fix(live-preview): client-side live preview cannot clear all array rows (#9439)

When using Client-side Live Preview, array fields are unable to clear
all their rows. This is because `reduceFieldsToValues` sets the array's
value as 0 within form-state when no rows exist, as opposed to an empty
array as one might expect. For now, we can simply handle this data shape
within Live Preview's merge logic. In the future we may want to take to
consider changing the behavior of empty arrays within form-state itself.

packages/live-preview/src/traverseFields.ts

@@ -25,6 +25,14 @@ export const traverseFields = <T>(args: {
 
       switch (fieldSchema.type) {
         case 'array':
+          if (
+            !incomingData[fieldName] &&
+            incomingData[fieldName] !== undefined &&
+            result?.[fieldName] !== undefined
+          ) {
+            result[fieldName] = []
+          }
+
           if (Array.isArray(incomingData[fieldName])) {
             result[fieldName] = incomingData[fieldName].map((incomingRow, i) => {
               if (!result[fieldName]) {
@@ -85,7 +93,7 @@ export const traverseFields = <T>(args: {
           break
 
         case 'group':
-
+        // falls through
         case 'tabs':
           if (!result[fieldName]) {
             result[fieldName] = {}
@@ -100,8 +108,9 @@ export const traverseFields = <T>(args: {
           })
 
           break
-        case 'relationship':
 
+        case 'relationship':
+        // falls through
         case 'upload':
           // Handle `hasMany` relationships
           if (fieldSchema.hasMany && Array.isArray(incomingData[fieldName])) {

test/live-preview/int.spec.ts

@@ -169,6 +169,53 @@ describe('Collections - Live Preview', () => {
     expect(mergedData._numberOfRequests).toEqual(0)
   })
 
+  it('— arrays - can clear all rows', async () => {
+    const initialData: Partial<Page> = {
+      title: 'Test Page',
+      arrayOfRelationships: [
+        {
+          id: '123',
+          relationshipInArrayMonoHasOne: testPost.id,
+        },
+      ],
+    }
+
+    const mergedData = await mergeData({
+      depth: 1,
+      fieldSchema: schemaJSON,
+      incomingData: {
+        ...initialData,
+        arrayOfRelationships: [],
+      },
+      initialData,
+      serverURL,
+      returnNumberOfRequests: true,
+      collectionPopulationRequestHandler,
+    })
+
+    expect(mergedData.arrayOfRelationships).toEqual([])
+    expect(mergedData._numberOfRequests).toEqual(0)
+
+    // do the same but with arrayOfRelationships: 0
+
+    const mergedData2 = await mergeData({
+      depth: 1,
+      fieldSchema: schemaJSON,
+      incomingData: {
+        ...initialData,
+        // @ts-expect-error eslint-disable-next-line
+        arrayOfRelationships: 0, // this is how form state represents an empty array
+      },
+      initialData,
+      serverURL,
+      returnNumberOfRequests: true,
+      collectionPopulationRequestHandler,
+    })
+
+    expect(mergedData2.arrayOfRelationships).toEqual([])
+    expect(mergedData2._numberOfRequests).toEqual(0)
+  })
+
   it('— uploads - adds and removes media', async () => {
     const initialData: Partial<Page> = {
       title: 'Test Page',