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vscode/Chapter10/WorkingWithEFCore/Program.Queries.cs

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+using Microsoft.EntityFrameworkCore; // Include extension method
+using Microsoft.EntityFrameworkCore.ChangeTracking; // CollectionEntry
+using Packt.Shared; // Northwind, Category, Product
+
+partial class Program
+{
+  static void QueryingCategories()
+  {
+    using (Northwind db = new())
+    {
+      SectionTitle("Categories and how many products they have:");
+
+      // a query to get all categories and their related products
+      IQueryable<Category>? categories;
+      // = db.Categories;
+      // .Include(c => c.Products);
+
+      db.ChangeTracker.LazyLoadingEnabled = false;
+
+      Write("Enable eager loading? (Y/N): ");
+      bool eagerLoading = (ReadKey(intercept: true).Key == ConsoleKey.Y);
+      bool explicitLoading = false;
+      WriteLine();
+
+      if (eagerLoading)
+      {
+        categories = db.Categories?.Include(c => c.Products);
+      }
+      else
+      {
+        categories = db.Categories;
+        Write("Enable explicit loading? (Y/N): ");
+        explicitLoading = (ReadKey(intercept: true).Key == ConsoleKey.Y);
+        WriteLine();
+      }
+
+      if (categories is null)
+      {
+        Fail("No categories found.");
+        return;
+      }
+
+      // execute query and enumerate results
+      foreach (Category c in categories)
+      {
+        if (explicitLoading)
+        {
+          Write($"Explicitly load products for {c.CategoryName}? (Y/N): ");
+          ConsoleKeyInfo key = ReadKey(intercept: true);
+          WriteLine();
+
+          if (key.Key == ConsoleKey.Y)
+          {
+            CollectionEntry<Category, Product> products =
+              db.Entry(c).Collection(c2 => c2.Products);
+
+            if (!products.IsLoaded) products.Load();
+          }
+        }
+
+        WriteLine($"{c.CategoryName} has {c.Products.Count} products.");
+      }
+    }
+  }
+
+  static void FilteredIncludes()
+  {
+    using (Northwind db = new())
+    {
+      SectionTitle("Products with a minimum number of units in stock.");
+
+      string? input;
+      int stock;
+
+      do
+      {
+        Write("Enter a minimum for units in stock: ");
+        input = ReadLine();
+      } while (!int.TryParse(input, out stock));
+
+      IQueryable<Category>? categories = db.Categories?
+        .Include(c => c.Products.Where(p => p.Stock >= stock));
+
+      if (categories is null)
+      {
+        Fail("No categories found.");
+        return;
+      }
+
+      Info($"ToQueryString: {categories.ToQueryString()}");
+
+      foreach (Category c in categories)
+      {
+        WriteLine($"{c.CategoryName} has {c.Products.Count} products with a minimum of {stock} units in stock.");
+        foreach (Product p in c.Products)
+        {
+          WriteLine($"  {p.ProductName} has {p.Stock} units in stock.");
+        }
+      }
+    }
+  }
+
+  static void QueryingProducts()
+  {
+    using (Northwind db = new())
+    {
+      SectionTitle("Products that cost more than a price, highest at top.");
+
+      string? input;
+      decimal price;
+
+      do
+      {
+        Write("Enter a product price: ");
+        input = ReadLine();
+      } while (!decimal.TryParse(input, out price));
+
+      IQueryable<Product>? products = db.Products?
+        .Where(product => product.Cost > price)
+        .OrderByDescending(product => product.Cost);
+
+      if (products is null)
+      {
+        Fail("No products found.");
+        return;
+      }
+
+      Info($"ToQueryString: {products.ToQueryString()}");
+
+      foreach (Product p in products)
+      {
+        WriteLine(
+          "{0}: {1} costs {2:$#,##0.00} and has {3} in stock.",
+          p.ProductId, p.ProductName, p.Cost, p.Stock);
+      }
+    }
+  }
+
+  static void QueryingWithLike()
+  {
+    using (Northwind db = new())
+    {
+      SectionTitle("Pattern matching with LIKE.");
+
+      Write("Enter part of a product name: ");
+      string? input = ReadLine();
+
+      if (string.IsNullOrWhiteSpace(input))
+      {
+        Fail("You did not enter part of a product name.");
+        return;
+      }
+
+      IQueryable<Product>? products = db.Products?
+        .Where(p => EF.Functions.Like(p.ProductName, $"%{input}%"));
+
+      if (products is null)
+      {
+        Fail("No products found.");
+        return;
+      }
+
+      foreach (Product p in products)
+      {
+        WriteLine("{0} has {1} units in stock. Discontinued? {2}",
+          p.ProductName, p.Stock, p.Discontinued);
+      }
+    }
+  }
+
+  static void GetRandomProduct()
+  {
+    using (Northwind db = new())
+    {
+      SectionTitle("Get a random product.");
+
+      int? rowCount = db.Products?.Count();
+
+      if (rowCount == null)
+      {
+        Fail("Products table is empty.");
+        return;
+      }
+
+      Product? p = db.Products?.FirstOrDefault(
+        p => p.ProductId == (int)(EF.Functions.Random() * rowCount));
+
+      if (p == null)
+      {
+        Fail("Product not found.");
+        return;
+      }
+
+      WriteLine($"Random product: {p.ProductId} {p.ProductName}");
+    }
+  }
+
+}
+