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node_modules/graphql/validation/rules/SingleFieldSubscriptions.mjs

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