Question

我需要在表中添加列，此列不存在。原因：在某些数据库中我们有这样的列，而在某些数据库中则没有。我已经为我的模型添加了字段并创建了迁移：

    public override void Up()
    {           
        AddColumn("dbo.NavFilters", "Promo", c => c.String(maxLength: 100, nullable:true));
    }

        public override void Down()
        {
            DropColumn("dbo.NavFilters", "Promo");
        }

我可以查看现有的列吗？

Answer 1

我一直致力于创建自定义迁移方法AddColumnIfNotExists

您需要自定义MigrationOperation类：

public class AddColumnIfNotExistsOperation : MigrationOperation
{
    public readonly string Table;
    public readonly string Name;
    public readonly ColumnModel ColumnModel;

    public AddColumnIfNotExistsOperation(string table, string name, Func<ColumnBuilder, ColumnModel> columnAction, object anonymousArguments) : base(anonymousArguments)
    {
        ArgumentValidator.CheckForEmptyArgument(table, nameof(table));
        ArgumentValidator.CheckForEmptyArgument(name, nameof(name));
        ArgumentValidator.CheckForNullArgument(columnAction, nameof(columnAction));

        Table = table;
        Name = name;

        ColumnModel = columnAction(new ColumnBuilder());
        ColumnModel.Name = name;

    }

    public override bool IsDestructiveChange => false;

    public override MigrationOperation Inverse => new DropColumnOperation(Table, Name, removedAnnotations: ColumnModel.Annotations.ToDictionary(s => s.Key,s => (object)s.Value) , anonymousArguments: null);
}

您还需要一个自定义SqlGenerator类：

public class AddColumnIfNotExistsSqlGenerator : SqlServerMigrationSqlGenerator
{
    protected override void Generate(MigrationOperation migrationOperation)
    {
        var operation = migrationOperation as AddColumnIfNotExistsOperation;
        if (operation == null) return;

        using (var writer = Writer())
        {
            writer.WriteLine("IF NOT EXISTS(SELECT 1 FROM sys.columns");
            writer.WriteLine($"WHERE Name = N'{operation.Name}' AND Object_ID = Object_ID(N'{Name(operation.Table)}'))");
            writer.WriteLine("BEGIN");
            writer.WriteLine("ALTER TABLE ");
            writer.WriteLine(Name(operation.Table));
            writer.Write(" ADD ");

            var column = operation.ColumnModel;
            Generate(column, writer);

            if (column.IsNullable != null
                && !column.IsNullable.Value
                && (column.DefaultValue == null)
                && (string.IsNullOrWhiteSpace(column.DefaultValueSql))
                && !column.IsIdentity
                && !column.IsTimestamp
                && !column.StoreType.EqualsIgnoreCase("rowversion")
                && !column.StoreType.EqualsIgnoreCase("timestamp"))
            {
                writer.Write(" DEFAULT ");

                if (column.Type == PrimitiveTypeKind.DateTime)
                {
                    writer.Write(Generate(DateTime.Parse("1900-01-01 00:00:00", CultureInfo.InvariantCulture)));
                }
                else
                {
                    writer.Write(Generate((dynamic)column.ClrDefaultValue));
                }
            }

            writer.WriteLine("END");



            Statement(writer);
        }
    }
}

和扩展方法，为您提供＆＃34; AddColumnIfNotExists＆＃34;功能：

public static class MigrationExtensions
{
    public static void AddColumnIfNotExists(this DbMigration migration, string table, string name, Func<ColumnBuilder, ColumnModel> columnAction, object anonymousArguments = null)
    {
        ((IDbMigration)migration)
          .AddOperation(new AddColumnIfNotExistsOperation(table, name, columnAction, anonymousArguments));
    }
}

在EF迁移配置文件中，您需要注册自定义SQL生成器：

[ExcludeFromCodeCoverage]
internal sealed class Configuration : DbMigrationsConfiguration<YourDbContext>
{
    public Configuration()
    {
        AutomaticMigrationsEnabled = false;

        // Register our custom generator
        SetSqlGenerator("System.Data.SqlClient", new AddColumnIfNotExistsSqlGenerator());
    }
}

然后您应该可以使用它代替AddColum（请注意此关键字）：

[ExcludeFromCodeCoverage]
public partial class AddVersionAndChangeActivity : DbMigration
{
    public override void Up()
    {
        this.AddColumnIfNotExists("dbo.Action", "VersionId", c => c.Guid(nullable: false));
        AlterColumn("dbo.Action", "Activity", c => c.String(nullable: false, maxLength: 8000, unicode: false));
    }

    public override void Down()
    {
        AlterColumn("dbo.Action", "Activity", c => c.String(nullable: false, maxLength: 50));
        DropColumn("dbo.Action", "VersionId");
    }
}

