//Hook into the createSchemaCustomization API
//This hook runs after all our nodes have been created
exports.createSchemaCustomization = ({ actions, schema }) => {
  //The createTypes action allows us to create custom types
  //and modify existing ones
  const { createTypes } = actions
  
  // Create our schema customizations
  const typeDefs = [
    // Replace "SanityReview" with your _typename of your post type
    "type SanityReview implements Node { related: [SanityReview] }",
    schema.buildObjectType({
      name: "SanityReview",
      fields: {
        related: {
          type: "[SanityReview]",
          //The resolve field is called when your page query looks for related posts
          //Here we can query our data for posts we deem 'related'
          //Exactly how you do this is up to you
          //I'm querying purely by category
          //But you could pull every single post and do a text match if you really wanted
          //(note that might slow down your build time a bit)
          //You could even query an external API if you needed
          resolve: async (source, args, context, info) => {
            //source is the current (post) object
            //context provides some methods to interact with the data store
            
            //Map a simple array of category IDs from our source object
            //In my data each category in the array is an object with a _id field
            //We're just flattening that to an array of those _id values
            //E.g. categories = ["1234", "4567", "4534"]
            const categories = source.categories.map((c) => c._id)
            
            //If this post has no categories, return an empty array
            if (!categories.length) return ["uncategorized"]
            
            //Query the data store for posts in our target categories
            const post = await context.nodeModel.runQuery({
              query: {
                filter: {
                  //We're filtering for categories that are sharedby our source node
                  categories: { elemMatch: { _id: { in: categories } } },
                  //Dont forget to exclude the current post node!
                  _id: { ne: source._id },
                },
              },
              //Change this to match the data type of your post
              //This will vary depending on how you source content
              type: "SanityReview",
            })

            //Gatsby gets unhappy if we return "null" here
            //So check the result and either return an array of post,
            //or an empty array
            return post && post.length ? post : []
          },
        },
      },
    }),
  ]
  
  createTypes(typeDefs)
}