How to join referenced category documents for a post with GROQ

post.groq

// Try it out here: https://groq.dev/iZPFWScPZXANqgFGQgoUH2

*[_type == "post"]{
  ...,
  // join into an array of just the titles
  "categoryTitles": categories[]->title,  
  // join the completedocuments
  "categoryDocs": categories[]->,
  // join and project specific fields
    categories[]->{
    title
  },
}

schema.ts

export const post = defineType({
  name: 'post',
  type: 'document',
  fields: [
    {
      name: 'title',
      type: 'string',
      title: 'Title',
    },
    {
      name: 'categories',
      type: 'array',
      title: 'Categories',
      of: [
        {
          type: 'reference',
          to: [{ type: 'category' }],
        }
      ]
    },
  ]
})

export const category = defineType({
  name: 'category',
  type: 'document',
  fields: [
    {
      name: 'title',
      type: 'string',
      title: 'Title',
    },
  ]
})