当然，您需要对操作进行一些测试：

[TestClass]
public class AddColumnIfNotExistsOperationTests
{
    [TestMethod]
    public void Can_get_and_set_table_and_column_info()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal(name: "T");

        var addColumnOperation = new AddColumnIfNotExistsOperation("T", "C", action, null);

        Assert.AreEqual("T", addColumnOperation.Table);
        Assert.AreEqual("C", addColumnOperation.Name);
    }

    [TestMethod]
    public void Inverse_should_produce_drop_column_operation()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal(name: "C", annotations: new Dictionary<string, AnnotationValues> { { "A1", new AnnotationValues(null, "V1") } });

        var addColumnOperation = new AddColumnIfNotExistsOperation("T", "C", action, null);

        var dropColumnOperation = (DropColumnOperation)addColumnOperation.Inverse;

        Assert.AreEqual("C", dropColumnOperation.Name);
        Assert.AreEqual("T", dropColumnOperation.Table);
        Assert.AreEqual("V1", ((AnnotationValues)dropColumnOperation.RemovedAnnotations["A1"]).NewValue);
        Assert.IsNull(((AnnotationValues)dropColumnOperation.RemovedAnnotations["A1"]).OldValue);
    }

    [TestMethod]
    [ExpectedException(typeof(ArgumentNullException))]
    public void Ctor_should_validate_preconditions_tableName()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal(name: "T");
        // ReSharper disable once ObjectCreationAsStatement
        new AddColumnIfNotExistsOperation(null, "T", action, null);
    }

    [TestMethod]
    [ExpectedException(typeof(ArgumentNullException))]
    public void Ctor_should_validate_preconditions_columnName()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal();
        // ReSharper disable once ObjectCreationAsStatement
        new AddColumnIfNotExistsOperation("T", null, action, null);
    }

    [TestMethod]
    [ExpectedException(typeof(ArgumentNullException))]
    public void Ctor_should_validate_preconditions_columnAction()
    {
        // ReSharper disable once ObjectCreationAsStatement
        new AddColumnIfNotExistsOperation("T", "C", null, null);
    }
}

测试SQL Generator：

[TestClass]
public class AddColumnIfNotExistsSqlGeneratorTests
{
    [TestMethod]
    public void AddColumnIfNotExistsSqlGenerator_Generate_can_output_add_column_statement_for_GUID_and_uses_newid()
    {
        var migrationSqlGenerator = new AddColumnIfNotExistsSqlGenerator();


        Func<ColumnBuilder, ColumnModel> action = c => c.Guid(nullable: false, identity: true, name: "Bar");


        var addColumnOperation = new AddColumnIfNotExistsOperation("Foo", "bar", action, null);

        var sql = string.Join(Environment.NewLine, migrationSqlGenerator.Generate(new[] {addColumnOperation}, "2005")
            .Select(s => s.Sql));


        Assert.IsTrue(sql.Contains("IF NOT EXISTS(SELECT 1 FROM sys.columns"));
        Assert.IsTrue(sql.Contains("WHERE Name = N\'bar\' AND Object_ID = Object_ID(N\'[Foo]\'))"));
        Assert.IsTrue(sql.Contains("BEGIN"));
        Assert.IsTrue(sql.Contains("ALTER TABLE"));
        Assert.IsTrue(sql.Contains("[Foo]"));
        Assert.IsTrue(sql.Contains("ADD [bar] [uniqueidentifier] NOT NULL DEFAULT newsequentialid()END"));
    }
}

Answer 2

使用标准DbMigration方法无法做到这一点最好的方法是在myTable中包含一个＆＃34; select fieldToCheck，其中1 = 2＆＃34;进入try catch然后根据需要添加字段（在catch中）。

另一种方法是编写一个扩展Migration生成器的自定义迁移生成器（即添加AddColumnIfNotExists方法）。你可以看看这里怎么做：
http://romiller.com/2013/02/27/ef6-writing-your-own-code-first-migration-operations/

Answer 3

sql 的基本示例：

// add colun if not exists
migrationBuilder.Sql(
@"IF COL_LENGTH('schemaName.TableName', 'ColumnName') IS NULL
     ALTER TABLE[TableName] ADD[ColumnName] int NULL
     GO
");

EF迁移代码优先。如果列不存在，则将列添加到数据库

3 个答案: