Fix to #32192 - Remove lookup of JsonTypeMapping via JsonElement (#35788)

When adding support for JSON mapped types, we decided to use JsonElement as a CLR type marker for JsonTypeMapping. This decision was in hindsight incorrect, it makes it hard for providers that actually support weak-typed json (npgsql) and throws cryptic exception (No coercion operator is defined between types 'System.IO.MemoryStream' and 'System.Text.Json.JsonElement) for providers that don't support it, when customers try to do it regardless.
Fix is to create a dummy type and use that instead, so JsonElement is no longer recognized by base EFCore.

Fixes #32192
also fixes #34752

Author: maumar

src/EFCore.Relational/Metadata/Internal/JsonColumn.cs

@@ -36,7 +36,7 @@ public JsonColumn(
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
     protected override RelationalTypeMapping GetDefaultStoreTypeMapping()
-        => (RelationalTypeMapping)Table.Model.Model.GetModelDependencies().TypeMappingSource.FindMapping(typeof(JsonElement))!;
+        => (RelationalTypeMapping)Table.Model.Model.GetModelDependencies().TypeMappingSource.FindMapping(typeof(JsonTypePlaceholder))!;
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to

src/EFCore.Relational/Metadata/Internal/JsonColumnBase.cs

@@ -36,5 +36,5 @@ public JsonColumnBase(
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
     protected override RelationalTypeMapping GetDefaultStoreTypeMapping()
-        => (RelationalTypeMapping)Table.Model.Model.GetModelDependencies().TypeMappingSource.FindMapping(typeof(JsonElement))!;
+        => (RelationalTypeMapping)Table.Model.Model.GetModelDependencies().TypeMappingSource.FindMapping(typeof(JsonTypePlaceholder))!;
 }

src/EFCore.Relational/Metadata/Internal/JsonViewColumn.cs

@@ -36,5 +36,5 @@ public JsonViewColumn(
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
     protected override RelationalTypeMapping GetDefaultStoreTypeMapping()
-        => (RelationalTypeMapping)Table.Model.Model.GetModelDependencies().TypeMappingSource.FindMapping(typeof(JsonElement))!;
+        => (RelationalTypeMapping)Table.Model.Model.GetModelDependencies().TypeMappingSource.FindMapping(typeof(JsonTypePlaceholder))!;
 }

src/EFCore.Relational/Metadata/Internal/RelationalModel.cs

@@ -582,7 +582,7 @@ private static void CreateContainerColumn<TColumnMappingBase>(
         {
             Check.DebugAssert(tableBase.FindColumn(containerColumnName) == null, $"Table does not have column '{containerColumnName}'.");
 
-            var jsonColumnTypeMapping = relationalTypeMappingSource.FindMapping(typeof(JsonElement), storeTypeName: containerColumnType)!;
+            var jsonColumnTypeMapping = relationalTypeMappingSource.FindMapping(typeof(JsonTypePlaceholder), storeTypeName: containerColumnType)!;
             var jsonColumn = createColumn(containerColumnName, containerColumnType, tableBase, jsonColumnTypeMapping);
             tableBase.Columns.Add(containerColumnName, jsonColumn);
             jsonColumn.IsNullable = !ownership.IsRequiredDependent || !ownership.IsUnique;

src/EFCore.Relational/Storage/JsonTypeMapping.cs

@@ -2,13 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Data;
-using System.Text.Json;
 
 namespace Microsoft.EntityFrameworkCore.Storage;
 
 /// <summary>
 ///     <para>
-///         Represents the mapping between a <see cref="JsonElement" /> type and a database type.
+///         Represents the mapping between a JSON object and a database type.
 ///     </para>
 ///     <para>
 ///         This type is typically used by database providers (and other extensions). It is generally

src/EFCore.Relational/Storage/JsonTypePlaceholder.cs

@@ -0,0 +1,14 @@
+// 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.Storage;
+
+/// <summary>
+/// A type representing CLR type of the JsonTypeMapping.
+/// </summary>
+public sealed class JsonTypePlaceholder
+{
+    private JsonTypePlaceholder()
+    {
+    }
+}

src/EFCore.SqlServer/Storage/Internal/SqlServerOwnedJsonTypeMapping.cs

@@ -45,7 +45,7 @@ private static readonly MethodInfo GetStringMethod
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
     public SqlServerOwnedJsonTypeMapping(string storeType)
-        : base(storeType, typeof(JsonElement), System.Data.DbType.String)
+        : base(storeType, typeof(JsonTypePlaceholder), System.Data.DbType.String)
     {
     }
 

src/EFCore.SqlServer/Storage/Internal/SqlServerTypeMappingSource.cs

@@ -3,7 +3,6 @@
 
 using System.Collections;
 using System.Data;
-using System.Text.Json;
 
 namespace Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal;
 
@@ -239,7 +238,7 @@ public SqlServerTypeMappingSource(
         var clrType = mappingInfo.ClrType;
         var storeTypeName = mappingInfo.StoreTypeName;
 
