实体框架迁移中必填字段的默认值？

我已经将[Required]数据注释添加到ASP.NET MVC应用程序中的其中一个模型中。创建迁移后，运行Update-Database命令将导致以下错误：

无法将值NULL插入表'MOVIES_cf7bad808fa94f89afa2e5dae1161e78.dbo.Movies'的“ Director”列中；列不允许为空。更新失败。该语句已终止。

这是由于某些记录的Director列中包含NULL 。如何将这些值自动更改为某些默认值（例如“ John Doe”）导演？

这是我的模型：

  public class Movie
    {
        public int ID { get; set; }
        [Required]
        public string Title { get; set; }

        [DataType(DataType.Date)]
        public DateTime ReleaseDate { get; set; }

        [Required]
        public string Genre { get; set; }

        [Range(1,100)]
        [DataType(DataType.Currency)]
        public decimal Price { get; set; }

        [StringLength(5)]
        public string Rating { get; set; }

        [Required]     /// <--- NEW
        public string Director { get; set; }
    }

这是我最新的迁移：

public partial class AddDataAnnotationsMig : DbMigration
{
    public override void Up()
    {
        AlterColumn("dbo.Movies", "Title", c => c.String(nullable: false));
        AlterColumn("dbo.Movies", "Genre", c => c.String(nullable: false));
        AlterColumn("dbo.Movies", "Rating", c => c.String(maxLength: 5));
        AlterColumn("dbo.Movies", "Director", c => c.String(nullable: false));
    }

    public override void Down()
    {
        AlterColumn("dbo.Movies", "Director", c => c.String());
        AlterColumn("dbo.Movies", "Rating", c => c.String());
        AlterColumn("dbo.Movies", "Genre", c => c.String());
        AlterColumn("dbo.Movies", "Title", c => c.String());
    }
}

如果我没记错的话，这样的事情应该起作用：

AlterColumn("dbo.Movies", "Director", c => c.String(nullable: false, defaultValueSql: "'John Doe'"));

注意：defaultValueSql参数值被视为逐字SQL语句，因此，如果所需的值实际上是一个字符串（如John Doe的示例），则该值周围需要单引号。

除了来自@webdeveloper和@Pushpendra的答案外，您还需要手动向迁移中添加更新以更新现有行。例如：

public override void Up()
{
    Sql("UPDATE [dbo].[Movies] SET Title = 'No Title' WHERE Title IS NULL");
    AlterColumn("dbo.Movies", "Title", c => c.String(nullable: false,defaultValue:"MyTitle"));
}

这是因为AlterColumn产生DDL来将列的默认值设置为表规范中的某个特定值。DDL不会影响数据库中的现有行。

实际上，您实际上是在同时进行两项更改（设置默认值并使列NOT NULL），并且每项都分别有效，但是由于您同时进行两项更改，因此可以期望系统将“明智地'实现您的意图，并将所有NULL值设置为默认值，但这并不是一直期望的。

假设您只是设置列的默认值，而不是将其设置为NOT NULL。您显然不希望所有NULL记录都使用您提供的默认值进行更新。

因此，我认为这不是错误，并且我不希望EF以我未明确告知它的方式更新数据。开发人员负责指导系统如何处理数据。

public partial class AddDataAnnotationsMig : DbMigration
{
    public override void Up()
    {
        AlterColumn("dbo.Movies", "Title", c => c.String(nullable: false,defaultValue:"MyTitle"));
        AlterColumn("dbo.Movies", "Genre", c => c.String(nullable: false,defaultValue:"Genre"));
        AlterColumn("dbo.Movies", "Rating", c => c.String(maxLength: 5));
        AlterColumn("dbo.Movies", "Director", c => c.String(nullable: false,defaultValue:"Director"));

    }

    public override void Down()
    {       
        AlterColumn("dbo.Movies", "Director", c => c.String());
        AlterColumn("dbo.Movies", "Rating", c => c.String());
        AlterColumn("dbo.Movies", "Genre", c => c.String());
        AlterColumn("dbo.Movies", "Title", c => c.String());       
    }
}

