In this article I will tell how to build "Related Reads" section for a [gatsby](https://www.gatsbyjs.org/) blog, like the one you can find at the bottom of this page.

As a prerequisite I expect, that you are already familiar with gatsby or even have an app with some source `[-plugin]`where you pull your articles from.

I use [netlify cms](https://www.netlifycms.org/) and store all my articles next to the source code [in git](https://github.com/kitos/kitos.github.io/tree/develop/src/_content/blog) as markdown files. So `gatsby-source-filesystem` and `gatsby-transformer-remark` plugins are responsible for making them available in graphql. E.g. here is how I can query all articles:

```graphql
query BlogPostList {
  allMarkdownRemark {
    nodes {
      frontmatter {
        title
      }
    }
  }
}
```

But enough about me, let's get back to business.

## Setup local plugin

First of all, I want to implement this feature as a local plugin (instead of root `gatsby-node.js`). I find this approach more idiomatic and clear. Creating one is a piece of cake and gatsby has [a decent documentation about it](https://www.gatsbyjs.org/docs/creating-a-local-plugin/), so check it out if you haven't yet.

As a result, we should have `/plugins/related-reads/gatsby-node.js` along with empty `package.json` and name (equal to name of the folder!) of our plugin listed in `gatsby-config.js`.

## Extend graphql

The next step would be extending graphql schema, generated by `gatsby-transformer-remark`. So along with a content of an article, we can also request a list of articles about similar topic[s]. Like this:

```graphql
query BlogPostList {
  allMarkdownRemark {
    nodes {
      frontmatter {
        title
      }

      // highlight-start
      relatedReads {
        slug
        title
      }
      // highlight-end
    }
  }
}
```

[A while ego](https://www.gatsbyjs.org/blog/2019-03-04-new-schema-customization/) gatsby team provided very nice way to achieve this - [schema customization API](https://www.gatsbyjs.org/docs/schema-customization). And we are going to leverage it, particularly `createResolvers`:

```js
exports.createResolvers = ({ createResolvers }) =>
  createResolvers({
    // node type we want to extend
    MarkdownRemark: {
      // name of the field we want to add
      relatedReads: {
        // we return an array of other articles
        type: '[MarkdownRemark!]',
        // let's allow to specify number of articles returned
        args: { limit: 'Int' },

        async resolve(
          source, // the node we extend
          args,
          // collection of helpers
          // including access to internal store - nodeModel
          ctx

        ) {
          return []
        },
      },
    },
  })
```

No we can implement the last bit - the logic of _Relative Reads_. In my blog each post have a list of tags, so I gonna use it to measure similarity.

```js
const intersection = require('lodash.intersection')

// I've extracted the resolve function just to get rid of extra noise
let resolve = async (source, args, ctx) {
  let { frontmatter: { slug, lang, tags } } = source
  let { limit = Number.MAX_SAFE_INTEGER } = args

  // request all other posts
  let otherPosts = await ctx.nodeModel.runQuery({
    firstOnly: false, // we want to get an array
    type: 'MarkdownRemark',
    query: {
      filter: {
        frontmatter: {
          slug: { ne: slug }, // not current article or translation
          lang: { eq: lang }, // same lang
        },
      },
    },
  })

  return otherPosts
    .map((p) => ({
      ...p,
      // measure the similarity by the size of tags intersection
      similarity: intersection(p.frontmatter.tags, tags).length,
    }))
    .filter(({ similarity }) => similarity !== 0)
    .sort(
      (a, b) =>
        b.similarity - a.similarity ||
        b.frontmatter.date.localeCompare(a.frontmatter.date)
    )
    .slice(0, limit)
}
```

That is it! Pretty easy implementation and zero runtime cost - everything is requested ahead of time and build into your html pages, that is the strongest point if gatsby and JAMStack.

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Blog #plugin

Build "Related Reads" for your gatsby blog