Embeddables - update query gets executed even without change in some cases

[ml1nk]

Describe the bug
If there is an ManyToOne relation inside a embeddable and the related entity uses foreign key as a primary key and the embeddable is nullable, then it generates an update on flush without even accessing any of the properties of the entity.

The faulty testcase produces the following sql:

update `b` set `a_c_node_id` = NULL where `id` = 1

To Reproduce

import { Embeddable, Embedded, Entity, EventArgs, EventSubscriber, ManyToOne, OneToOne, PrimaryKey, PrimaryKeyProp, PrimaryKeyType, Property, Ref } from '@mikro-orm/core';
import { MikroORM } from '@mikro-orm/sqlite';

@Embeddable()
export class A {

  @Property()
  alpha!: number | null;

  @ManyToOne(() => C, { onDelete: 'CASCADE', onUpdateIntegrity: 'cascade' })
  c!: Ref<C>

}

@Entity()
export class B {

  @PrimaryKey()
  id!: number;

  @Embedded({ entity: () => A, object: false, nullable: true })
  a!: A  | null;


}

@Entity()
export class C {

  [PrimaryKeyType]!: number
  [PrimaryKeyProp]!: 'node'
  @OneToOne({ entity: ()=> D, primary: true, onDelete: 'cascade', onUpdateIntegrity: 'cascade' })
    node!: Ref<D>

}


@Entity()
export class D {
  @PrimaryKey()
  id!: number;
}




@Embeddable()
export class A2 {

  @Property()
  alpha!: number | null;

  @ManyToOne(() => C2, { onDelete: 'CASCADE', onUpdateIntegrity: 'cascade' })
  c2!: Ref<C2>

}

@Entity()
export class B2 {

  @PrimaryKey()
  id!: number;

  @Embedded({ entity: () => A2, object: false, nullable: true })
  a2!: A2  | null;


}

@Entity()
export class C2 {

  @PrimaryKey()
  id!: number;

}


export class FooBarSubscriber implements EventSubscriber {
  async afterUpdate(args: EventArgs<any>): Promise<void> {
    throw new Error("afterUpdate called but nothing changed");
  }
}

describe.only('GH issue ', () => {

  let orm: MikroORM;

  beforeAll(async () => {
    orm = await MikroORM.init({
      entities: [A, B, C, D, A2, B2, C2],
      dbName: ':memory:',
      debug: true,
      subscribers:[new FooBarSubscriber()]
    });
    await orm.getSchemaGenerator().ensureDatabase();
    await orm.getSchemaGenerator().dropSchema();
    await orm.getSchemaGenerator().createSchema();
  });

  afterAll(() => orm.close(true));


  test('embedded with ManyToOne to foreign key as primary key entity', async () => {
    orm.em.create(B, { a: null });
    await orm.em.flush();
    orm.em.clear();
    await orm.em.getRepository(B).findAll()
    await orm.em.flush(); // update query is created
    orm.em.clear()
  });


  test('embedded with ManyToOne to entity', async () => {
    orm.em.create(B2, { a2: null });
    await orm.em.flush();
    orm.em.clear();
    await orm.em.getRepository(B2).findAll()
    await orm.em.flush(); // no update query is created
    orm.em.clear()
  });
});

Additional context
Trying to reproduce the behavior lead to #4787

Versions

Dependency	Version
mikro-orm	cbc0c50