IDEA: You can create Categories (e.g. Programming Basics). Inside these you can create individual lessons (e.g. What are variables?, what are recursions?). If I want to create a Lesson inside I category now, I don't want to reference the category manually but want the category already selected for reference in the new lesson document. So there s…

category.js

import { FaListAlt } from 'react-icons/lib/fa'

export default {
    title: "Category",
    name: "category",
    type: "document",
    icon: FaListAlt,
    fields: [
        {
            title: "Name",
            name: "name",
            type: "string",
            validation: Rule => [
                Rule.required()
            ]
        },
        {
            title: "Description",
            name: "description",
            description: "Describe, what users can expect to learn from completing this category.",
            type: "string",
            validation: Rule => [
                Rule.required()
            ]
        },
    ],
    //...
}

deskStructure.js

import S from "@sanity/desk-tool/structure-builder"
import { FaFileText, FaEdit, FaComment, FaPlayCircle, FaQuestionCircle, FaCheckCircle } from 'react-icons/lib/fa'
 
export default () =>
    S.list()
        .title("App")
        .items([
            S.listItem()
                .title("Content")
                .icon(FaFileText)
                .child(
                    S.documentList()
                        .id('categories')
                        .title('Categories')
                        .schemaType('category')
                        .filter('_type == "category" && !(_id in path("drafts.**"))')
                        .defaultOrdering([{field: 'position', direction: 'asc'}])
                        .menuItems([
                            S.orderingMenuItem({title: 'Position ascending', by: [{ field: "position", direction: "asc" }]}),
                            S.orderingMenuItem({title: 'Position descending', by: [{ field: "position", direction: "desc" }]})
                            ])
                        .child(categoryId =>
                            S.documentList()
                            .id('lessons')
                            .title('Lessons of Category')
                            .schemaType('lesson')
                            .filter('_type == "lesson" && category._ref == $categoryId && !(_id in path("drafts.**"))')
                            .params({types: ['lesson-for-category'], categoryId})
                            //not this works:
                            .initialValueTemplates([
                                S.initialValueTemplateItem('lesson-for-category', {categoryId: categoryId})
                              ])
                            .defaultOrdering([{field: 'position', direction: 'asc'}])
                            .menuItems([
                                //nor this:
                                S.menuItemsFromInitialValueTemplateItems([
                                    S.initialValueTemplateItem('lesson-for-category', {categoryId: categoryId}),
                                  ]),
                                S.menuItem()
                                    .title('edit category')
                                    .icon(FaEdit)
                                    .intent({
                                        type: "edit",
                                        params: {
                                            type: "category",
                                            id: categoryId
                                        }
                                    }),
                                S.orderingMenuItem({title: 'Position ascending', by: [{ field: "position", direction: "asc" }]}),
                                S.orderingMenuItem({title: 'Position descending', by: [{ field: "position", direction: "desc" }]})
                                ])

initialValueTemplates.js

import T from '@sanity/base/initial-value-template-builder'

export default [
  ...T.defaults(),

  T.template({
    id: 'lesson-for-category',
    title: 'Lesson for Category',
    description: 'Creates a lesson for selected category',
    schemaType: 'lesson',
    parameters: [
      {
        name: 'categoryId',
        type: 'string'
      }
    ],
    value: parameters => ({
      category: {_type: 'reference', _ref: parameters.categoryId} //if i put here the categoryID manually for testing, the templating works (of course only manually for one specific category). So it has to be the parameter-passing from deskStructure to template.
    })
  })
]

lesson.js

export default {
    title: "Lesson",
    name: "lesson",
    type: "document",
    fields: [
        {
            title: "Belongs to Category",
            name: "category",
            type: "reference",
            to: [{type: "category"}],
            validation: Rule => [
                Rule.required()
            ]
        },
        {
            title: "Name",
            name: "name",
            type: "string",
            validation: Rule => [
                Rule.required()
            ]
        },
        {
            title: "Description",
            name: "description",
            type: "text",
            validation: Rule => [
                Rule.required()
            ]
        },
    ],
  //...
}

sanity.json

{
  //...
  "parts": [
    {
      "name": "part:@sanity/base/schema",
      "path": "./schemas/schema.js"
    },
    {
      "name": "part:@sanity/desk-tool/structure",
      "path": "./deskStructure.js"
    },
    {
      "name": "part:@sanity/base/initial-value-templates",
      "path": "./initialValueTemplates.js"
    }
  ]
}