Query posts by category reference - accessing nested post titles

I can see the issue! The problem is in how you're accessing the posts array in your JSX. Let me break down what's happening:

In your GROQ query, "posts": *[_type == 'post' && references(^._id)]{title} returns an array of post objects, not a single object. So categoryData[0].posts is an array like [{title: "Post 1"}, {title: "Post 2"}].

When you write categoryData[0].posts.title, you're trying to access .title on an array, which is undefined.

Here's the fix:

export default function Category() {
  let { categoryData } = useLoaderData();

  return (
    <div>
      <div>{categoryData[0].title}</div>
      {/* Map over the posts array */}
      {categoryData[0].posts.map((post) => (
        <div key={post._id}>{post.title}</div>
      ))}
    </div>
  );
}

Additional improvements to your query:

1. Add _id to your posts projection so you have a unique key for React:

"posts": *[_type == 'post' && references(^._id)]{_id, title}

2. Your order(publishedAt desc) is currently ordering the categories, not the posts. If you want to order posts by publish date, move it inside the posts subquery:

*[_type == "category" && slug.current == $slug]{ 
  _id, 
  title, 
  slug,
  description,
  "posts": *[_type == 'post' && references(^._id)] | order(publishedAt desc) {_id, title, publishedAt}
}

3. Since you're filtering by slug (which should be unique), you could simplify by using [0] in the query itself:

*[_type == "category" && slug.current == $slug][0]{ 
  _id, 
  title, 
  slug,
  description,
  "posts": *[_type == 'post' && references(^._id)] | order(publishedAt desc) {_id, title, publishedAt}
}

Then access it as categoryData.posts instead of categoryData[0].posts.

The key concept here is understanding GROQ projections - when you create a subquery like your posts query, it returns an array of documents, and you need to handle that array appropriately in your frontend code.