不知道此选项是否一直存在，但是遇到了类似的问题，发现我可以使用以下命令设置默认值而无需运行任何手动更新

defaultValueSql: "'NY'"

当提供的值当时是错误的，"NY"我意识到他们期望像"GETDATE()"这样的SQL值，所以我尝试"'NY'"了

整行看起来像这样

AddColumn("TABLE_NAME", "State", c => c.String(maxLength: 2, nullable: false, defaultValueSql: "'NY'"));

多亏了这个答案，我才能走上正轨

从EF Core 2.1开始，您可以MigrationBuilder.UpdateData在更改列之前使用更改值（比使用原始SQL更干净）：

protected override void Up(MigrationBuilder migrationBuilder)
{
    // Change existing NULL values to NOT NULL values
    migrationBuilder.UpdateData(
        table: tableName,
        column: columnName,
        value: valueInsteadOfNull,
        keyColumn: columnName,
        keyValue: null);

    // Change column type to NOT NULL
    migrationBuilder.AlterColumn<ColumnType>(
        table: tableName,
        name: columnName,
        nullable: false,
        oldClrType: typeof(ColumnType),
        oldNullable: true);
}

我发现仅对实体属性使用自动属性初始化程序就足以完成工作。

例如：

public class Thing {
    public bool IsBigThing { get; set; } = false;
}

其他许多响应集中在发生这些问题时如何进行手动干预。

生成迁移后，对迁移执行以下任一更改：

1. 修改列定义以包括defaultValue或defaultSql语句：
   AlterColumn("dbo.Movies", "Director", c => c.String(nullable: false, default: ""));

2. 在AlterColumn之前插入一条SQL语句以预填充现有列：
   Sql("UPDATE dbo.Movies SET Director = '' WHERE Director IS NULL");

请记住，如果您重新构建迁移，则应用于迁移脚本的手动更改将被覆盖。对于第一个解决方案，很容易扩展EF以在迁移生成过程中自动在字段上定义默认值。

注意：EF不会自动为您执行此操作，因为每个RDBMS提供程序的默认值实现会有所不同，而且因为在纯EF运行时中默认值的意义较小，因为每个行插入将为每个属性提供当前值，即使它为null，也不会评估默认值约束。
该AlterColumn语句是默认约束生效的唯一时间，我想这对于设计SQL Server迁移实现的团队来说将成为较低的优先级。

以下解决方案结合了属性符号，模型配置约定和列注释，以将元数据传递给自定义的迁移代码生成器。如果不使用属性符号，则可以对每个受影响的字段使用流利的符号替换步骤1和2。
这里有很多技巧，您可以随意使用其中的一些或全部，我希望这里对每个人都有价值

1. 声明默认值
   创建或重新使用现有属性来定义要使用的默认值，在本示例中，我们将创建一个名为DefaultValue的新属性，该属性继承自ComponentModel.DefaultValueAttribute，因为用法很直观，并且存在存在的可能性代码库已经实现了此属性。通过此实现，您只需要使用此特定属性即可访问DefaultValueSql，该属性对日期和其他自定义方案很有用。

   实作

   [DefaultValue("Insert DefaultValue Here")]
   [Required]     /// <--- NEW
   public string Director { get; set; }
   
   // Example of default value sql
   [DefaultValue(DefaultValueSql: "GetDate()")]
   [Required]
   public string LastModified { get; set; }

   属性定义

   namespace EFExtensions
   {
       /// <summary>
       /// Specifies the default value for a property but allows a custom SQL statement to be provided as well. <see cref="MiniTuber.Database.Conventions.DefaultValueConvention"/>
       /// </summary>
       public class DefaultValueAttribute : System.ComponentModel.DefaultValueAttribute
       {
           /// <summary>
           /// Specifies the default value for a property but allows a custom SQL statement to be provided as well. <see cref="MiniTuber.Database.Conventions.DefaultValueConvention"/>
           /// </summary>
           public DefaultValueAttribute() : base("")
           {
           }
   
