flow like the river

BACK_BACK/node_modules/graphql/validation/rules/SingleFieldSubscriptionsRule.js.flow

@@ -0,0 +1,33 @@
+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+
+import type { ASTVisitor } from '../../language/visitor';
+import type { OperationDefinitionNode } from '../../language/ast';
+
+import type { ASTValidationContext } from '../ValidationContext';
+
+/**
+ * Subscriptions must only include one field.
+ *
+ * A GraphQL subscription is valid only if it contains a single root field.
+ */
+export function SingleFieldSubscriptionsRule(
+  context: ASTValidationContext,
+): ASTVisitor {
+  return {
+    OperationDefinition(node: OperationDefinitionNode) {
+      if (node.operation === 'subscription') {
+        if (node.selectionSet.selections.length !== 1) {
+          context.reportError(
+            new GraphQLError(
+              node.name
+                ? `Subscription "${node.name.value}" must select only one top level field.`
+                : 'Anonymous Subscription must select only one top level field.',
+              node.selectionSet.selections.slice(1),
+            ),
+          );
+        }
+      }
+    },
+  };
+}