Gatsby blog template query not filtering by slug for individual posts

You're absolutely right about the issue! Your GraphQL query in templates/blog.js is not filtering by slug, so it's just returning one post (likely the first or most recent one) regardless of which URL you're on.

The fix involves two parts:

1. Update your template query to accept a slug parameter

In templates/blog.js, modify your query to filter by the slug:

export const query = graphql`
  query ($slug: String!) {
    sanityPost(slug: {current: {eq: $slug}}) {
      id
      slug {
        current
      }
      title
      publishedAt(formatString: "MMM Do, YYYY")
      _rawBody
    }
  }
`

The key changes:

• Added query ($slug: String!) to accept a slug parameter
• Added (slug: {current: {eq: $slug}}) to filter the specific post

2. Create pages programmatically in gatsby-node.js

You need to use Gatsby's createPages API to generate individual pages for each blog post and pass the slug as context. In your gatsby-node.js:

const path = require('path')

exports.createPages = async ({ graphql, actions }) => {
  const { createPage } = actions
  
  const result = await graphql(`
    {
      allSanityPost {
        nodes {
          slug {
            current
          }
        }
      }
    }
  `)

  result.data.allSanityPost.nodes.forEach((node) => {
    createPage({
      path: `/blog/${node.slug.current}`,
      component: path.resolve('./src/templates/blog.js'),
      context: {
        slug: node.slug.current
      }
    })
  })
}

This creates a page for each post and passes the slug as context, which becomes the $slug variable in your template query.

After making these changes, restart your Gatsby dev server (gatsby clean && gatsby develop is recommended to clear the cache).

The gatsby-source-sanity plugin handles all the data fetching from Sanity, and this pattern of using createPages with context variables is the standard Gatsby approach for dynamic pages from any CMS. The _raw fields you're using (like _rawBody) are specific to the Gatsby integration and work perfectly for rendering Portable Text content!