实体框架 6：迁移删除列（如果存在）

Question

在删除列的迁移操作期间，如何生成 SQL 以在尝试删除列之前先检查该列的存在？

对于删除列操作实体框架目前生成这样的sql来删除列：

// Migration Operation:
DropColumn("dbo.Table", "Column");

// TSQL generated:
// Dependency management logic ...
ALTER TABLE [dbo].[Table] DROP COLUMN [Column]

如何更改 SQL 以首先检查列的存在：

// Migration Operation:
DropColumn("dbo.Table", "Column");

// TSQL desired:
IF EXISTS (SELECT * FROM sys.columns WHERE object_id = Object_id('dbo.Table') AND name = 'Column')
BEGIN
    // Dependency management logic ...
    ALTER TABLE [dbo].[Table] DROP COLUMN [Column]
END

我知道可以通过继承SqlServerMigrationSqlGenerator 来自定义迁移SQL。我这样做的尝试未能将默认的删除列逻辑包装在 IF 块中。请参见下面的示例：

public class CustomSqlServerMigrationSqlGenerator: SqlServerMigrationSqlGenerator
{
    /// <summary>
    /// Drop column only if it exists.
    /// </summary>
    protected override void Generate(System.Data.Entity.Migrations.Model.DropColumnOperation dropColumnOperation)
    {
        using (var writer = Writer())
        {
            writer.WriteLine(
              "IF EXISTS (SELECT * FROM sys.columns WHERE object_id = Object_id('{0}') AND name = '{1}')",
              dropColumnOperation.Table,
              dropColumnOperation.Name);
            writer.WriteLine("BEGIN");
            Statement(writer);
        }

        // Default drop column logic
        base.Generate(dropColumnOperation);

        using (var writer = Writer())
        {
            writer.WriteLine("END");
            Statement(writer);
        }
    }
}

来源：

• Entity Framework 6: How to override SQL generator?
• Romiller on customizing code first migrations
• EF 6 SqlServerMigrationSqlGenerator drop column logic

Answer 1

如果您正确配置了CustomSqlServerMigrationSqlGenerator，那么在执行Update-Database 期间，您应该会遇到以下错误消息：

System.Data.SqlClient.SqlException (0x80131904): Incorrect syntax near 'BEGIN'.

问题是您已经构建并执行了 3 个部分语句，默认情况下对 Statement() 的调用作为单个批处理操作执行，并且您的批处理在实际 DropColumn 之前是无效语法语句和End都包含在语句中。

由于基础实现不允许我们通过文本编写器（我们需要的方法标记为protected），因此我们不得不完全忽略基础实现：

/// <summary>
/// Drop column only if it exists.
/// </summary>
/// <remarks>This helps when we're stuffed up a previous migration or someone has already executed the drop in the DB direct.</remarks>
protected override void Generate(System.Data.Entity.Migrations.Model.DropColumnOperation dropColumnOperation)
{
    using (var writer = Writer())
    {
        writer.WriteLine(
          "IF EXISTS (SELECT * FROM sys.columns WHERE object_id = Object_id('{0}') AND name = '{1}')",
          dropColumnOperation.Table,
          dropColumnOperation.Name);
        writer.WriteLine("BEGIN");

        // Base Implementation, increase the indentation
        writer.Indent++;
        writer.WriteLine("ALTER TABLE {0}", Quote(dropColumnOperation.Table));
        writer.WriteLine("DROP COLUMN {0}", Quote(dropColumnOperation.Name));
        writer.Indent--;

        writer.WriteLine("END");

        Statement(writer);
    }
}

如果您没有看到当前的错误，可能是 CustomSqlServerMigrationSqlGenerator 没有正确注册，请确保您在 Configuration.cs 的 Configuration 类的构造函数中设置它，例如：

public Configuration()
{
    AutomaticMigrationsEnabled = false;
    AutomaticMigrationDataLossAllowed = false;
    // Register the Customized SQL Generator to use
    this.SetSqlGenerator("System.Data.SqlClient", new CustomSqlServerMigrationSqlGenerator());
}

---

如果你需要调试这个过程，按照advice in this post这个类似的场景，你可以在你的生成器类的构造函数中设置一个断点：

public class CustomSqlServerMigrationSqlGenerator: SqlServerMigrationSqlGenerator
{
    public CustomSqlServerMigrationSqlGenerator()
        : base()
    {
        if (!System.Diagnostics.Debugger.IsAttached)
            System.Diagnostics.Debugger.Launch();
    }

    ...
}