           /// <i
           /// <summary>
           /// Optional SQL to use to specify the default value.
           /// </summary>
           public string DefaultSql { get; set; }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a Unicode character.
           /// </summary>
           /// <param name="value">
           /// A Unicode character that is the default value.
           /// </param>
           public DefaultValueAttribute(char value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using an 8-bit unsigned integer.
           /// </summary>
           /// <param name="value">
           /// An 8-bit unsigned integer that is the default value.
           /// </param>
           public DefaultValueAttribute(byte value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a 16-bit signed integer.
           /// </summary>
           /// <param name="value">
           /// A 16-bit signed integer that is the default value.
           /// </param>
           public DefaultValueAttribute(short value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a 32-bit signed integer.
           /// </summary>
           /// <param name="value">
           /// A 32-bit signed integer that is the default value.
           /// </param>
           public DefaultValueAttribute(int value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a 64-bit signed integer.
           /// </summary>
           /// <param name="value">
           /// A 64-bit signed integer that is the default value.
           /// </param>
           public DefaultValueAttribute(long value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a single-precision floating point number.
           /// </summary>
           /// <param name="value">
           /// A single-precision floating point number that is the default value.
           /// </param>
           public DefaultValueAttribute(float value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a double-precision floating point number.
           /// </summary>
           /// <param name="value">
           /// A double-precision floating point number that is the default value.
           /// </param>
           public DefaultValueAttribute(double value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a System.Boolean value.
           /// </summary>
           /// <param name="value">
           /// A System.Boolean that is the default value.
           /// </param>
           public DefaultValueAttribute(bool value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class using a System.String.
           /// </summary>
           /// <param name="value">
           /// A System.String that is the default value.
           /// </param>
           public DefaultValueAttribute(string value) : base(value) { }
   
           /// <summary>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class.
           /// </summary>
           /// <param name="value">
           /// An System.Object that represents the default value.
           /// </param>
           public DefaultValueAttribute(object value) : base(value) { }
   
           /// /// <inheritdoc/>
           /// Initializes a new instance of the System.ComponentModel.DefaultValueAttribute
           /// class, converting the specified value to the specified type, and using an invariant
           /// culture as the translation context.
           /// </summary>
           /// <param name="type">
           /// A System.Type that represents the type to convert the value to.
           /// </param>
           /// <param name="value">
           /// A System.String that can be converted to the type using the System.ComponentModel.TypeConverter
           /// for the type and the U.S. English culture.
           /// </param>
           public DefaultValueAttribute(Type type, string value) : base(value) { }
       }
   }

2. 创建约定以将默认值注入到列批注中
   列批注用于将有关列的自定义元数据传递到迁移脚本生成器。
   使用约定来执行此操作演示了属性符号背后的强大功能，该符号简化了如何为许多属性定义和操作流利的元数据，而不是为每个字段单独指定它。

   namespace EFExtensions
   {
   
       /// <summary>
       /// Implement SQL Default Values from System.ComponentModel.DefaultValueAttribute
       /// </summary>
       public class DefaultValueConvention : Convention
       {
           /// <summary>
           /// Annotation Key to use for Default Values specified directly as an object
           /// </summary>
           public const string DirectValueAnnotationKey = "DefaultValue";
           /// <summary>
           /// Annotation Key to use for Default Values specified as SQL Strings
           /// </summary>
           public const string SqlValueAnnotationKey = "DefaultSql";
   
           /// <summary>
           /// Implement SQL Default Values from System.ComponentModel.DefaultValueAttribute
           /// </summary>
           public DefaultValueConvention()
           {
               // Implement SO Default Value Attributes first
               this.Properties()
                       .Where(x => x.HasAttribute<EFExtensions.DefaultValueAttribute>())
                       .Configure(c => c.HasColumnAnnotation(
                           c.GetAttribute<EFExtensions.DefaultValueAttribute>().GetDefaultValueAttributeKey(),
                           c.GetAttribute<EFExtensions.DefaultValueAttribute>().GetDefaultValueAttributeValue()
                           ));
   
