Fix: Override GetDataReaderMethod in MySqlJsonTypeMapping to use GetString

MySQL stores JSON as strings, but EF Core's default behavior tries to read them as MemoryStream for complex types. This causes a "No coercion operator is defined between types 'System.String' and 'System.IO.MemoryStream'" error.

The fix overrides GetDataReaderMethod() to explicitly return DbDataReader.GetString, ensuring JSON values are read as strings.

Co-authored-by: renemadsen <76994+renemadsen@users.noreply.github.com>

Author: Copilot

src/EFCore.MySql/Storage/Internal/MySqlJsonTypeMapping.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Data.Common;
 using System.Linq;
+using System.Reflection;
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.ChangeTracking;
@@ -57,6 +58,9 @@ protected override RelationalTypeMapping Clone(bool? noBackslashEscapes = null,
 
     public abstract class MySqlJsonTypeMapping : MySqlStringTypeMapping, IMySqlCSharpRuntimeAnnotationTypeMappingCodeGenerator
     {
+        private static readonly MethodInfo _getString
+            = typeof(DbDataReader).GetRuntimeMethod(nameof(DbDataReader.GetString), new[] { typeof(int) });
+
         public MySqlJsonTypeMapping(
             [NotNull] string storeType,
             [NotNull] Type clrType,
@@ -118,6 +122,14 @@ protected override void ConfigureParameter(DbParameter parameter)
             }
         }
 
+        /// <summary>
+        /// Returns the method to be used for reading JSON values from the database.
+        /// MySQL stores JSON as strings, so we use GetString instead of the default GetFieldValue&lt;T&gt;.
+        /// This prevents EF Core from trying to convert from string to MemoryStream for complex JSON types.
+        /// </summary>
+        public override MethodInfo GetDataReaderMethod()
+            => _getString;
+
         void IMySqlCSharpRuntimeAnnotationTypeMappingCodeGenerator.Create(
             CSharpRuntimeAnnotationCodeGeneratorParameters codeGeneratorParameters,
             CSharpRuntimeAnnotationCodeGeneratorDependencies codeGeneratorDependencies)