typeorm/docs/faq.md

FAQ and best practices

• How to do validation?
• What does "owner side" in relations mean or why we need to put @JoinColumn and @JoinTable decorators?
• How to handle outDir TypeScript compiler option?
• How to use TypeORM with ts-node?

How to do validation?

Validation is not part of TypeORM because validation is a separate process don't really related to what ORM does. If you want to use validation use class-validator library - it works perfectly with TypeORM.

What does "owner side" in relations mean or why we need to put @JoinColumn and @JoinTable decorators?

Let's start with one-to-one relation. Let's say we have two entities: User and Photo:

@Entity()
export class User {
    
    @PrimaryGeneratedColumn()
    id: number;
    
    @Column()
    name: string;
    
    @OneToOne()
    photo: Photo;
    
}

@Entity()
export class Photo {
    
    @PrimaryGeneratedColumn()
    id: number;
    
    @Column()
    url: string;
    
    @OneToOne()
    user: User;
    
}

This example does not have @JoinColumn decorator which is not correct. Why? Because to make a real relation we need to create a column in the database. We need to create a column userId in photo table or photoId in user table. But which exactly column should be created - userId or photoId? ORM cannot decide it for you. To make a decision you must @JoinColumn decorator on one of the side. If you put @JoinColumn in Photo entity then userId column will be created in photo table. If you put @JoinColumn in User entity then photoId column will be created in user table. Where you put @JoinColumn decorator that side will be called "owner side of the relationship". Other side of relation, without @JoinColumn decorator is called "inverse (non-owner) side of relationship".

Same in @ManyToMany relation you use @JoinTable decorator to show owner side of relation.

In @ManyToOne or @OneToMany relations @JoinColumn decorator is not necessary because both decorators are different and where you put @ManyToOne decorator that table will have relational column.

@JoinColumn and @JoinTable decorators are also can be used to specify additional join column / junction table settings, like join column name or junction table name.

How to handle outDir TypeScript compiler option?

When you are using outDir compiler option don't forget to copy into output directory assets and resources your app is using. Or make sure to setup correct paths to those assets.

One thing important to know is when you remove or move entities old entities left unremoved inside ouput directory. For example you create Post entity and renamed it to Blog. You don't have Post.ts file in your project, however Post.js file is left inside output directory. Now when TypeORM reads entities from your output directory it sees two entities - both Post and Blog. This may be a source of bugs. That's why when you remove and move entities with outDir enabled its strongly recommended to remove your output directory and recompile the project again.

How to use TypeORM with ts-node?

You can prevent compiling files each time using ts-node. If you are using ts-node you can specify ts entities inside your connection options:

{
    entities: "src/entity/*.ts",
    subscribers: "src/subscriber/*.ts"
}

Also if you want to use CLI via ts-node you can execute typeorm following way:

ts-node ./node_modules/bin/typeorm schema:sync