Question

在更改实体上的属性类型时出现问题，创建了默认约束，但EF在更改列时未删除它，因此，update-database失败。

该实体以前拥有DateTime属性。

public DateTime ImportDate { get; set; }

它的迁移包含在Up()方法

中

AddColumn("dbo.Table", "ImportDate", c => c.DateTime(nullable: false));

但是，它还创建了默认约束

ALTER TABLE [dbo].[Table] ADD  DEFAULT ('1900-01-01T00:00:00.000') FOR [ImportDate]

ImportDate正在更改为DateTimeOffset

public DateTimeOffset ImportDate { get; set; }

生成的迁移有一个AlterColumn

AlterColumn("dbo.Table", "ImportDate", c => c.DateTimeOffset(nullable: false, precision: 7));

但是，这会失败，因为默认约束存在。

我能想到的唯一方法是将Sql(...)与DROP CONSTRAINT一起使用，但是，CONSTRAINT有一个看似随机的名称DF__Table__Import__5441852A，因此硬编码的SQL无处不在。

思想？

Answer 1

您可以创建扩展程序并在生成的迁移中使用它：

 internal static class MigrationExtensions
    {
        public static void DeleteDefaultContraint(this IDbMigration migration, string tableName, string colName, bool suppressTransaction = false)
        {
            var sql = new SqlOperation(String.Format(@"DECLARE @SQL varchar(1000)
                SET @SQL='ALTER TABLE {0} DROP CONSTRAINT ['+(SELECT name
                FROM sys.default_constraints
                WHERE parent_object_id = object_id('{0}')
                AND col_name(parent_object_id, parent_column_id) = '{1}')+']';
                PRINT @SQL;
                EXEC(@SQL);", tableName, colName)) { SuppressTransaction = suppressTransaction };
            migration.AddOperation(sql);
        }
    }

并像这样使用

this.DeleteDefaultContraint("dbo.Table", "ImportDate");

从这篇文章：Group by in LINQ

Answer 2

更好的方法是永远解决问题。

您可以从System.Data.Entity.SqlServer命名空间实现从SqlServerMigrationSqlGenerator派生的自定义sql生成器类：

using System.Data.Entity.Migrations.Model;
using System.Data.Entity.SqlServer;

namespace System.Data.Entity.Migrations.Sql{
    internal class FixedSqlServerMigrationSqlGenerator : SqlServerMigrationSqlGenerator {
        protected override void Generate(AlterColumnOperation alterColumnOperation){
            ColumnModel column = alterColumnOperation.Column;
            var sql = String.Format(@"DECLARE @ConstraintName varchar(1000);
            DECLARE @sql varchar(1000);
            SELECT @ConstraintName = name   FROM sys.default_constraints
                WHERE parent_object_id = object_id('{0}')
                AND col_name(parent_object_id, parent_column_id) = '{1}';
            IF(@ConstraintName is NOT Null)
                BEGIN
                set @sql='ALTER TABLE {0} DROP CONSTRAINT [' + @ConstraintName+ ']';
            exec(@sql);
            END", alterColumnOperation.Table, column.Name);
                this.Statement(sql);
            base.Generate(alterColumnOperation);
            return;
        }
        protected override void Generate(DropColumnOperation dropColumnOperation){
            var sql = String.Format(@"DECLARE @SQL varchar(1000)
                SET @SQL='ALTER TABLE {0} DROP CONSTRAINT [' + (SELECT name
                    FROM sys.default_constraints
                    WHERE parent_object_id = object_id('{0}')
                    AND col_name(parent_object_id, parent_column_id) = '{1}') + ']';
            PRINT @SQL;
                EXEC(@SQL); ", dropColumnOperation.Table, dropColumnOperation.Name);

                    this.Statement(sql);
            base.Generate(dropColumnOperation);
        }
    }
}

并设置此配置：

internal sealed class Configuration : DbMigrationsConfiguration<MyDbContext>
{
    public Configuration()
    {
        AutomaticMigrationsEnabled = true;

        SetSqlGenerator("System.Data.SqlClient", new FixedSqlServerMigrationSqlGenerator ());
    }
    ...
}

由于默认值约束，代码优先迁移失败

2 个答案: