fix(ui): component AddNewRelation saves relationTo as first collection instead of selected collection (#14836)

### What?

Currently when you use the create button in a relationship field that
has more than one collection in its `relationTo` and a collection that
isn't first in the list is selected, the selected value in the field
will always be pointing to the first collection in the list, causing
incorrect results or validation errors.

### Why?

The selected value in the field should be applied to the collection the
user has selected, not just the first collection in the `relationTo`
array.

### How?

Use the selected collection in the `onChange` handler instead of
`relatedCollections[0]`.

Fixes: #14728

---------

Co-authored-by: Patrik Kozak <35232443+PatrikKozak@users.noreply.github.com>

Author: rilrom

docs/access-control/collections.mdx

@@ -339,8 +339,9 @@ export const CollectionWithVersionsAccess: CollectionConfig = {
 ```
 
 <Banner type="warning">
-  **Note:** Returning a [Query](../queries/overview) will apply the constraint to the
-  [`versions` collection](../versions/overview#database-impact), not the original Collection.
+  **Note:** Returning a [Query](../queries/overview) will apply the constraint
+  to the [`versions` collection](../versions/overview#database-impact), not the
+  original Collection.
 </Banner>
 
 The following arguments are provided to the `readVersions` function:

docs/access-control/globals.mdx

@@ -137,8 +137,9 @@ export const GlobalWithVersionsAccess: GlobalConfig = {
 ```
 
 <Banner type="warning">
-  **Note:** Returning a [Query](../queries/overview) will apply the constraint to the
-  [`versions` collection](../versions/overview#database-impact), not the original Global.
+  **Note:** Returning a [Query](../queries/overview) will apply the constraint
+  to the [`versions` collection](../versions/overview#database-impact), not the
+  original Global.
 </Banner>
 
 The following arguments are provided to the `readVersions` function:

packages/ui/src/elements/AddNewRelation/index.tsx

@@ -79,7 +79,7 @@ export const AddNewRelation: React.FC<Props> = ({
             ])
           } else {
             onChange({
-              relationTo: relatedCollections[0].slug,
+              relationTo: collectionConfig?.slug,
               value: doc.id,
             })
           }
@@ -88,7 +88,7 @@ export const AddNewRelation: React.FC<Props> = ({
         setSelectedCollection(undefined)
       }
     },
-    [collectionConfig, hasMany, onChange, value, relatedCollections],
+    [collectionConfig, hasMany, onChange, value],
   )
 
   const onPopupToggle = useCallback((state) => {

test/fields/collections/Relationship/e2e.spec.ts

@@ -98,6 +98,32 @@ describe('relationship', () => {
     await expect(page.locator('.payload-toast-container')).toContainText('successfully')
   })
 
+  test('should save correct relationTo when creating doc in second collection (bug #14728)', async () => {
+    await loadCreatePage()
+
+    await openCreateDocDrawer({ page, fieldSelector: '#field-relationship' })
+
+    // Select the SECOND collection (array-fields) instead of the first (text-fields)
+    await page
+      .locator('#field-relationship .relationship-add-new__relation-button--array-fields')
+      .click()
+
+    await page.locator('[id^=doc-drawer_array-fields_1_] #action-save').click()
+    await expect(page.locator('.payload-toast-container')).toContainText('successfully')
+    await page.locator('[id^=close-drawer__doc-drawer_array-fields_1_]').click()
+
+    const relationshipValue = page.locator('#field-relationship .relationship--single-value__text')
+    await expect(relationshipValue).toBeVisible()
+    await expect(async () => {
+      const valueText = await relationshipValue.textContent()
+      expect(valueText).not.toContain('Another text document')
+      expect(valueText).not.toContain('Untitled')
+    }).toPass()
+
+    await page.locator('#action-save').click()
+    await expect(page.locator('.payload-toast-container')).toContainText('successfully')
+  })
+
   test('should create nested inline relationships', async () => {
     await loadCreatePage()