fix(plugin-import-export): csv export column order (#12258)

### What?
The order of fields, when specified for the create export function was
not used for constructing the data. Now the fields order will be used.

### Why?
This is important to building CSV data for consumption in other systems.

### How?
Adds logic to handle ordering the field values assigned to the export
data prior to building the CSV.

packages/plugin-import-export/src/export/createExport.ts

@@ -87,7 +87,7 @@ export const createExport = async (args: CreateExportArgs) => {
         let isFirstBatch = true
 
         while (result.docs.length > 0) {
-          const csvInput = result.docs.map((doc) => flattenObject(doc))
+          const csvInput = result.docs.map((doc) => flattenObject({ doc, fields }))
           const csvString = stringify(csvInput, { header: isFirstBatch })
           this.push(encoder.encode(csvString))
           isFirstBatch = false
@@ -119,7 +119,7 @@ export const createExport = async (args: CreateExportArgs) => {
     result = await payload.find(findArgs)
 
     if (isCSV) {
-      const csvInput = result.docs.map((doc) => flattenObject(doc))
+      const csvInput = result.docs.map((doc) => flattenObject({ doc, fields }))
       outputData.push(stringify(csvInput, { header: isFirstBatch }))
       isFirstBatch = false
     } else {

packages/plugin-import-export/src/export/flattenObject.ts

@@ -1,23 +1,61 @@
-export const flattenObject = (obj: any, prefix: string = ''): Record<string, unknown> => {
+import type { Document } from 'payload'
+
+type Args = {
+  doc: Document
+  fields?: string[]
+  prefix?: string
+}
+
+export const flattenObject = ({ doc, fields, prefix }: Args): Record<string, unknown> => {
   const result: Record<string, unknown> = {}
 
-  Object.entries(obj).forEach(([key, value]) => {
+  const flatten = (doc: Document, prefix?: string) => {
+    Object.entries(doc).forEach(([key, value]) => {
       const newKey = prefix ? `${prefix}_${key}` : key
 
       if (Array.isArray(value)) {
         value.forEach((item, index) => {
           if (typeof item === 'object' && item !== null) {
-          Object.assign(result, flattenObject(item, `${newKey}_${index}`))
+            flatten(item, `${newKey}_${index}`)
           } else {
             result[`${newKey}_${index}`] = item
           }
         })
       } else if (typeof value === 'object' && value !== null) {
-      Object.assign(result, flattenObject(value, newKey))
+        flatten(value, newKey)
       } else {
         result[newKey] = value
       }
     })
+  }
+
+  flatten(doc, prefix)
+
+  if (fields) {
+    const orderedResult: Record<string, unknown> = {}
+
+    const fieldToRegex = (field: string): RegExp => {
+      const parts = field.split('.').map((part) => `${part}(?:_\\d+)?`)
+      const pattern = `^${parts.join('_')}`
+      return new RegExp(pattern)
+    }
+
+    fields.forEach((field) => {
+      if (result[field.replace(/\./g, '_')]) {
+        const sanitizedField = field.replace(/\./g, '_')
+        orderedResult[sanitizedField] = result[sanitizedField]
+      } else {
+        const regex = fieldToRegex(field)
+        Object.keys(result).forEach((key) => {
+          if (regex.test(key)) {
+            orderedResult[key] = result[key]
+          }
+        })
+      }
+    })
+
+    return orderedResult
+  }
 
   return result
 }

test/plugin-import-export/int.spec.ts

@@ -1,5 +1,6 @@
 import type { CollectionSlug, Payload } from 'payload'
 
+import fs from 'fs'
 import path from 'path'
 import { fileURLToPath } from 'url'
 
@@ -221,6 +222,39 @@ describe('@payloadcms/plugin-import-export', () => {
       expect(data[0].array_1_field2).toStrictEqual('baz')
     })
 
+    it('should create a CSV file with columns matching the order of the fields array', async () => {
+      const fields = ['id', 'group.value', 'group.array.field1', 'title', 'createdAt', 'updatedAt']
+      const doc = await payload.create({
+        collection: 'exports',
+        user,
+        data: {
+          collectionSlug: 'pages',
+          fields,
+          format: 'csv',
+          where: {
+            title: { contains: 'Title ' },
+          },
+        },
+      })
+
+      const exportDoc = await payload.findByID({
+        collection: 'exports',
+        id: doc.id,
+      })
+
+      expect(exportDoc.filename).toBeDefined()
+      const expectedPath = path.join(dirname, './uploads', exportDoc.filename as string)
+      const buffer = fs.readFileSync(expectedPath)
+      const str = buffer.toString()
+
+      // Assert that the header row matches the fields array
+      expect(str.indexOf('id')).toBeLessThan(str.indexOf('title'))
+      expect(str.indexOf('group_value')).toBeLessThan(str.indexOf('title'))
+      expect(str.indexOf('group_value')).toBeLessThan(str.indexOf('group_array'))
+      expect(str.indexOf('title')).toBeLessThan(str.indexOf('createdAt'))
+      expect(str.indexOf('createdAt')).toBeLessThan(str.indexOf('updatedAt'))
+    })
+
     it('should create a file for collection csv from array.subfield', async () => {
       let doc = await payload.create({
         collection: 'exports',