typeorm: @BeforeInsert() hook not executed for cascade inserts

Issue type: [x] bug report

Database system/driver: [x] postgres

TypeORM version:

[x] 0.2.17

Context I have an User which has Pictures. For the user and the pictures there should be the ID generated automatically with the @BeforeInsert() hook, if it’s not set. Its working fine for the user - but not for the cascade inserted Picture.

@Entity()
export class User {
  @PrimaryColumn()
  id: string;

  @Column()
  name: string;

  @OneToMany(() => Picture, picture => picture.user, {
    cascade: true
  })
  pictures: Picture[];

  @BeforeInsert()
  generateId = async () => {
    console.log('Is working fine');
    this.id = isEmpty(this.id) ? generate() : this.id;
  };
}

@Entity()
export class Picture {
  @PrimaryColumn()
  id: string;

  @Column()
  title: string;

  @ManyToOne(() => User, user => user.pictures, {
    onDelete: 'CASCADE'
  })
  user: User;

  @BeforeInsert()
  generateId = async () => {
    console.log('Is not working fine here', this.id);
    this.id = isEmpty(this.id) ? generate() : this.id;
  };
}

Problem This isn’t working (The id for picture is missing):

this.userRepository.save({name: 'foo', pictures: [{title: 'bar'}]);

This is working (because the id is not missing):

this.userRepository.save({name: 'foo', pictures: [{id: '123', title: 'bar'}]);

Expected behaviour The ID should be generated also for pictures, because its a cascaded insert happening here for it. The docs for @BeforeInsert() specify that this hook is executed for the repositories save, but i expect it to be executed also for its sub-entities.