Move source code into "Source" folder

Source/DataLayer/EfClasses/BookContributor.cs

@@ -0,0 +1,31 @@
+using Dinah.Core;
+
+namespace DataLayer
+{
+    public enum Role { Author = 1, Narrator = 2, Publisher = 3 }
+
+    public class BookContributor
+    {
+        internal int BookId { get; private set; }
+        internal int ContributorId { get; private set; }
+        public Role Role { get; private set; }
+        public byte Order { get; private set; }
+
+        public Book Book { get; private set; }
+        public Contributor Contributor { get; private set; }
+
+        private BookContributor() { }
+        internal BookContributor(Book book, Contributor contributor, Role role, byte order)
+        {
+            ArgumentValidator.EnsureNotNull(book, nameof(book));
+            ArgumentValidator.EnsureNotNull(contributor, nameof(contributor));
+
+            Book = book;
+            Contributor = contributor;
+            Role = role;
+            Order = order;
+        }
+
+		public override string ToString() => $"{Book} {Contributor} {Role} {Order}";
+	}
+}