Cascading persist, merge and remove

When persisting or removing entity, all your references are by default cascade persisted. This means that by persisting any entity, ORM will automatically persist all of its associations.

You can control this behaviour via cascade attribute of @ManyToOne, @ManyToMany, @OneToMany and @OneToOne fields.

New entities without primary key will be always persisted, regardless of cascade value.

// cascade persist & merge is default value
@OneToMany({ entity: () => Book, mappedBy: 'author' })
books = new Collection<Book>(this);

// same as previous definition
@OneToMany({ entity: () => Book, mappedBy: 'author', cascade: [Cascade.PERSIST, Cascade.MERGE] })
books = new Collection<Book>(this);

// only cascade remove
@OneToMany({ entity: () => Book, mappedBy: 'author', cascade: [Cascade.REMOVE] })
books = new Collection<Book>(this);

// cascade persist and remove (but not merge)
@OneToMany({ entity: () => Book, mappedBy: 'author', cascade: [Cascade.PERSIST, Cascade.REMOVE] })
books = new Collection<Book>(this);

// no cascade
@OneToMany({ entity: () => Book, mappedBy: 'author', cascade: [] })
books = new Collection<Book>(this);

// cascade all (persist, merge and remove)
@OneToMany({ entity: () => Book, mappedBy: 'author', cascade: [Cascade.ALL] })
books = new Collection<Book>(this);

// same as previous definition
@OneToMany({ entity: () => Book, mappedBy: 'author', cascade: [Cascade.PERSIST, Cascade.MERGE, Cascade.REMOVE] })
books = new Collection<Book>(this);

Cascade persist

Here is example of how cascade persist works:

const book = await orm.em.findOne(Book, 'id', ['author', 'tags']);
book.author.name = 'Foo Bar';
book.tags[0].name = 'new name 1';
book.tags[1].name = 'new name 2';
await orm.em.persist(book); // all book tags and author will be persisted too

When cascade persisting collections, keep in mind only fully initialized collections will be cascade persisted.

Cascade merge

When you want to merge entity and all its associations, you can use Cascade.MERGE. This comes handy when you want to clear identity map (e.g. when importing large number of entities), but you also have to keep your parent entities managed (because otherwise they would be considered as new entities and insert-persisted, which would fail with non-unique identifier).

In following example, without having Author.favouriteBook set to cascade merge, you would get an error because it would be cascade-inserted with already taken ID.

const a1 = new Author(...);
a1.favouriteBook = new Book('the best', ...);
await orm.em.persistAndFlush(a1); // cascade persists favourite book as well

for (let i = 1; i < 1000; i++) {
  const book = new Book('...', a1);
  orm.em.persistLater(book);
  
  // persist every 100 records
  if (i % 100 === 0) {
    await orm.em.flush();
    orm.em.clear(); // this makes both a1 and his favourite book detached
    orm.em.merge(a1); // so we need to merge them to prevent cascade-inserts
    
    // without cascade merge, you would need to manually merge all his associations
    orm.em.merge(a1.favouriteBook); // not needed with Cascade.MERGE
  }
}

await orm.em.flush();

Cascade remove

Cascade remove works same way as cascade persist, just for removing entities. Following example assumes that Book.publisher is set to Cascade.REMOVE:

Note that cascade remove for collections can be inefficient as it will fire 1 query for each entity in collection.

await orm.em.removeEntity(book); // this will also remove book.publisher

Keep in mind that cascade remove can be dangerous when used on @ManyToOne fields, as cascade removed entity can stay referenced in another entities that were not removed.

const publisher = new Publisher(...);
// all books with same publisher
book1.publisher = book2.publisher = book3.publisher = publisher;
await orm.em.removeEntity(book1); // this will remove book1 and its publisher

// but we still have reference to removed publisher here
console.log(book2.publisher, book3.publisher);