               // Implement Component Model Default Value Attributes, but only if it is not the SO implementation
               this.Properties()
                       .Where(x => x.HasAttribute<System.ComponentModel.DefaultValueAttribute>())
                       .Where(x => !x.HasAttribute<MiniTuber.DataAnnotations.DefaultValueAttribute>())
                       .Configure(c => c.HasColumnAnnotation(
                           DefaultValueConvention.DirectValueAnnotationKey, 
                           c.GetAttribute<System.ComponentModel.DefaultValueAttribute>().Value
                           ));
           }
       }
   
       /// <summary>
       /// Extension Methods to simplify the logic for building column annotations for Default Value processing
       /// </summary>
       public static partial class PropertyInfoAttributeExtensions
       {
           /// <summary>
           /// Wrapper to simplify the lookup for a specific attribute on a property info.
           /// </summary>
           /// <typeparam name="T">Type of attribute to lookup</typeparam>
           /// <param name="self">PropertyInfo to inspect</param>
           /// <returns>True if an attribute of the requested type exists</returns>
           public static bool HasAttribute<T>(this PropertyInfo self) where T : Attribute
           {
               return self.GetCustomAttributes(false).OfType<T>().Any();
           }
   
           /// <summary>
           /// Wrapper to return the first attribute of the specified type
           /// </summary>
           /// <typeparam name="T">Type of attribute to return</typeparam>
           /// <param name="self">PropertyInfo to inspect</param>
           /// <returns>First attribuite that matches the requested type</returns>
           public static T GetAttribute<T>(this System.Data.Entity.ModelConfiguration.Configuration.ConventionPrimitivePropertyConfiguration self) where T : Attribute
           {
               return self.ClrPropertyInfo.GetCustomAttributes(false).OfType<T>().First();
           }
   
           /// <summary>
           /// Helper to select the correct DefaultValue annotation key based on the attribute values
           /// </summary>
           /// <param name="self"></param>
           /// <returns></returns>
           public static string GetDefaultValueAttributeKey(this EFExtensions.DefaultValueAttribute self)
           {
               return String.IsNullOrWhiteSpace(self.DefaultSql) ? DefaultValueConvention.DirectValueAnnotationKey : DefaultValueConvention.SqlValueAnnotationKey;
           }
   
           /// <summary>
           /// Helper to select the correct attribute property to send as a DefaultValue annotation value
           /// </summary>
           /// <param name="self"></param>
           /// <returns></returns>
           public static object GetDefaultValueAttributeValue(this EFExtensions.DefaultValueAttribute self)
           {
               return String.IsNullOrWhiteSpace(self.DefaultSql) ? self.Value : self.DefaultSql;
           }
       }
   
   }

3. 将约定添加到DbContext中
   有多种方法可以实现，我想将约定声明为ModelCreation逻辑中的第一个自定义步骤，这将在您的DbContext类中。

   protected override void OnModelCreating(DbModelBuilder modelBuilder)
   {
       base.OnModelCreating(modelBuilder);
       // Use our new DefaultValueConvention
       modelBuilder.Conventions.Add<EFExtensions.DefaultValueConvention>();
   
       // My personal favourites ;)
       modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
       modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();
   
   }

4. 覆盖MigrationCodeGenerator
   现在，这些注释已应用于模型中的列定义，我们需要修改迁移脚本生成器以使用这些注释。为此，我们将继承自，System.Data.Entity.Migrations.Design.CSharpMigrationCodeGenerator因为我们只需要注入最少的更改即可。
   处理完自定义注释后，需要将其从列定义中删除，以防止将其序列化为最终输出。

