Prefetch extension for QueryResult

- extension itself
- IMaterializingReader extended with Session property
- access to reader for internal use

Author: alex-kulakov

Orm/Xtensive.Orm/Orm/Linq/Materialization/MaterializingReader.cs

@@ -13,6 +13,7 @@ namespace Xtensive.Orm.Linq.Materialization
 {
   internal interface IMaterializingReader<out TItem>
   {
+    Session Session { get; }
     IEnumerator<TItem> AsEnumerator();
     IAsyncEnumerator<TItem> AsAsyncEnumerator();
   }
@@ -25,6 +26,8 @@ internal class MaterializingReader<TItem>: IMaterializingReader<TItem>, IEnumera
     private readonly IItemMaterializer<TItem> itemMaterializer;
     private readonly Queue<Action> materializationQueue;
 
+    public Session Session => context.Session;
+
     public IEnumerator<TItem> AsEnumerator()
     {
       recordSetReader.Reset();

Orm/Xtensive.Orm/Orm/PrefetchExtensions.cs

@@ -52,6 +52,27 @@ public static PrefetchQuery<TElement> Prefetch<TElement, TFieldValue>(
       return new PrefetchQuery<TElement>(source.Session, source).RegisterPath(expression);
     }
 
+    /// <summary>
+    /// Registers fields specified by <paramref name="expression"/> for prefetch.
+    /// </summary>
+    /// <typeparam name="TElement">The type of the element of the source sequence.</typeparam>
+    /// <typeparam name="TFieldValue">The type of the field's value to be prefetched.</typeparam>
+    /// <param name="source">The source query.</param>
+    /// <param name="expression">The expression specifying a field to be prefetched.</param>
+    /// <returns>An <see cref="IEnumerable{TElement}"/> of source items.</returns>
+    public static PrefetchQuery<TElement> Prefetch<TElement, TFieldValue>(
+      this QueryResult<TElement> source,
+      Expression<Func<TElement, TFieldValue>> expression)
+    {
+      var session = source.Reader.Session;
+      if (session != null) {
+        return new PrefetchQuery<TElement>(session, source).RegisterPath(expression);
+      }
+      return new PrefetchQuery<TElement>(Session.Demand(), source).RegisterPath(expression);
+    }
+
+
+
     /// <summary>
     /// Registers fields specified by <paramref name="expression"/> for prefetch.
     /// </summary>

Orm/Xtensive.Orm/Orm/QueryResult.cs

@@ -1,4 +1,4 @@
-// Copyright (C) 2020 Xtensive LLC.
+// Copyright (C) 2020-2021 Xtensive LLC.
 // This code is distributed under MIT license terms.
 // See the License.txt file in the project root for more information.
 
@@ -19,6 +19,8 @@ private class EnumerableReader : IMaterializingReader<TItem>
     {
       private readonly IEnumerable<TItem> items;
 
+      public Session Session => null;
+
       public IEnumerator<TItem> AsEnumerator() => items.GetEnumerator();
 
       public IAsyncEnumerator<TItem> AsAsyncEnumerator() => throw new System.NotSupportedException();
@@ -29,17 +31,20 @@ public EnumerableReader(IEnumerable<TItem> items)
       }
     }
 
-    private readonly IMaterializingReader<TItem> reader;
     private readonly StateLifetimeToken lifetimeToken;
 
+
+    // DO NOT ENUMERATE this reader anywhere outside this class
+    internal IMaterializingReader<TItem> Reader { get; }
+
     /// <inheritdoc/>
     IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
     /// <inheritdoc/>
     public IEnumerator<TItem> GetEnumerator()
     {
       EnsureResultsAlive();
-      return reader.AsEnumerator();
+      return Reader.AsEnumerator();
     }
 
     /// <summary>
@@ -48,7 +53,7 @@ public IEnumerator<TItem> GetEnumerator()
     public async IAsyncEnumerable<TItem> AsAsyncEnumerable()
     {
       EnsureResultsAlive();
-      var enumerator = reader.AsAsyncEnumerator();
+      var enumerator = Reader.AsAsyncEnumerator();
       while (await enumerator.MoveNextAsync().ConfigureAwait(false)) {
         yield return enumerator.Current;
       }
@@ -63,13 +68,13 @@ private void EnsureResultsAlive()
 
     internal QueryResult(IMaterializingReader<TItem> reader, StateLifetimeToken lifetimeToken)
     {
-      this.reader = reader;
+      this.Reader = reader;
       this.lifetimeToken = lifetimeToken;
     }
 
     internal QueryResult(IEnumerable<TItem> items)
     {
-      reader = new EnumerableReader(items);
+      Reader = new EnumerableReader(items);
       this.lifetimeToken = default;
     }
   }