Allow for single field indexes in 'google_firestore_index' resources to support __name__ DESC indexes

.changelog/12121.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+firestore: allowed single field indexes to support `__name__ DESC` indexes in `google_firestore_index` resources
+```

google-beta/services/firestore/resource_firestore_index.go

@@ -107,7 +107,7 @@ specified as the last field, it will be added automatically with the same
 direction as that of the last field defined. If the final field in a
 composite index is not directional, the '__name__' will be ordered
 '"ASCENDING"' (unless explicitly specified otherwise).`,
-				MinItems: 2,
+				MinItems: 1,
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
 						"array_config": {

google-beta/services/firestore/resource_firestore_index_generated_test.go

@@ -214,6 +214,57 @@ resource "google_firestore_index" "my-index" {
 `, context)
 }
 
+func TestAccFirestoreIndex_firestoreIndexNameDescendingExample(t *testing.T) {
+	t.Parallel()
+
+	context := map[string]interface{}{
+		"project_id":    envvar.GetTestProjectFromEnv(),
+		"random_suffix": acctest.RandString(t, 10),
+	}
+
+	acctest.VcrTest(t, resource.TestCase{
+		PreCheck:                 func() { acctest.AccTestPreCheck(t) },
+		ProtoV5ProviderFactories: acctest.ProtoV5ProviderFactories(t),
+		CheckDestroy:             testAccCheckFirestoreIndexDestroyProducer(t),
+		Steps: []resource.TestStep{
+			{
+				Config: testAccFirestoreIndex_firestoreIndexNameDescendingExample(context),
+			},
+			{
+				ResourceName:            "google_firestore_index.my-index",
+				ImportState:             true,
+				ImportStateVerify:       true,
+				ImportStateVerifyIgnore: []string{"collection", "database"},
+			},
+		},
+	})
+}
+
+func testAccFirestoreIndex_firestoreIndexNameDescendingExample(context map[string]interface{}) string {
+	return acctest.Nprintf(`
+resource "google_firestore_database" "database" {
+  project     = "%{project_id}"
+  name        = "tf-test-database-id%{random_suffix}"
+  location_id = "nam5"
+  type        = "FIRESTORE_NATIVE"
+
+  delete_protection_state = "DELETE_PROTECTION_DISABLED"
+  deletion_policy         = "DELETE"
+}
+
+resource "google_firestore_index" "my-index" {
+  project     = "%{project_id}"
+  database   = google_firestore_database.database.name
+  collection = "atestcollection"
+
+  fields {
+    field_path = "__name__"
+    order      = "DESCENDING"
+  }
+}
+`, context)
+}
+
 func testAccCheckFirestoreIndexDestroyProducer(t *testing.T) func(s *terraform.State) error {
 	return func(s *terraform.State) error {
 		for name, rs := range s.RootModule().Resources {

website/docs/r/firestore_index.html.markdown

@@ -139,6 +139,31 @@ resource "google_firestore_index" "my-index" {
   }
 }
 ```
+## Example Usage - Firestore Index Name Descending
+
+
+```hcl
+resource "google_firestore_database" "database" {
+  project     = "my-project-name"
+  name        = "database-id"
+  location_id = "nam5"
+  type        = "FIRESTORE_NATIVE"
+
+  delete_protection_state = "DELETE_PROTECTION_DISABLED"
+  deletion_policy         = "DELETE"
+}
+
+resource "google_firestore_index" "my-index" {
+  project     = "my-project-name"
+  database   = google_firestore_database.database.name
+  collection = "atestcollection"
+
+  fields {
+    field_path = "__name__"
+    order      = "DESCENDING"
+  }
+}
+```
 
 ## Argument Reference