Apollo is not letting me edit an object field because it is readonly, but I cannot just make a copy of it - graphql

Question

I am not sure how I should set cart.items to a new array, I have already made a copy of the original cache because I learned Apollo does not let you directly edit the cache, but I am still getting the following error
Error: Cannot assign to read only property 'items' of object '#'
Do I need to make a copy of the items array? And if so how do I go about changing the array on the current objects item field?
Here are my console.logs
You can ignore the typename fields as they are irrelevant to the problem
addItem
{
"__typename": "Cart",
"items": [
{
"__typename": "CartItem",
"name": "Item 3"
},
{
"__typename": "CartItem",
"name": "Item 4"
},
{
"__typename": "CartItem",
"name": "New Item!"
}
]
}
carts
{
"carts": [
{
"__typename": "Cart",
"id": "1",
"items": [
{
"__typename": "CartItem",
"id": "1",
"name": "Item 1"
},
{
"__typename": "CartItem",
"id": "2",
"name": "Item 2"
}
]
},
{
"__typename": "Cart",
"id": "2",
"items": [
{
"__typename": "CartItem",
"id": "3",
"name": "Item 3"
},
{
"__typename": "CartItem",
"id": "4",
"name": "Item 4"
}
]
}
]
}

Answer 1

So it seems you need to remake the items array as well since items is its own gql object type the easiest way to do this was to do it all at once with a map.
Please note the comment as that was an important detail I learned
// IMPORTANT NOTE when updating the cache of a query you must return the
// same fields as the original query even if you aren't using them in the code
const GET_CARTS = gql`
query {
carts{
id
items{
id
name
}}} `;
const MUTATION = gql`
mutation AddItem($input:MutationAddItemInput!) {
addItem(input: $input){
items{
id
name
}
}
}
`;
const { loading, error, data } = useQuery(GET_CARTS)
const [addItem] = useMutation(MUTATION, {
// refetchQueries: [{ query: GET_CARTS }]
update(cache, { data: { addItem } }) {
// addItem is the response of the query of add item function
console.log({ addItem });
// #ts-ignore
let { carts } = cache.readQuery({ query: GET_CARTS });
console.log({ carts })
// make a new array out of the carts array and add the new item to the array if the id of the cart is 2
let newCarts = carts.map((cart: Cart) => {
if (cart.id === "2") {
return { ...cart, items: [...addItem.items] }
} else {
return cart
}
})
console.log({ newCarts });
cache.writeQuery({
query: GET_CARTS,
data: { carts: newCarts }
// data: { carts: [{ id: "1", items: [{ id: "2", name: "an item" }] }] }
})
}
})
And lastly you will call the addItem function from the use mutation hook