GraphQL enables efficient and flexible data querying, reducing payload size and improving application performance.

In this lesson, we use Apollo Server to explore nested resolvers and data relationships in GraphQL. By the end, you should be able to create a GraphQL schema with nested types and use nested resolvers to handle complex data relationships.

A GraphQL schema defines the structure of the API and the types of data it can return.

Here’s an example defining two types, Author and Book, which have a nested relationship:

• Author has an id, name, and a list of books.
• Book has an id, title, and an author.
• Query fetches lists of both books and authors.

GraphQL queries are executed in a depth-first manner, meaning that the server resolves one field completely (including all its nested fields) before moving to the next.

• Top-Level Resolvers handle fields in the Query or Mutation type. They Fetch data at the "root level" and return lists or objects that include nested fields.
• Nested Resolvers handle fields within other types, such as the fields inside Author or Book and are responsible for resolving fields inside types that appear in a query. These are executed after the top-level resolver has fetched the parent object.

GraphQL uses a resolver chain:

• The top-level resolver is executed first.
• For each nested field, the resolver for that field is executed recursively.
• The process continues until all requested fields are resolved.

This also prevents infinite loops because the nested calls are handled recursively within the resolution lifecycle, ensuring each level of the query is processed before returning data to the client.

Here’s an example:

In the example above:

• The Query resolver returns the full lists of books and authors.
• The Author resolver fetches books associated with an author.
• The Book resolver retrieves the author associated with a book.

The nested resolvers work seamlessly because GraphQL calls the resolvers depth-first. For example, when querying a book's author, it first resolves the book and then the associated author field. This also prevents infinite loops because the nested calls are handled recursively within the resolution lifecycle.

Given the previously defined schema and resolvers, let's define our sample data:

And initialize and start the Apollo Server:

When you run your server script, it should print:

Now that your GraphQL server is up and running, let's execute a query to fetch nested data.

Example Query:

Fetching Data:

Executing this script fetches the nested data relationships and logs them. You should see an output similar to:

In this lesson, we covered how to define GraphQL schemas with nested types and implement nested resolvers for complex data relationships. We also demonstrated executing queries to fetch nested data.

As you move on to the practice exercises, try creating your own schemas and resolvers. Practice is essential to solidify your understanding and improve your skills. This will set a strong foundation for tackling more advanced topics in GraphQL.

Good luck, and enjoy coding!