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Entity Cloning

Joist supports cloning entities, to easily implement feature requests like "duplicate this author", or "duplicate this author and all of their books".

To clone an entity, call em.clone and pass a load-hint of the subgraph you want to be included in the clone operation.

For example, to clone an Author plus all of their Books and all of the Book's BookReviews, you can call:

const a1 = await em.load(Author, "a:1");
const a2 = await em.clone(a1, { books: "reviews" })

After the em.clone is finished:

  • a2 will be a copy of a1 with all the same primitive field values, but a new primary key/new identity
  • Each Book in a1.books will have a new Book instance created, and be correctly hooked up to a2 instead of the original a1
  • Each BookReview in each will have a new BookReview instance created, and again be correctly up to the right newly-created Book instance in a2.books

Besides setting the correct "parent" to a2 for each cloned child Book, any other references/FKs in the newly-created entities that happened to point to also-cloned input entities (like a1.favoriteBook pointing to a1.books.get[0]) are adjusted to point to the correct/corresponding newly-cloned output entity.

Basically Joist will keep the subgraph of cloned entities intact.

Advanced Features

When calling em.clone, you can provide three config options to customize the behavior:

  • opts.deep is the load hint from above, i.e. { books: "reviews" }, that specifies the subgraph to clone.

  • opts.skipIf is a function that accepts an entity and returns true if that entity should be skipped/not cloned:

    // This will duplicate the author's books, but skip any book where the title includes `sea`
    const duplicatedBooks = await em.clone(
    { skipIf: (original) => original.title.includes("sea") }
  • opts.postClone is a function that accepts both the original entity and its new clone, to allow customizing to the clone:

    // This will duplicate the author's books, and assign them to a different author
    const duplicatedBooks = await em.clone(
    { postClone: (_original, clone) => }