Graphql cache: Specify a merge function to a query which is nested under a common type

When working with GraphQL, it is common to have queries that are nested under a common type. In some cases, you may want to specify a merge function for these nested queries to control how the data is merged into the cache. In this blog post, we will explore different solutions to this problem.

Solution 1: Using the @merge directive

One way to specify a merge function for a nested query is by using the @merge directive provided by Apollo Client. The @merge directive allows you to specify a custom merge function for a specific field in your GraphQL schema.

Here’s an example of how you can use the @merge directive:


  type Query {
    user(id: ID!): User @merge(keyField: "id")
  }

In the above example, we have a user query that is nested under the Query type. We specify the @merge directive with the keyField parameter set to “id”. This tells Apollo Client to use the “id” field as the key for merging the data into the cache.

Solution 2: Using the cache.modify method

Another way to specify a merge function for a nested query is by using the cache.modify method provided by Apollo Client. The cache.modify method allows you to directly modify the cache and specify a custom merge function.

Here’s an example of how you can use the cache.modify method:


  cache.modify({
    fields: {
      user(existingUser, { toReference }) {
        return existingUser ? existingUser : toReference({ __typename: 'User', id: '123' });
      }
    }
  });

In the above example, we use the cache.modify method to modify the user field in the cache. If the existingUser is null or undefined, we create a new reference using the toReference function and return it. This allows us to specify a custom merge function for the user query.

Conclusion

Specifying a merge function for a query that is nested under a common type in GraphQL can be achieved using different approaches. You can use the @merge directive provided by Apollo Client or the cache.modify method to control how the data is merged into the cache.

By utilizing these solutions, you can have more control over the caching behavior of your GraphQL queries and ensure that the data is merged in the desired way.

Graphql cache: Specify a merge function to a query which is nested under a common type