em.persist(entity) under specific conditions empties removeStack and causes em.flush() to error.

[Jcoke]

Describe the bug
Deleting a child entity with a relation to parent X, then persisting a new child entity with a relation to the same parent causes the removeStack to be emptied, and em.flush() to error.

I load a parent (OneToMany) and its children (ManyToOne) from the database:

const storedParent = em.findOneOrFail(Parent, "parentId", {populate: ["children"]})

I delete one of the children using em.remove():

em.remove(storedParent.children[0])
console.log(em.getUnitOfWork().getRemoveStack().size) // 1

When I persist a new Child entity with a relation to the Parent entity, the removeStack is emptied:

const newChild = new Child().assign(
  { id: "newChildId", parent: storedParent },
  { em }
);
em.persist(newChild)
console.log(em.getUnitOfWork().getRemoveStack().size) // 0

After this, em.flush() errors on duplicate primary key as it tries to insert the removed child:

em.flush() // Error, duplicate PrimaryKey on storedParent.children[0].id

To Reproduce

Entity definition:

@Entity()
export class Parent{
  @PrimaryKey()
  id: string;

  @OneToMany({
    entity: () => Child,
    mappedBy: (c) => c.parent,
  })
  children = new Collection<Child>(this);
}

@Entity()
export class Child{
  @PrimaryKey()
  id: string;

  @ManyToOne({
    entity: () => Parent,
    wrappedReference: true,
    index: true,
    onDelete: "cascade",
  })
  parent: IdentifiedReference<Parent>;

  @Field()
  @Property({ persist: false })
  get parentId(): string {
    return this.parent.id;
  }
}

Create intial dataset:

const parent = new Parent().assign(
  { id: "parentId" },
  { em }
);
em.persist(parent);

const child1 = new Child().assign(
  { id: "childId1", parent },
  { em }
);
em.persist(child1);

const child2 = new Child().assign(
  { id: "childId2", parent },
  { em }
);
em.persist(child2);

await em.flush();

Query created data, delete a child, create and persist a new child

const storedParent = await em.findOneOrFail(Parent, "parentId", {
  populate: ["children"],
});
const removeChild = storedParent.children[0]

// Remove child
em.remove(removeChild)

// Remove stack gets + 1 element
console.log(em.getUnitOfWork().getRemoveStack().size);

// Add unrelated child to same parent
const newChild = new Child().assign(
  {
    id: "newChildId",
    parent: storedParent,
  }
  { em }
);

// Remove stack size is still 1
console.log(em.getUnitOfWork().getRemoveStack().size);

// Persist 
em.persist(newChild);

// Remove stack is empty after persist
console.log(em.getUnitOfWork().getRemoveStack().size);

// Flushing errors on trying to create a child with the id of the removed child
em.flush()

Current fix: explicitly removing the child from the parent as well, but this feels unintuitive.

em.remove(removeChild)
storedParent.children.remove(removeChild);

Expected behavior
I would expect that a child can be added (persisted) to a parent, after another child has been deleted from the same parent using em.remove(child)

Versions

Dependency	Version
node	12.16.2
typescript	3.9.7
mikro-orm	4.2.1
postgres	12

[B4nan]

Might be issue with the assign helper, it is meant for updating existing entities, not really for inserting new entities. Better to use em.create() (it will call the ctor for you, with the right parameters).

But the issue will be probably cascade persist, as persisting the child is then cascaded to the parent property, and persisting an entity marked for removal will clear the entity from remove stack automatically.

edit: I can confirm the behaviour you described (as well as that em.create() won't help here). Thanks for the repro with detailed description! Only if all the bug reports looked like this 👍

[B4nan]

You can try the fix in 4.2.4-dev.73 before 4.3 is out.