   请参阅基类代码以探索其他用法：http : //entityframework.codeplex.com/sourcecontrol/latest#src/EntityFramework/Migrations/Design/CSharpMigrationCodeGenerator.cs

   namespace EFExtensions
   {
       /// <summary>
       /// Implement DefaultValue constraint definition in Migration Scripts.
       /// </summary>
       /// <remarks>
       /// Original guide that provided inspiration for this https://romiller.com/2012/11/30/code-first-migrations-customizing-scaffolded-code/
       /// </remarks>
       public class CustomCodeGenerator : System.Data.Entity.Migrations.Design.CSharpMigrationCodeGenerator
       {
           /// <summary>
           /// Inject Default values from the DefaultValue attribute, if the DefaultValueConvention has been enabled.
           /// </summary>
           /// <seealso cref="DefaultValueConvention"/>
           /// <param name="column"></param>
           /// <param name="writer"></param>
           /// <param name="emitName"></param>
           protected override void Generate(ColumnModel column, IndentedTextWriter writer, bool emitName = false)
           {
               var annotations = column.Annotations?.ToList();
               if (annotations != null && annotations.Any())
               {
                   for (int index = 0; index < annotations.Count; index ++)
                   {
                       var annotation = annotations[index];
                       bool handled = true;
   
                       try
                       {
                           switch (annotation.Key)
                           {
                               case DefaultValueConvention.SqlValueAnnotationKey:
                                   if (annotation.Value?.NewValue != null)
                                   {
                                       column.DefaultValueSql = $"{annotation.Value.NewValue}";
                                   }
                                   break;
                               case DefaultValueConvention.DirectValueAnnotationKey:
                                   if (annotation.Value?.NewValue != null)
                                   {
                                       column.DefaultValue = Convert.ChangeType(annotation.Value.NewValue, column.ClrType);
                                   }
                                   break;
                               default:
                                   handled = false;
                                   break;
                           }
                       }
                       catch(Exception ex)
                       {
                           // re-throw with specific debug information
                           throw new ApplicationException($"Failed to Implement Column Annotation for column: {column.Name} with key: {annotation.Key} and new value: {annotation.Value.NewValue}", ex);
                       }
   
                       if(handled)
                       {
                           // remove the annotation, it has been applied
                           column.Annotations.Remove(annotation.Key);
                       }
                   }
               }
               base.Generate(column, writer, emitName);
           }
   
           /// <summary>
           /// Generates class summary comments and default attributes
           /// </summary>
           /// <param name="writer"> Text writer to add the generated code to. </param>
           /// <param name="designer"> A value indicating if this class is being generated for a code-behind file. </param>
           protected override void WriteClassAttributes(IndentedTextWriter writer, bool designer)
           {
               writer.WriteLine("/// <summary>");
               writer.WriteLine("/// Definition of the Migration: {0}", this.ClassName);
               writer.WriteLine("/// </summary>");
               writer.WriteLine("/// <remarks>");
               writer.WriteLine("/// Generated Time: {0}", DateTime.Now);
               writer.WriteLine("/// Generated By: {0}", Environment.UserName);
               writer.WriteLine("/// </remarks>");
               base.WriteClassAttributes(writer, designer);
           }
   
   
       }
   }

5. 注册CustomCodeGenerator的
   最后一步，在DbMigration配置文件中，我们需要指定要使用的代码生成器，默认情况下，在Migration文件夹中查找Configuration.cs ...

   internal sealed class Configuration : DbMigrationsConfiguration<YourApplication.Database.Context>
   {
       public Configuration()
       {
           // I recommend that auto-migrations be disabled so that we control
           // the migrations explicitly 
           AutomaticMigrationsEnabled = false;
           CodeGenerator = new EFExtensions.CustomCodeGenerator();
       }
   
       protected override void Seed(YourApplication.Database.Context context)
       {
           //   Your custom seed logic here
       }
   }

由于某种原因，我无法向自己解释批准的答案对我而言不再有效。

它可以在另一款应用程序上工作，而在我正在使用的应用程序上却没有。

因此，另一种但效率不高的解决方案是，如下所示重写SaveChanges（）方法。此方法应该在Context类上。

    public override int SaveChanges()
    {
        foreach (var entry in ChangeTracker.Entries().Where(entry => entry.Entity.GetType().GetProperty("ColumnName") != null))
        {
            if (entry.State == EntityState.Added)
            {
                entry.Property("ColumnName").CurrentValue = "DefaultValue";
            }
        }