Update the cache of Apollo client 3 when polling not working - caching

Question

I am playing with the cache of #apollo/client v3. Here's the codesandbox.
I am adding a user to a cached list of users using client.writeQuery, and the query has a pollInterval to refetch every few seconds.
I am able to add the user to the list, it does refresh the UI, and I can see the pollInterval working in the network tab of Chrome.
THE PROBLEM
I would expect the list of users to return to its initial state when the polling kicks in, and overwrite the user I added manually to the cache, but it does not.
Apollo config
export const cache = new InMemoryCache();
const client = new ApolloClient({
cache,
link: new HttpLink({
uri: "https://fakeql.com/graphql/218375d695835e0850a14a3c505a6447"
})
});
UserList
export const UserList = () => {
const { optimisticAddUserToCache, data, loading } = useUserList();
if (loading) {
return <div>Loading...</div>;
}
return (
<div>
<button onClick={() => optimisticAddUserToCache()}>Add User to cache</button>
<ol>
{data?.users.map(user => {
return <li key={user.id}>{user.firstname}</li>;
})}
</ol>
</div>
);
}
useUserList
const GET_USER_LIST = gql`
query Users {
users {
id
firstname
}
}
`;
export const useUserList = () => {
const { loading, error, data, refetch } = useQuery(GET_USER_LIST, {
pollInterval: 4000 // It does poll (check chromes's network tab), but it doesn't seem to overwrite the cache
});
const client = useApolloClient();
const optimisticAddUserToCache = () => {
const newUser: any = {
id: `userId-${Math.random()}`,
firstname: "JOHN DOE",
__typename: "User"
};
const currentUserList = client.readQuery({ query: GET_USER_LIST }).users;
// This works, it does add a user, and UI refreshes.
client.writeQuery({
query: GET_USER_LIST,
data: {
users: [newUser, ...currentUserList]
}
});
};
return { optimisticAddUserToCache, loading, error, data, refetch };
};

Answer 1

Working as expected (almost)
Polled response arrives always with the same data ...
... doesn't result in write to cache (no content compared) ...
... no data change in cache ...
... data property (from useQuery) not updated ...
... no data updated, no component rerendering.
For optimistic update working you need a real mutation, real [persisted] change on remote datasource ... propagated to next polled responses.