Introduce exclusive one-to-one linking

Just like how a post can link to an author as a singular link, now the
author can link back to a single post.

Schema's linked-field definitions now have to use array notation to
allow linking back to many entries. If it is in the form of:

soulmate: +Person/foo

then the field will be populated with a single entry.

This is a preparation step for making linking more abstract. Make the
entry schema responsible for the linked-field's type.

Previously all backlinks were collected in 'bucket' fields and schema
had no power to change that. Now, it has.

Author: scriptype

src/compiler/contentModel/index.js

@@ -63,14 +63,19 @@ const linkBack = (post, entry, key) => {
   if (entry.schema) {
     Object.keys(entry.schema).forEach(schemaKey => {
       const schemaValue = entry.schema[schemaKey]
+      const isSchemaValueArray = Array.isArray(schemaValue)
       const re = new RegExp(`^\\+(${post.contentType}|):${key}$`)
-      const match = Array.isArray(schemaValue) ?
+      const match = isSchemaValueArray ?
         schemaValue.find(v => re.test(v)) :
         re.test(schemaValue)
       if (match) {
-        // console.log('linking', post.title, 'to', schemaKey, 'field of', entry.title)
-        entry[schemaKey] = entry[schemaKey] || []
-        entry[schemaKey].push(post)
+        if (isSchemaValueArray) {
+          // console.log('linking', post.title, 'to', schemaKey, 'field of', entry.title)
+          entry[schemaKey] = entry[schemaKey] || []
+          entry[schemaKey].push(post)
+        } else {
+          entry[schemaKey] = post
+        }
       }
     })
     return
@@ -372,4 +377,4 @@ class ContentModel extends ContentModelEntryNode {
   }
 }
 
-module.exports = ContentModel
+module.exports = ContentModel

src/e2e-tests/magazine/fixtures/fs/content/authors/Sir Tim Berners Lee.md

@@ -1,4 +1,5 @@
 ---
 date: 2025-11-07
 slug: tim
+twin: +authors/enes
 ---

src/e2e-tests/magazine/fixtures/fs/schema/Person.md

@@ -1,7 +1,8 @@
 ---
 name: Person
-articles: +Article:author
-demos: +Demo:maker
+articles: [+Article:author]
+demos: [+Demo:maker]
 events: [+Event:organizers, +Event:participants]
-books: +Book:author
+books: [+Book:author]
+twin: +Person:twin
 ---

src/e2e-tests/magazine/fixtures/fs/theme/keep/templates/pages/post/Person.hbs

@@ -65,6 +65,12 @@
       {{/if}}
     </div>
 
+    <div>
+      {{#if twin}}
+        Twin: <a data-twin-link href="{{twin.permalink}}">{{twin.title}}</a>
+      {{/if}}
+    </div>
+
     <div>
     {{#if links.relations}}
       Relations:

src/e2e-tests/magazine/fixtures/model.js

@@ -317,6 +317,11 @@ const FIXTURE_CONTENT_MODEL = {
             },
             { title: 'Vue good', slug: 'vue-good', permalink: '/articles/vue-good.html' }
           ],
+          twin: {
+            title: 'Sir Tim Berners Lee',
+            slug: 'tim',
+            permalink: '/authors/tim.html'
+          },
           content: "Hey, it's me.",
           context: {
             collection: {
@@ -423,6 +428,11 @@ const FIXTURE_CONTENT_MODEL = {
               permalink: '/articles/pure-js-very-good.html'
             }
           ],
+          twin: {
+            title: 'Mustafa Enes',
+            slug: 'enes',
+            permalink: '/authors/enes.html'
+          },
           content: '',
           context: {
             collection: {

src/e2e-tests/magazine/post.test.js

@@ -207,6 +207,18 @@ test('E2E Magazine - Post Pages', async t => {
             `${post.permalink} displays all events with correct links`
           )
         }
+
+        if (post.twin) {
+          const twinLink = $('[data-twin-link]')
+          const linkText = $(twinLink).text()
+          const linkHref = $(twinLink).attr('href')
+          const twinLinked = linkText === post.twin.title && linkHref === post.twin.permalink
+
+          t.ok(
+            twinLinked,
+            `${post.permalink} displays twin with correct link`
+          )
+        }
       } catch (err) {
         t.fail(
           `${post.permalink} Person content links: ${err.message}`
@@ -373,4 +385,4 @@ test('E2E Magazine - Post Pages', async t => {
     }
   })
 
-})
+})