@Entity()
export class City {
  @PrimaryGeneratedColumn('uuid')
  id: string;

  @Column()
  caption: string;

  @RelationId('zips')
  zipCodes: string[];

  @ManyToMany(() => Zip, zip => zip.cities, {
    cascade: true,
  })
  @JoinTable()
  zips: Zip[];
}

Entity()
export class Country {
  @PrimaryColumn({ length: 2 })
  code: string;

  @Column()
  caption: string;

  @OneToMany(() => Zip, zip => zip.country)
  zips: Zip[];
}

@Entity()
export class Zip {
  @RelationId('country')
  @PrimaryColumn({ length: 2 })
  countryCode: string;

  @ManyToOne(() => Country, country => country.zips, {
      createForeignKeyConstraints: true,
  })
  @JoinColumn({ name: 'countryCode' })
  country: Country;

  @PrimaryColumn()
  code: string;

  @RelationId('cities')
  cityIds: string[];

  @ManyToMany(() => City, city => city.zips)
  @JoinTable({ name: 'city_zips_zip' })
  cities: City[];
}

await connection.getRepository(Country).save({
  code: 'de',
  caption: 'Germany',
});

// create works great 
const city = await connection.getRepository(City).save({
  id: 'INSERT_ID_OF_TEST_CITY_HERE',
  caption: 'Test city',
  zips: [
    {
      countryCode: 'de',
      code: '60311',
    },
    {
      countryCode: 'de',
      code: '35781',
    },
  ],
}, {
  reload: true,
});

// but if we update the relations the exception throws

await connection.getRepository(City).save({
  id: city.id,
  zips: [
    {
      countryCode: 'de',
      code: '60311',
    },
  ],
});