-        if (clrType == typeof(JsonElement))
+        if (clrType == typeof(JsonTypePlaceholder))
         {
             return storeTypeName == "json"
                 ? SqlServerOwnedJsonTypeMapping.OwnedJsonTypeDefault

src/EFCore.Sqlite.Core/Storage/Internal/SqliteJsonTypeMapping.cs

@@ -39,7 +39,7 @@ private static readonly ConstructorInfo MemoryStreamConstructor
     /// </summary>
     /// <param name="storeType">The name of the database type.</param>
     public SqliteJsonTypeMapping(string storeType)
-        : base(storeType, typeof(JsonElement), System.Data.DbType.String)
+        : base(storeType, typeof(JsonTypePlaceholder), System.Data.DbType.String)
     {
     }
 

src/EFCore.Sqlite.Core/Storage/Internal/SqliteTypeMappingSource.cs

@@ -83,7 +83,7 @@ private static readonly HashSet<string> SpatialiteTypes
         { typeof(double), Real },
         { typeof(float), new FloatTypeMapping(RealTypeName) },
         { typeof(Guid), SqliteGuidTypeMapping.Default },
-        { typeof(JsonElement), SqliteJsonTypeMapping.Default }
+        { typeof(JsonTypePlaceholder), SqliteJsonTypeMapping.Default }
     };
 
     private readonly Dictionary<string, RelationalTypeMapping> _storeTypeMappings = new(StringComparer.OrdinalIgnoreCase)

test/EFCore.Relational.Specification.Tests/Query/AdHocMiscellaneousQueryRelationalTestBase.cs

@@ -4,6 +4,7 @@
 #nullable disable
 
 using System.ComponentModel.DataAnnotations.Schema;
+using System.Text.Json;
 using NameSpace1;
 
 namespace Microsoft.EntityFrameworkCore.Query
@@ -239,6 +240,32 @@ public static DateTime Modify(DateTime date)
 
         #endregion
 
+        #region 34752
+
+        [ConditionalFact]
+        public virtual async Task Mapping_JsonElement_property_throws_a_meaningful_exception()
+        {
+            var message = (await Assert.ThrowsAsync<InvalidOperationException>(
+                () => InitializeAsync<Context34752>())).Message;
+
+            Assert.Equal(
+                CoreStrings.PropertyNotAdded(nameof(Context34752.Entity), nameof(Context34752.Entity.Json), nameof(JsonElement)),
+                message);
+        }
+
+        protected class Context34752(DbContextOptions options) : DbContext(options)
+        {
+            public DbSet<Entity> Entities { get; set; }
+
+            public class Entity
+            {
+                public int Id { get; set; }
+                public JsonElement Json { get; set; }
+            }
+        }
+
+        #endregion
+
         #region Inlined redacting
 
         protected abstract DbContextOptionsBuilder SetTranslateParameterizedCollectionsToConstants(DbContextOptionsBuilder optionsBuilder);

test/EFCore.SqlServer.FunctionalTests/Scaffolding/Baselines/BigModel_with_JSON_columns/DbContextModelBuilder.cs

@@ -1,12 +1,12 @@
 // <auto-generated />
 using System;
 using System.Collections.Generic;
-using System.Text.Json;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Metadata;
 using Microsoft.EntityFrameworkCore.Metadata.Internal;
 using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage;
 using Microsoft.EntityFrameworkCore.Update.Internal;
 
 #pragma warning disable 219, 612, 618
@@ -2258,10 +2258,10 @@ private IRelationalModel CreateRelationalModel()
                 IsNullable = true
             };
             principalBaseTable.Columns.Add("ManyOwned", manyOwnedColumn);
-            manyOwnedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonElement>(manyOwnedColumn);
+            manyOwnedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonTypePlaceholder>(manyOwnedColumn);
             var ownedColumn = new JsonColumn("Owned", "nvarchar(max)", principalBaseTable);
             principalBaseTable.Columns.Add("Owned", ownedColumn);
-            ownedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonElement>(ownedColumn);
+            ownedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonTypePlaceholder>(ownedColumn);
             var refTypeArrayColumn = new Column("RefTypeArray", "nvarchar(max)", principalBaseTable)
             {
                 IsNullable = true

test/EFCore.Sqlite.FunctionalTests/Scaffolding/Baselines/BigModel_with_JSON_columns/DbContextModelBuilder.cs

@@ -1,12 +1,12 @@
 // <auto-generated />
 using System;
 using System.Collections.Generic;
-using System.Text.Json;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Metadata;
 using Microsoft.EntityFrameworkCore.Metadata.Internal;
 using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage;
 using Microsoft.EntityFrameworkCore.Update.Internal;
 using NetTopologySuite.Geometries;
 
@@ -2325,10 +2325,10 @@ private IRelationalModel CreateRelationalModel()
                 IsNullable = true
             };
             principalBaseTable.Columns.Add("ManyOwned", manyOwnedColumn);
-            manyOwnedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonElement>(manyOwnedColumn);
+            manyOwnedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonTypePlaceholder>(manyOwnedColumn);
             var ownedColumn = new JsonColumn("Owned", "TEXT", principalBaseTable);
             principalBaseTable.Columns.Add("Owned", ownedColumn);
-            ownedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonElement>(ownedColumn);
+            ownedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonTypePlaceholder>(ownedColumn);
             var pointColumn0 = new Column("Point", "geometry", principalBaseTable)
             {
                 IsNullable = true