在删除列的迁移操作期间,如何在尝试删除列之前先生成 SQL 以检查该列是否存在?
对于删除列操作, Entity Framework 当前生成这样的 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 以首先检查列是否存在:
// 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删除列逻辑包装在 IF
块中。请参阅下面的示例: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);
}
}
}
资料来源:
最佳答案
如果您已正确配置您的 CustomSqlServerMigrationSqlGenerator
然后在执行 Update-Database
期间您应该遇到过此错误消息:
System.Data.SqlClient.SqlException (0x80131904): Incorrect syntax near 'BEGIN'.
问题是您已经构造并执行了 3 个部分语句,即对 Statement()
的调用。默认情况下,作为单个批处理操作执行,并且在语句中包含实际的 DropColumn 语句和 End 之前,您的批处理是无效的语法。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);
}
}
如果您没有看到当前的错误,可能是 CustomSqlServerMigrationSqlGenerator
未正确注册,请确保您已在 Configuration.cs 中的 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());
}
如果您需要调试这个过程,请按照advice in this post这与此场景类似,您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();
}
...
}
关于entity-framework - Entity Framework 6 : Migration Drop column if exists,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/46933717/