Writing new fragment or query to cache

Help
,
1

It seems like all the useMutation update examples change an already existing fragment or query in the cache. I’d like to use useMutation’s update parameter to add a completely new fragment to the cache, but when I try to use writeFragment or even writeQuery to add to the cache, nothing changes. Any help would be much appreciated.

here’s some pseudo code that explains how I’m going about it:

const [createBook] = useMutation(CREATE_BOOK)

const book = {
  id: "1234",
  __typename: "Book",
  name: "The Guns of August",
  author: "Barbara W. Tuchman"
}
createBook({
  variables: {
    book
  },
  optimisticResponse: {
    createBook: {
      ...book,
    },
  },
  update: (cache, { data: { createBook } }) => {
    const bookId = "Book:" + createBook._id
    // doesn't work
    cache.writeQuery({
      query: GET_BOOK,
      data: {
        book: {
          ...createBook,
        },
      },
      variables: {
        _id: createBook._id,
      },
    })
    // neither does this
    cache.writeFragment({
      fragment: gql`
        fragment BookFragment on Book {
          _id
          name
          author
        }
      `,
      id: bookId,
      data: {
        ...createBook,
      },
    })
  }
})

also, I’m logging cache.data.data to look for my updates.

2

okay, it seems like I can read directly from the cache with cache.readQuery, but in a component that loads later and uses useQuery with the same query and variables, data is undefined if the fetch policy is “cache-only” why can’t useQuery read my new query?