Move source code into "Source" folder

Source/DataLayer/QueryObjects/BookQueries.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore;
+
+namespace DataLayer
+{
+    // only library importing should use tracking. All else should be NoTracking.
+    // only library importing should directly query Book. All else should use LibraryBook
+    public static class BookQueries
+    {
+        public static Book GetBook_Flat_NoTracking(this LibationContext context, string productId)
+            => context
+                .Books
+                .AsNoTrackingWithIdentityResolution()
+                .GetBook(productId);
+
+        public static Book GetBook(this IQueryable<Book> books, string productId)
+            => books
+                .GetBooks()
+                // 'Single' is more accurate but 'First' is faster and less error prone
+                .FirstOrDefault(b => b.AudibleProductId == productId);
+
+        /// <summary>This is still IQueryable. YOU MUST CALL ToList() YOURSELF</summary>
+        public static IQueryable<Book> GetBooks(this IQueryable<Book> books, Expression<Func<Book, bool>> predicate)
+            => books
+                .GetBooks()
+                .Where(predicate);
+
+        /// <summary>This is still IQueryable. YOU MUST CALL ToList() YOURSELF</summary>
+        public static IQueryable<Book> GetBooks(this IQueryable<Book> books)
+            => books
+                // owned items are always loaded. eg: book.UserDefinedItem, book.Supplements
+                .Include(b => b.SeriesLink).ThenInclude(sb => sb.Series)
+                .Include(b => b.ContributorsLink).ThenInclude(c => c.Contributor)
+                .Include(b => b.Category).ThenInclude(c => c.ParentCategory);
+    }
+}