EF Core中怎么实现自动更新实体的属性值到数据库

我们在开发系统的时候,经常会遇到这种需求数据库表中的行被更新时需要自动更新某些列,比如下面的Person表有一列UpdateTime,这列数据要求在行被更新后自动更新为系统的当前时间。

 

Person表:

CREATE TABLE [dbo].[Person](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [Name] [nvarchar](50) NULL,
    [Age] [int] NULL,
    [CreateTime] [datetime] NULL,
    [UpdateTime] [datetime] NULL,
 CONSTRAINT [PK_Person] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[Person] ADD  CONSTRAINT [DF_Person_CreateTime]  DEFAULT (getdate()) FOR [CreateTime]
GO

 

我们还有一个Book表,它没有UpdateTime列,那么这个表的数据在行更新时不要求自动更新任何列

Book表:

CREATE TABLE [dbo].[Book](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [Name] [nvarchar](50) NULL,
    [BookDescription] [nvarchar](100) NULL,
    [ISBN] [nvarchar](50) NULL,
    [CreateTime] [datetime] NULL,
 CONSTRAINT [PK_Book] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[Book] ADD  CONSTRAINT [DF_Book_CreateTime]  DEFAULT (getdate()) FOR [CreateTime]
GO

 

那么Person表的UpdateTime列如果映射到了EF Core的实体上的话,有办法在Person实体被Update的时候自动设置为系统当前时间吗?答案是当然有!

 

首先我们将这两张表映射到EF Core的实体对象上:

Person实体:

public partial class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int? Age { get; set; }
    public DateTime? CreateTime { get; set; }
    public DateTime? UpdateTime { get; set; }
}

 

Book实体:

public partial class Book
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string BookDescription { get; set; }
    public string Isbn { get; set; }
    public DateTime? CreateTime { get; set; }
}

 

EF Core的DB First生成的DbContext类EFDemoContext

public partial class EFDemoContext : DbContext
{
    public EFDemoContext()
    {
    }

    public EFDemoContext(DbContextOptions<EFDemoContext> options)
        : base(options)
    {
    }

    public virtual DbSet<Book> Book { get; set; }
    public virtual DbSet<Person> Person { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            optionsBuilder.UseSqlServer("Server=localhost;User Id=sa;Password=1qaz!QAZ;Database=EFDemo");
        }
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Book>(entity =>
        {
            entity.Property(e => e.Id).HasColumnName("ID");

            entity.Property(e => e.BookDescription).HasMaxLength(100);

            entity.Property(e => e.CreateTime)
                .HasColumnType("datetime")
                .HasDefaultValueSql("(getdate())");

            entity.Property(e => e.Isbn)
                .HasColumnName("ISBN")
                .HasMaxLength(50);

            entity.Property(e => e.Name).HasMaxLength(50);
        });

        modelBuilder.Entity<Person>(entity =>
        {
            entity.Property(e => e.Id).HasColumnName("ID");

            entity.Property(e => e.CreateTime)
                .HasColumnType("datetime")
                .HasDefaultValueSql("(getdate())");

            entity.Property(e => e.Name).HasMaxLength(50);

            entity.Property(e => e.UpdateTime).HasColumnType("datetime");
        });
    }
}

 

之后最关键的一点到了,我们需要用到DbContext.ChangeTracker.StateChanged这个事件,这个事件会在DbContext中被Track的实体对象的EntityState状态发生变化时被触发,有多少个实体的EntityState状态变化了,它就会被触发多少次。

为此,我们需要再定义一个自定义的DbContext类EFDbContext,来继承DB First自动生成的EFDemoContext类:

//EFDbContext继承自EFDemoContext,EFDemoContext又继承自DbContext
public class EFDbContext: EFDemoContext
{
    public EFDbContext()
    {
        //设置数据库Command永不超时
        this.Database.SetCommandTimeout(0);

        //DbContext.ChangeTracker.StateChanged事件,会在DbContext中被Track的实体其EntityState状态值发生变化时被触发
        this.ChangeTracker.StateChanged += (sender, entityStateChangedEventArgs) =>
        {
            //如果实体状态变为了EntityState.Modified,那么就尝试设置其UpdateTime属性为当前系统时间DateTime.Now,如果实体没有UpdateTime属性,会抛出InvalidOperationException异常,所以下面要用try catch来捕获异常避免系统报错
            if (entityStateChangedEventArgs.NewState == EntityState.Modified)
            {
                try
                {
                    //如果是Person表的实体那么下面的Entry.Property("UpdateTime")就不会抛出异常
                    entityStateChangedEventArgs.Entry.Property("UpdateTime").CurrentValue = DateTime.Now;
                }
                catch(InvalidOperationException)
                {
                    //如果上面try中抛出InvalidOperationException,就是实体没有属性UpdateTime,应该是表Book的实体
                }
            }

