Fix to #28648 - Json/Query: translate element access of a json array

src/EFCore.Relational/Properties/RelationalStrings.resx

@@ -950,6 +950,9 @@
   <data name="PendingAmbientTransaction" xml:space="preserve">
     <value>This connection was used with an ambient transaction. The original ambient transaction needs to be completed before this connection can be used outside of it.</value>
   </data>
+  <data name="ProjectingJsonCollectionElementRequiresNoTracking" xml:space="preserve">
+    <value>The query projects an entity mapped to JSON and accesses a JSON collection element. Such queries require 'AsNoTracking' option, even when the parent entity is projected.</value>
+  </data>
   <data name="ProjectionMappingCountMismatch" xml:space="preserve">
     <value>Unable to translate set operations when both sides don't assign values to the same properties in the nominal type. Please make sure that the same properties are included on both sides, and consider assigning default values if a property doesn't require a specific value.</value>
   </data>

src/EFCore.Relational/Query/Internal/CollectionIndexerToElementAtNormalizingExpressionVisitor.cs

@@ -0,0 +1,66 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.EntityFrameworkCore.Query.Internal;
+
+/// <summary>
+///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+///     any release. You should only use it directly in your code with extreme caution and knowing that
+///     doing so can result in application failures when updating to a new Entity Framework Core release.
+/// </summary>
+public class CollectionIndexerToElementAtNormalizingExpressionVisitor : ExpressionVisitor
+{
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    protected override Expression VisitMethodCall(MethodCallExpression methodCallExpression)
+    {
+        // Convert list[x] to list.ElementAt(x)
+        if (methodCallExpression.Method is { Name: "get_Item", IsStatic: false, DeclaringType: { IsGenericType: true } declaringType }
+            && declaringType.GetGenericTypeDefinition() == typeof(List<>))
+        {
+            var source = Visit(methodCallExpression.Object!);
+            var index = Visit(methodCallExpression.Arguments[0]);
+            var sourceTypeArgument = source.Type.GetSequenceType();
+
+            return Expression.Call(
+                QueryableMethods.ElementAt.MakeGenericMethod(sourceTypeArgument),
+                    Expression.Call(
+                        QueryableMethods.AsQueryable.MakeGenericMethod(sourceTypeArgument),
+                        source),
+                    index);
+        }
+
+        return base.VisitMethodCall(methodCallExpression);
+    }
+
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    protected override Expression VisitBinary(BinaryExpression binaryExpression)
+    {
+        // Convert array[x] to array.ElementAt(x)
+        if (binaryExpression.NodeType == ExpressionType.ArrayIndex)
+        {
+            var source = Visit(binaryExpression.Left);
+            var index = Visit(binaryExpression.Right);
+            var sourceTypeArgument = source.Type.GetSequenceType();
+
+            return Expression.Call(
+                QueryableMethods.ElementAt.MakeGenericMethod(sourceTypeArgument),
+                    Expression.Call(
+                        QueryableMethods.AsQueryable.MakeGenericMethod(sourceTypeArgument),
+                        source),
+                    index);
+        }
+
+        return base.VisitBinary(binaryExpression);
+    }
+}

src/EFCore.Relational/Query/JsonQueryExpression.cs

@@ -161,6 +161,33 @@ public virtual JsonQueryExpression BindNavigation(INavigation navigation)
             IsNullable || !navigation.ForeignKey.IsRequiredDependent);
     }
 
+    /// <summary>
+    ///     Binds a collection element access with this JSON query expression to get the SQL representation.
+    /// </summary>
+    /// <param name="collectionIndexExpression">The collection index to bind.</param>
+    public virtual JsonQueryExpression BindCollectionElement(SqlExpression collectionIndexExpression)
+    {
+        // this needs to be changed IF JsonQueryExpression will also be used for collection of primitives
+        // see issue #28688
+        Debug.Assert(
+            Path.Last().ArrayIndex == null,
+            "Already accessing JSON array element.");
+
+        var newPath = Path.ToList();
+        newPath.Add(new PathSegment(collectionIndexExpression));
+
+        return new JsonQueryExpression(
+            EntityType,
+            JsonColumn,
+            _keyPropertyMap,
+            newPath,
+            EntityType.ClrType,
+            collection: false,
+            // TODO: computing nullability might be more complicated when we allow strict mode
+            // see issue #28656
+            nullable: true);
+    }
+
     /// <summary>
     ///     Makes this JSON query expression nullable.
     /// </summary>

src/EFCore.Relational/Query/PathSegment.cs

@@ -7,44 +7,69 @@ namespace Microsoft.EntityFrameworkCore.Query;
 
 /// <summary>
 ///     <para>
-///         A class representing a component of JSON path used in <see cref="JsonQueryExpression" /> or <see cref="JsonScalarExpression" />.
+///         A struct representing a component of JSON path used in <see cref="JsonQueryExpression" /> or <see cref="JsonScalarExpression" />.
 ///     </para>
 ///     <para>
 ///         This type is typically used by database providers (and other extensions). It is generally
 ///         not used in application code.
 ///     </para>
 /// </summary>
-public class PathSegment
+public readonly struct PathSegment
 {
     /// <summary>
-    ///     Creates a new instance of the <see cref="PathSegment" /> class.
+    ///     Creates a new <see cref="PathSegment" /> struct representing JSON property access.
     /// </summary>
-    /// <param name="key">A key which is being accessed in the JSON.</param>
-    public PathSegment(string key)
+    /// <param name="propertyName">A name of JSON property which is being accessed.</param>
+    public PathSegment(string propertyName)
     {
-        Key = key;
+        PropertyName = propertyName;
+        ArrayIndex = null;
     }
 
     /// <summary>
-    ///     The key which is being accessed in the JSON.
+    ///     Creates a new <see cref="PathSegment" /> struct representing JSON array element access.
     /// </summary>
-    public virtual string Key { get; }
+    /// <param name="arrayIndex"><see langword="abstract"/>An index of an element which is being accessed in the JSON array.</param>
+    public PathSegment(SqlExpression arrayIndex)
+    {
+        ArrayIndex = arrayIndex;
+        PropertyName = null;
+    }
+
+    /// <summary>
+    ///     The name of JSON property which is being accessed.
+    /// </summary>
+    public string? PropertyName { get; }
+
+    /// <summary>
+    ///     The index of an element which is being accessed in the JSON array.
+    /// </summary>
+    public SqlExpression? ArrayIndex { get; }
 
     /// <inheritdoc />
     public override string ToString()
-        => (Key == "$" ? "" : ".") + Key;
+    {
+        var arrayIndex = ArrayIndex switch
+        {
+            null => "",
+            SqlConstantExpression { Value: not null } sqlConstant => $"[{sqlConstant.Value}]",
+            SqlParameterExpression sqlParameter => $"[{sqlParameter.Name}]",
+            _ => "[(...)]"
+        };
+
+        return (PropertyName == "$" ? "" : ".") + (PropertyName ?? arrayIndex);
+    }
 
     /// <inheritdoc />
     public override bool Equals(object? obj)
-        => obj != null
-            && (ReferenceEquals(this, obj)
-                || obj is PathSegment pathSegment
-                && Equals(pathSegment));
+        => obj is PathSegment pathSegment && Equals(pathSegment);
 
     private bool Equals(PathSegment pathSegment)
-        => Key == pathSegment.Key;
+        => PropertyName == pathSegment.PropertyName
+            && ((ArrayIndex == null && pathSegment.ArrayIndex == null)
+                || (ArrayIndex != null && ArrayIndex.Equals(pathSegment.ArrayIndex)));
 
     /// <inheritdoc />
     public override int GetHashCode()
-        => HashCode.Combine(Key);
+        => HashCode.Combine(PropertyName, ArrayIndex);
 }

src/EFCore.Relational/Query/RelationalQueryTranslationPreprocessor.cs

@@ -37,6 +37,7 @@ public override Expression NormalizeQueryableMethod(Expression expression)
         expression = new RelationalQueryMetadataExtractingExpressionVisitor(_relationalQueryCompilationContext).Visit(expression);
         expression = base.NormalizeQueryableMethod(expression);
         expression = new TableValuedFunctionToQueryRootConvertingExpressionVisitor(QueryCompilationContext.Model).Visit(expression);
+        expression = new CollectionIndexerToElementAtNormalizingExpressionVisitor().Visit(expression);
 
         return expression;
     }