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JSON Column Store Type Fix for EF Core 10

Problem

EF Core 10 requires complex collections mapped with .ToJson() to have a provider-specific store type set. Tests were failing with:

The store type 'null' specified for JSON column 'Departments' is not supported by the current provider.

Investigation

Compared with SQLite and SQL Server

Both work without setting ContainerColumnType annotation because:

  • SQLite: Default string mapping is "TEXT" (valid for JSON)
  • SQL Server: Default string mapping is "nvarchar(max)" (valid for JSON)
  • MySQL: Default string mapping is "longtext" (EF Core doesn't accept this for JSON)

Root Causes Identified

  1. Missing annotation: ContainerColumnType not set to "json" on complex types
  2. Type mapping restriction: MySqlTypeMappingSource only returned JSON mapping for string or MySqlJsonString CLR types, but EF Core requests mappings with the actual collection type (e.g., List<Department>)

Solution

1. MySqlJsonColumnConvention

File: src/EFCore.MySql/Metadata/Conventions/MySqlJsonColumnConvention.cs

  • Implements IComplexPropertyAddedConvention and IComplexPropertyAnnotationChangedConvention
  • Detects when .ToJson() is called by checking for JsonPropertyName annotation
  • Uses SetContainerColumnType("json") extension method (not generic HasAnnotation())
  • Sets annotation on the complex type (not the complex property)

Key insight: Must use the specific extension method SetContainerColumnType() for the annotation to persist properly.

2. MySqlTypeMappingSource

File: src/EFCore.MySql/Storage/Internal/MySqlTypeMappingSource.cs

Changed line 333 from:

if (storeTypeName.Equals("json", StringComparison.OrdinalIgnoreCase) &&
    (clrType == null || clrType == typeof(string) || clrType == typeof(MySqlJsonString)))

To:

if (storeTypeName.Equals("json", StringComparison.OrdinalIgnoreCase))

Rationale: JSON can serialize any object, so the type mapping should work for any CLR type when store type is explicitly "json".

MySQL/MariaDB JSON Support

  • MySQL: 5.7.8+ (native JSON type)
  • MariaDB: 10.2.4+ (JSON functions, alias for LONGTEXT with validation)

Both support storing JSON in "json" column type.

Status

Code changes complete and committed (615e974)

Remaining work:

  • Verify convention is being triggered by EF Core convention pipeline
  • Run actual failing tests with MySQL database to confirm fix
  • May need to adjust convention timing if not triggering

The solution is theoretically sound based on investigation, but needs practical testing with the actual test suite.