如何在 EF Core 中添加自定义添加迁移行为？

Question

我的目标是创建一个自定义 Attribute 并允许。 Add-Migration 基于它生成自定义代码。

模型和Attribute 类

public class MyAttribute: Attribute {}

public class MyModel
{
    public int Id { get; set; }

    [MyAttribute]
    public string Name { get; set; }
}

MigrationProvider 和 AnnotationProvider：

internal class MyMigrationsAnnotationProvider : SqliteMigrationsAnnotationProvider
{
    public override IEnumerable<IAnnotation> For( IProperty property )
    {
        MemberInfo MInfo = property.PropertyInfo ?? ( MemberInfo ) property.FieldInfo;
        MyAttribute MyAttr = MInfo?.GetCustomAttribute<MyAttribute>();

        if ( MyAttr != null )
        {
            return base.For( property ).Concat( new IAnnotation[] { new Annotation( "MyAttribute", true ) } );
        }

        return base.For( property );
    }
}

internal class MyMigrationsSqlGenerator : SqliteMigrationsSqlGenerator 
{
    public MyMigrationsSqlGenerator( IRelationalCommandBuilderFactory IRFactory, ISqlGenerationHelper ISHelper,  IRelationalTypeMapper Mapper, IRelationalAnnotationProvider AnProvider )
        : base( IRFactory, ISHelper, Mapper, AnProvider ) {}

    protected override void Generate( AddColumnOperation operation, IModel model, MigrationCommandListBuilder builder )
    {
        throw new Exception( "Hello world" );
        // Here's where I got it wrong.
        // I thought I should be able to read the "MyAttribute" annotation from here and generate extra code in the Up method
        /*
        if( operation.FindAnnotation( "MyAttribute" ) != null )
        {
            builder.AppendLine( "Hello there, not sure if this would work." );
        }
        */
    }
}

class MyContext : DbContext
{
    public DbSet<MyModel> MModel { get; set; }

    protected override void OnConfiguring( DbContextOptionsBuilder optionsBuilder )
    {
        optionsBuilder.UseSqlite( "Data Source=mydata.db" );
        optionsBuilder.ReplaceService<IMigrationsSqlGenerator, MyMigrationsSqlGenerator>();
        optionsBuilder.ReplaceService<IMigrationsAnnotationProvider, MyMigrationsAnnotationProvider>();
    }
}

生成的迁移代码（经过一些清理）

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "MyModel",
        columns: table => new
        {
            Id = table.Column<string>(nullable: false),
            Name = table.Column<string>(nullable: false)
                .Annotation("MyAttribute", true),
        });
    // The following line is what I want it to be generated
    migrationBuilder.Sql( "I WANT MY CUSTOM QUERY BE GENERATED HERE" );
}

protected override void Down(MigrationBuilder migrationBuilder)
{
    migrationBuilder.DropTable( name: "MyModel" );
    // The following line is what I want it to be generated
    migrationBuilder.Sql( "I WANT MY CUSTOM QUERY BE GENERATED HERE" );
}

如您所见，MyAttribute 注释已成功添加到Up 方法中。但是，我似乎无法覆盖 Generate 方法，因为在运行 Add-Migration 时没有抛出 Hello world 异常。

我使用的是 EF Core 1.1.5

提前致谢！

Answer 1

IMigrationsSqlGenerator 只能处理已生成的MigrationOperation。要检测新 Attribute 中的更改，您可能需要替换 IMigrationsModelDiffer 服务。然后可以返回一个新的 SqlOperation（或自定义类型），其中包含两个模型之间的其他差异。

从好的方面来说，这意味着您也可以在 Down 过程中生成撤消操作。

Answer 2

问题是AddColumnOperation 方法只有在现有表中添加新列时才会被调用。

对于CreateTable，您需要改写void Generate(CreateTableOperation operation, IModel model, MigrationCommandListBuilder builder) 方法。 CreateTableOperation 包含相同 AddColumnOperation 类型的 Operations 属性。

这是一个完整的例子

protected override void Generate(CreateTableOperation operation, IModel model, MigrationCommandListBuilder builder)
{
    base.Generate(operation, model, builder);
    foreach (var columnOperation in operation.Columns) //columnOperation is AddColumnOperation
    {
        //operation.FindAnnotation("MyAttribute")
    }
}

希望这会有所帮助！