@Entity()
export Class EntityA extends BaseEntity {
  @PrimaryGeneratedColumn()
  EntityAId: number;

  @Column()
  SomeCol: string;

  @OneToMany(type => EntityB, entity => entity.EntityA, {
    cascadeInsert: true
  })
  EntityBs: EntityB[];
}

@Entity()
export Class EntityB extends BaseEntity {
  @PrimaryGeneratedColumn()
  EntityAId: number;

  @ManyToOne(type => EntityA, (entity: EntityA) => entity.EntityBs)
  @JoinColumn({ name: 'fkEntityA' })
  EntityA: EntityA;
}

...
let entity = await EntityA.findOne({
  SomeCol: 'SomeVal'
});

entity.EntityBs = [
  EntityB.create()
];

entity.save();