public IActionResult GetAuthor(int id)
        {
            var author = _controller.FindByID(id);

            if (author == null)
            {
                return(new NotFoundResult());
            }

            return(new JsonResult(author));
        }
        /// <summary>
        /// Creates a material entity based on the parameters passed
        /// </summary>
        /// <param name="isbn">The ISBN of the material</param>
        /// <param name="title">The title of the material</param>
        /// <param name="language">The language the material is written in</param>
        /// <param name="lendable">Whether the material is lendable or not</param>
        /// <param name="description">A description of the material</param>
        /// <param name="type">The type of the material</param>
        /// <param name="subjects">The subjects the material covers</param>
        ///
        /// <param name="authors">
        /// The authors of the material. If no AuthorId is supplied with any of the authors,
        /// that author WILL be inserted into the database as a new entry, even if there already exists an author with that full name.
        /// </param>
        ///
        /// <returns>A material object</returns>
        /// <exception cref="ArgumentNullException">Thrown if one of isbn, title, language or description is empty or null</exception>
        /// <exception cref="ArgumentException">Thrown if MaterialType, MaterialSubject or Author doesn't exist in the database</exception>
        public Material Create(string isbn, string title, string language, bool lendable, string description,
                               MaterialType type, List <MaterialSubject> subjects, List <Author> authors)
        {
            // validate material data
            if (isbn.IsNullOrEmpty())
            {
                throw new ArgumentNullException(nameof(isbn), "ISBN can't be null or empty");
            }
            if (title.IsNullOrEmpty())
            {
                throw new ArgumentNullException(nameof(title), "Title can't be null or empty");
            }
            if (language.IsNullOrEmpty())
            {
                throw new ArgumentNullException(nameof(language), "Language can't be null or empty");
            }
            if (description.IsNullOrEmpty())
            {
                throw new ArgumentNullException(nameof(description), "Description can't be null or empty");
            }

            var materialType = _context.MaterialTypes.Find(type.TypeId);

            if (materialType == null)
            {
                throw new ArgumentException("MaterialType must already exist", nameof(type));
            }

            // create material
            var material = new Material
            {
                Isbn        = isbn,
                Title       = title,
                Language    = language,
                Lendable    = lendable,
                Description = description,
                Type        = materialType,
            };

            // validate extra data
            var materialSubjects = subjects
                                   .Select(subject =>
            {
                var s = _context.MaterialSubjects.Find(subject.SubjectId);
                if (s == null)
                {
                    throw new ArgumentException("All MaterialSubjects must already exist", nameof(subjects));
                }

                return(new MaterialSubjects {
                    MaterialSubject = s, Material = material
                });
            }).ToList();

            var materialAuthors = authors
                                  .Select(author =>
            {
                var fetchedAuthor = _authorController.FindByID(author.AuthorId);

                if (fetchedAuthor == null)
                {
                    if (author.FirstName.IsNullOrEmpty() || author.LastName.IsNullOrEmpty())
                    {
                        throw new ArgumentNullException(
                            nameof(authors),
                            "New authors must be provided with FirstName AND LastName"
                            );
                    }

                    var newAuthor = _authorController.Create(author.FirstName, author.LastName);
                    _authorController.Insert(newAuthor);

                    return(new MaterialAuthor {
                        Author = newAuthor, Material = material
                    });
                }

                return(new MaterialAuthor {
                    Author = fetchedAuthor, Material = material
                });
            }).ToList();

            material.MaterialSubjects = materialSubjects;
            material.MaterialAuthors  = materialAuthors;

            return(material);
        }