text: const { body,validationResult } = require('express-validator');
  var Author = require('../models/author');
  var Book = require('../models/book');
  var async = require('async');
  
  // Display list of all Authors.
  exports.author_list = function(req, res, next) {
  
      Author.find()
        .sort([['family_name', 'ascending']])
        .exec(function (err, list_authors) {
          if (err) { return next(err); }
          //Successful, so render
          res.render('author_list', { title: 'Author List', author_list: list_authors });
        });
    
    };
  
  // Display detail page for a specific Author.
  exports.author_detail = function(req, res, next) {
  
      async.parallel({
          author: function(callback) {
              Author.findById(req.params.id)
                .exec(callback)
          },
          authors_books: function(callback) {
            Book.find({ 'author': req.params.id },'title summary')
            .exec(callback)
          },
      }, function(err, results) {
          if (err) { return next(err); } // Error in API usage.
          if (results.author==null) { // No results.
              var err = new Error('Author not found');
              err.status = 404;
              return next(err);
          }
          // Successful, so render.
          res.render('author_detail', { title: 'Author Detail', author: results.author, author_books: results.authors_books } );
      });
  
  };
  
  // Display Author create form on GET.
  exports.author_create_get = function(req, res, next) {
      res.render('author_form', { title: 'Create Author'});
  };
  
  // Handle Author create on POST.
  exports.author_create_post = [
  
      // Validate and sanitize fields.
      body('first_name').trim().isLength({ min: 1 }).escape().withMessage('First name must be specified.')
          .isAlphanumeric().withMessage('First name has non-alphanumeric characters.'),
      body('family_name').trim().isLength({ min: 1 }).escape().withMessage('Family name must be specified.')
          .isAlphanumeric().withMessage('Family name has non-alphanumeric characters.'),
      body('date_of_birth', 'Invalid date of birth').optional({ checkFalsy: true }).isISO8601().toDate(),
      body('date_of_death', 'Invalid date of death').optional({ checkFalsy: true }).isISO8601().toDate(),
  
      // Process request after validation and sanitization.
      (req, res, next) => {
  
          // Extract the validation errors from a request.
          const errors = validationResult(req);
  
          if (!errors.isEmpty()) {
              // There are errors. Render form again with sanitized values/errors messages.
              res.render('author_form', { title: 'Create Author', author: req.body, errors: errors.array() });
              return;
          }
          else {
              // Data from form is valid.
  
              // Create an Author object with escaped and trimmed data.
              var author = new Author(
                  {
                      first_name: req.body.first_name,
                      family_name: req.body.family_name,
                      date_of_birth: req.body.date_of_birth,
                      date_of_death: req.body.date_of_death
                  });
              author.save(function (err) {
                  if (err) { return next(err); }
                  // Successful - redirect to new author record.
                  res.redirect(author.url);
              });
          }
      }
  ];
  
  // Display Author delete form on GET.
  exports.author_delete_get = function(req, res, next) {
  
      async.parallel({
          author: function(callback) {
              Author.findById(req.params.id).exec(callback)
          },
          authors_books: function(callback) {
            Book.find({ 'author': req.params.id }).exec(callback)
          },
      }, function(err, results) {
          if (err) { return next(err); }
          if (results.author==null) { // No results.
              res.redirect('/catalog/authors');
          }
          // Successful, so render.
          res.render('author_delete', { title: 'Delete Author', author: results.author, author_books: results.authors_books } );
      });
  
  };
  
  // Handle Author delete on POST.
  exports.author_delete_post = function(req, res, next) {
  
      async.parallel({
          author: function(callback) {
            Author.findById(req.body.authorid).exec(callback)
          },
          authors_books: function(callback) {
            Book.find({ 'author': req.body.authorid }).exec(callback)
          },
      }, function(err, results) {
          if (err) { return next(err); }
          // Success
          if (results.authors_books.length > 0) {
              // Author has books. Render in same way as for GET route.
              res.render('author_delete', { title: 'Delete Author', author: results.author, author_books: results.authors_books } );
              return;
          }
          else {
              // Author has no books. Delete object and redirect to the list of authors.
              Author.findByIdAndRemove(req.body.authorid, function deleteAuthor(err) {
                  if (err) { return next(err); }
                  // Success - go to author list
                  res.redirect('/catalog/authors')
              })
          }
      });
  };
  
  // Display Author update form on GET.
  exports.author_update_get = function(req, res) {
      res.send('NOT IMPLEMENTED: Author update GET');
  };
  
  // Handle Author update on POST.
  exports.author_update_post = function(req, res) {
      res.send('NOT IMPLEMENTED: Author update POST');
  };
  