Question

在删除列的迁移操作期间，如何在尝试删除列之前生成SQL以检查列的存在？

对于drop column操作，Entity框架当前生成这样的sql以删除列：

// 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。我尝试这样做无法将IF块中的默认丢弃列逻辑包装起来。见下面的例子：

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：

Answer 1

如果您已正确配置CustomSqlServerMigrationSqlGenerator，则在执行Update-Database时，您应该会遇到以下错误消息：

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

问题是您已经构造并执行了3条局部语句，默认情况下对Statement()的调用是作为单个批处理操作执行的，并且直到实际的 DropColumn，您的批处理才是有效的语法语句和 End 。

由于基本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），我们被迫要做的是完全忽略基本实现：

/// <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); } }

如果没有看到当前错误，可能是CustomSqlServerMigrationSqlGenerator未正确注册，请确保在Configuration.cs的Configuration类的构造函数中进行了 set 设置。：

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

如果需要调试此过程，请遵循advice in this post（与本场景类似的场景），可以在生成器类的构造函数中放置一个断点：

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

实体框架6：迁移删除列（如果存在）

1 个答案: