fix(core): do not try to propagate changes to mapToPk relations

Closes mikro-orm#4254

tests/features/unit-of-work/GH4254.test.ts

@@ -0,0 +1,68 @@
+import { Cascade, Entity, LoadStrategy, OneToOne, PrimaryKey, Property } from '@mikro-orm/core';
+import { MikroORM } from '@mikro-orm/sqlite';
+
+@Entity()
+export class OrderSummary {
+
+  @OneToOne({
+    entity: () => Order,
+    onDelete: 'cascade',
+    primary: true,
+    mapToPk: true,
+  })
+  orderId!: number;
+
+  @Property()
+  prop!: string;
+
+}
+
+@Entity()
+export class Order {
+
+  @PrimaryKey()
+  id!: number;
+
+  @OneToOne(() => OrderSummary, 'orderId', {
+    cascade: [Cascade.ALL],
+    eager: true,
+  })
+  summary!: OrderSummary;
+
+}
+
+let orm: MikroORM;
+
+beforeAll(async () => {
+  orm = await MikroORM.init({
+    dbName: ':memory:',
+    entities: [Order, OrderSummary],
+  });
+  await orm.schema.createSchema();
+});
+
+afterAll(async () => {
+  await orm.close(true);
+});
+
+test(`GH issue 1352`, async () => {
+  const entity = orm.em.create(Order, {
+    id: 4,
+    summary: orm.em.create(OrderSummary, {
+      orderId: 4,
+      prop: '123',
+    }),
+  });
+  orm.em.persist(entity);
+  await orm.em.flush();
+});
+
+test(`GH issue 4254`, async () => {
+  orm.em.create(Order, {
+    id: 4,
+    summary: {
+      orderId: 4,
+      prop: '123',
+    },
+  });
+});