Should a query in Apollo Client look for the results cached by different queries before making a network request?

Question

I'm trying to figure out how queries in Apollo Client are supposed to interact with the cache.

Specifically, I want to know if we run a query that fetches all todos:

todos {
  title
  completed
}

And then later we run a query that fetches a single todo that was already fetched by the todos query and requests the exact same fields:

todo(id: $id) {
  title
  completed
} 

Should the second query a) fetch the data from the cache, or b) make a network request?

My assumption was that it would be case A. This is based on this quote from an official Apollo blog post:

https://www.apollographql.com/blog/demystifying-cache-normalization/

For example, if we were to:

1. Perform a GetAllTodos query, normalizing and caching all todos from a backend
2. Call GetTodoById on a todo that we had already retrieved with GetAllTodos

...then Apollo Client could just reach into the cache and get the object directly without making another request.

However, in my app I kept getting case B, it was always making an additional network request even though I had already requested all the data in a different query.

I assumed that I was doing something wrong, so I checked out this Apollo Full-stack Tutorial repo (https://github.com/apollographql/fullstack-tutorial) and updated the LaunchDetails query to only request the same data that was already requested in the GetLaunchList query. This replicated the same scenario I detailed above with the todos.

The queries now look like this:

export const GET_LAUNCHES = gql`
  query GetLaunchList($after: String) {
    launches(after: $after) {
      cursor
      hasMore
      launches {
        ...LaunchTile
      }
    }
  }
  ${LAUNCH_TILE_DATA}
`;

export const GET_LAUNCH_DETAILS = gql`
  query LaunchDetails($launchId: ID!) {
    launch(id: $launchId) {
      ...LaunchTile
    }
  }
  ${LAUNCH_TILE_DATA}
`;

I ran the application, and found that a new network request was made for the LaunchDetails query, even though all the required data was already in the cache after the GetLaunchList query was run.

I haven't been able to find any answer to this in the documentation, and the results I'm seeing from the example tutorial app seem to be at odds with the quote from the blog piece above.

Is it the case that a query will only look to the cache if the query has already been run before? Can it not fetch cached data if that data was cached by a different query? Am I missing something?

Answer 1

Please see this better (in my opinion) answer here:

https://stackoverflow.com/a/66053242/6423036

Copying directly from that answer, credit to the author:

This functionality exists, but it's hard to find if you don't know what you're looking for. In Apollo Client v2 you're looking for cache redirect functionality, in Apollo Client v3 this is replaced by type policies / field read policies (v3 docs).

Apollo doesn't 'know' your GraphQL schema and that makes it easy to set up and work with in day-to-day usage. However, this implies that given some query (e.g. getBooks) it doesn't know what the result type is going to be upfront. It does know it afterwards, as long as the __typename's are enabled. This is the default behaviour and is needed for normalized caching.

Let's assume you have a getBooks query that fetches a list of Books. If you inspect the cache after this request is finished using Apollo devtools, you should find the books in the cache using the Book:123 key in which Book is the typename and 123 is the id. If it exists (and is queried!) the id field is used as identifier for the cache. If your id field has another name, you can use the typePolicies of the cache to inform Apollo InMemoryCache about this field.

If you've set this up and you run a getBook query afterwards, using some id as input, you will not get any cached data. The reason is as described before: Apollo doesn't know upfront which type this query is going to return.

So in Apollo v2 you would use a cacheRedirect to 'redirect' Apollo to the right cache:

  cacheRedirects: {
    Query: {
      getBook(_, args, { getCacheKey }) {
        return getCacheKey({
          __typename: 'Book',
          id: args.id,
        });
      }
    },
  },

(args.id should be replaced by another identifier if you have specified another key in the typePolicy)

When using Apollo v3, you need a typepolicy / field read policy:

  typePolicies: {
    Query: {
      fields: {
        getBook(_, { args, toReference }) {
          return toReference({
            __typename: 'Book',
            id: args.id,
          });
        }
      }
    }
  }

Answer 2

the query will make a network query.

todo(id: $id) {
  title
  completed
} 

Apollo cache isn't very smart. It is just storage. You need to read/write for more complicated operations manually.

The reason for this is Apollo doesn't know about your schema and data structure. It doesn't know that todo(id: $id) will do DB search by, so it can't optimize to look in the cache.

If you don't want a second fetch, you have to implement your data fetch structure with fragment:

try {
  return client.readFragment({
    id: 'Todo:5', // The value of the to-do item's unique identifier
    fragment: gql`
      fragment TodoFragment on Todo {
      id
      title
      completed 
    }
    `,
  });
} catch(_e) { // if no fragment is found there will be an error
  client.query(QUERY, variables: { id: 5})
}

The way Apollo cache is that if you do two queries:

1. load todos

todos {
  id
  title
  completed
}

2. load single todo

todo(id: $id) {
  id
  title
  completed
} 

If you list a list of todos and load the second one - it will update the todo data.