public void ShouldReturnExistingBookFromRepositoryIfExists()
        {
            var existingBook = new Book {
                Id = 0, Title = "Gra Endera", Author = "Orson Scott Card", Localisation = "Gliwice", ISBN = "9788376482514"
            };
            var booksInLibrary = new List <Book>
            {
                existingBook,
                new Book {
                    Id = 1, Title = "Cieñ Endera", Author = "Orson Scott Card", Localisation = "Gliwice", ISBN = "9788378397649"
                }
            };

            _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.ElementType).Returns(booksInLibrary.AsQueryable().ElementType);
            _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.Expression).Returns(booksInLibrary.AsQueryable().Expression);
            _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.Provider).Returns(booksInLibrary.AsQueryable().Provider);
            _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.GetEnumerator()).Returns(booksInLibrary.GetEnumerator());


            var bookWithGivenIsbn = _libraryRepository.FindBookWithGivenIsbn("9788376482514");

            Assert.Equal(existingBook, bookWithGivenIsbn);
        }