    @Column()
    viewCount: number
}

This will create a single table called content and all instances of photos, questions and posts will be saved into this table.

Using embeddeds​

There is an amazing way to reduce duplication in your app (using composition over inheritance) by using embedded columns. Read more about embedded entities here.