flow like the river

BACK_BACK/node_modules/graphql/validation/rules/SingleFieldSubscriptionsRule.mjs

@@ -0,0 +1,18 @@
+import { GraphQLError } from "../../error/GraphQLError.mjs";
+
+/**
+ * Subscriptions must only include one field.
+ *
+ * A GraphQL subscription is valid only if it contains a single root field.
+ */
+export function SingleFieldSubscriptionsRule(context) {
+  return {
+    OperationDefinition: function OperationDefinition(node) {
+      if (node.operation === 'subscription') {
+        if (node.selectionSet.selections.length !== 1) {
+          context.reportError(new GraphQLError(node.name ? "Subscription \"".concat(node.name.value, "\" must select only one top level field.") : 'Anonymous Subscription must select only one top level field.', node.selectionSet.selections.slice(1)));
+        }
+      }
+    }
+  };
+}