            //如果要自动更新多列,比如还要自动更新实体的UpdateUser属性值到数据库,可以像下面这样再加一个try catch来更新UpdateUser属性
            //if (entityStateChangedEventArgs.NewState == EntityState.Modified)
            //{
            //    try
            //    {
            //        entityStateChangedEventArgs.Entry.Property("UpdateUser").CurrentValue = currentUser;
            //    }
            //    catch (InvalidOperationException)
            //    {
            //    }
            //}
        };
    }
        
}

 

然后我们在Program.cs的Main方法中(我在本例建立的是一个.Net Core控制台程序)先初始化Person表和Book表的数据,然后再修改Person表和Book表的数据,看看被修改的Person表数据其列UpdateTime的值是否设置为了系统当前时间:

class Program
{
    //初始化Person表和Book表的数据
    static void InitializeDataToDB()
    {
        var personJim = new Person() { Name="Jim", Age=20 };
        var personTom= new Person() { Name = "Tom", Age = 30 };
        var personSam = new Person() { Name = "Sam", Age = 25 };
        var personJerry = new Person() { Name = "Jerry", Age = 35 };
        var personHenry = new Person() { Name = "Henry ", Age = 26 };

        var bookScience = new Book() { Name = "Science", BookDescription= "Science", Isbn="0001" };
        var bookMath = new Book() { Name = "Math", BookDescription = "Math", Isbn = "0002" };
        var bookPhysics = new Book() { Name = "Physics", BookDescription = "Physics", Isbn = "0003" };
        var bookComputer = new Book() { Name = "Computer", BookDescription = "Computer", Isbn = "0004" };
        var bookEnglish = new Book() { Name = "English", BookDescription = "English", Isbn = "0005" };

        using (var efDbContext = new EFDbContext())
        {
            efDbContext.Person.Add(personJim);
            efDbContext.Person.Add(personTom);
            efDbContext.Person.Add(personSam);
            efDbContext.Person.Add(personJerry);
            efDbContext.Person.Add(personHenry);

            efDbContext.Book.Add(bookScience);
            efDbContext.Book.Add(bookMath);
            efDbContext.Book.Add(bookPhysics);
            efDbContext.Book.Add(bookComputer);
            efDbContext.Book.Add(bookEnglish);

            efDbContext.SaveChanges();
        }
    }

    static void Main(string[] args)
    {
        Console.WriteLine("Testing start!");

        //初始化Person表和Book表的数据
        InitializeDataToDB();

        //修改Person表和Book表的数据
        using (var efDbContext = new EFDbContext())
        {
            //更改Person.Name为Tom的实体的Age属性值,这会导致personTom这个Person实体的EntityState变为Modified
            Expression<Func<Person, bool>> expressionTom = p => p.Name == "Tom";
            var personTom = efDbContext.Person.First(expressionTom);
            personTom.Age = 50;


            //更改Book.Name为Computer的实体的Isbn属性值,这会导致bookComputer这个Book实体的EntityState变为Modified
            Expression<Func<Book, bool>> expressionComputer = b => b.Name == "Computer";
            var bookComputer = efDbContext.Book.First(expressionComputer);
            bookComputer.Isbn = "1000";

            //由于上面DbContext中有两个实体的EntityState改变了,下面的SaveChanges方法会触发两次DbContext.ChangeTracker.StateChanged事件,在实体数据保存到数据库之前,自动更新personTom这个Person实体的UpdateTime属性值为系统当前时间
            efDbContext.SaveChanges();
        }

        Console.WriteLine("Testing end!");
        Console.ReadLine();
    }
}

 

当执行完InitializeDataToDB方法后,数据库两张表的值:

Person表:

Book表:

 

当Program.cs的Main方法运行完毕后,数据库两张表的值:

Person表:

Book表:

 

我们可以看到Person表中列Name为Tom的行,其UpdateTime也被自动更新为了系统当前时间。