import { GraphQLError } from '../../error'; /**
                                             * Copyright (c) 2015-present, Facebook, Inc.
                                             *
                                             * This source code is licensed under the MIT license found in the
                                             * LICENSE file in the root directory of this source tree.
                                             *
                                             *  strict
                                             */

export function singleFieldOnlyMessage(name) {
  return (name ? 'Subscription "' + name + '" ' : '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)));
        }
      }
    }
  };
}