Fix DisplayPackage Perf regression (#6667)

Author: shishirx34

src/NuGetGallery/Controllers/PackagesController.cs

@@ -1222,16 +1222,25 @@ public virtual ActionResult DeleteSymbols(string id, string version)
 
             var model = new DeletePackageViewModel(package, currentUser, DeleteReasons);
 
+            // Fetch all the available symbols package for all the versions from the 
+            // database since the DisplayPackageViewModel(base class for DeletePackageViewModel) does not
+            // set the `LatestSymbolsPackage` data on the model(since it is an unnecessary and expensive db
+            // query). It is fine to do this here when invoking delete page. Note: this could also potentially 
+            // cause unbounded(high number) db calls based on the number of versions associated with a package.
+            var packageViewModelsForAllAvailableSymbolsPackage = package
+                .PackageRegistration
+                .Packages
+                .Where(p => p.PackageStatusKey != PackageStatus.Deleted)
+                .Select(p => p.LatestSymbolPackage())
+                .Where(sp => sp != null && sp.StatusKey == PackageStatus.Available)
+                .Select(sp => new PackageViewModel(sp.Package));
+
             model.VersionSelectList = new SelectList(
-                model
-                .PackageVersions
-                .Where(p => !p.Deleted
-                    && p.LatestSymbolsPackage != null
-                    && p.LatestSymbolsPackage.StatusKey == PackageStatus.Available)
-                .Select(p => new
+                packageViewModelsForAllAvailableSymbolsPackage
+                .Select(pvm => new
                 {
-                    text = p.NuGetVersion.ToFullString() + (p.LatestVersionSemVer2 ? " (Latest)" : string.Empty),
-                    url = Url.DeleteSymbolsPackage(p)
+                    text = pvm.NuGetVersion.ToFullString() + (pvm.LatestVersionSemVer2 ? " (Latest)" : string.Empty),
+                    url = Url.DeleteSymbolsPackage(pvm)
                 }), "url", "text", Url.DeleteSymbolsPackage(model));
 
             return View(model);

src/NuGetGallery/ViewModels/DisplayPackageViewModel.cs

@@ -55,8 +55,6 @@ public DisplayPackageViewModel(Package package, User currentUser, string pushedB
 
             PushedBy = pushedBy;
 
-            LatestSymbolsPackage = package.LatestSymbolPackage();
-
             InitializeRepositoryMetadata(package.RepositoryUrl, package.RepositoryType);
 
             if (PackageHelper.TryPrepareUrlForRendering(package.ProjectUrl, out string projectUrl))
@@ -65,7 +63,7 @@ public DisplayPackageViewModel(Package package, User currentUser, string pushedB
             }
 
             var licenseExpression = package.LicenseExpression;
-            if (!String.IsNullOrWhiteSpace(licenseExpression))
+            if (!string.IsNullOrWhiteSpace(licenseExpression))
             {
                 LicenseUrl = LicenseExpressionRedirectUrlHelper.GetLicenseExpressionRedirectUrl(licenseExpression);
             }

tests/NuGetGallery.Facts/ViewModels/DisplayPackageViewModelFacts.cs

@@ -167,6 +167,55 @@ public void TheCtorSortsPackageVersionsProperly()
             Assert.Equal("1.0.10", packageVersions[0].Version);
         }
 
+        [Fact]
+        public void TheCtorDoesNotPopulateLatestSymbolsPackageForHistory()
+        {
+            var package = new Package
+            {
+                Version = "1.0.0",
+                Dependencies = Enumerable.Empty<PackageDependency>().ToList(),
+                PackageRegistration = new PackageRegistration
+                {
+                    Owners = Enumerable.Empty<User>().ToList(),
+                }
+            };
+
+            package.SymbolPackages.Add(new SymbolPackage()
+            {
+                Package = package,
+                StatusKey = PackageStatus.Available
+            });
+
+            package.PackageRegistration.Packages = new[]
+                {
+                    new Package { Version = "1.0.0-alpha2", PackageRegistration = package.PackageRegistration },
+                    new Package { Version = "1.0.0", PackageRegistration = package.PackageRegistration },
+                    new Package { Version = "1.0.0-alpha", PackageRegistration = package.PackageRegistration },
+                    new Package { Version = "1.0.0-beta", PackageRegistration = package.PackageRegistration },
+                    new Package { Version = "1.0.2-beta", PackageRegistration = package.PackageRegistration },
+                    new Package { Version = "1.0.2", PackageRegistration = package.PackageRegistration },
+                    new Package { Version = "1.0.10", PackageRegistration = package.PackageRegistration }
+                };
+
+            foreach (var packageVersion in package.PackageRegistration.Packages)
+            {
+                packageVersion.SymbolPackages.Add(new SymbolPackage()
+                {
+                    Package = packageVersion,
+                    StatusKey = PackageStatus.Available
+                });
+            }
+
+            var viewModel = new DisplayPackageViewModel(package, null, package.PackageRegistration.Packages.OrderByDescending(p => new NuGetVersion(p.Version)));
+
+            // Descending
+            Assert.NotNull(viewModel.LatestSymbolsPackage);
+            foreach (var version in viewModel.PackageVersions)
+            {
+                Assert.Null(version.LatestSymbolsPackage);
+            }
+        }
+
         [Fact]
         public void TheCtorReturnsLatestSymbolPackageByDateCreated()
         {