【ASP.NET Core學習】Entity Framework Core

【ASP.NET Core學習】在ASP.NET Core 種使用Entity Framework Core介紹，包括如何添加Entity Framwork Core，創建模型和遷移到資料庫，查詢數據，保存數據，使用事務，處理併發衝突 ...

這裡介紹在ASP.NET Core中使用EF Core，這裡資料庫選的是Sql Server

如何使用Sql Server
添加模型 && 資料庫遷移
查詢數據
保存數據

如何使用Sql Server

1. 安裝dotnet-ef（已經安裝忽略）

dotnet tool install --global dotnet-ef

2. 添加包Microsoft.EntityFrameworkCore.Design

dotnet add package Microsoft.EntityFrameworkCore.Design

3. 添加包Microsoft.EntityFrameworkCore.SqlServer

dotnet add package Microsoft.EntityFrameworkCore.SqlServer

4. 添加DbContext

public class EFCoreDbContext : DbContext
                        {
                            public EFCoreDbContext(DbContextOptions<EFCoreDbContext> options)
                                : base(options)
                            {
                        
                            }
                        }

View Code

5.在ConfigureServices註入DbContext

public void ConfigureServices(IServiceCollection services)
                        {
                            services.AddRazorPages();
                        
                            services.AddDbContext<Data.EFCoreDbContext>(options =>
                                options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
                        }

View Code

經過上面5步，我們就可以在項目中使用資料庫，在需要的地方註入DbContext即可

添加模型

我們就以學校 -> 學生這樣的模型（一對多）為例，欄位也儘量簡潔，這裡不是展示設計，以展示操作EF Core為主，所以類定義未必是最合適的。學校類

[Table("School")]
                    public class School
                    {
                        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
                        public int Id { get; set; }
                    
                        [Display(Name = "學校名稱")]
                        [Required(ErrorMessage = "學校名稱不能為空")]
                        [StringLength(100, ErrorMessage = "學校名稱最大長度為100")]
                        public string Name { get; set; }
                    
                        [Display(Name = "學校地址")]
                        [Required(ErrorMessage = "學校地址不能為空")]
                        [StringLength(200, ErrorMessage = "學校地址最大長度為200")]
                        public string Address { get; set; }
                    
                        public List<Student> Students { get; set; }
                    
                        [Display(Name = "創建時間")]
                        [DataType(DataType.DateTime), DisplayFormat(DataFormatString = "{0:yyyy-MM-dd HH:mm:ss}")]
                        public DateTime CreateTime { get; set; }
                    
                        [Display(Name = "最後更新時間")]
                        [DataType(DataType.DateTime), DisplayFormat(DataFormatString = "{0:yyyy-MM-dd HH:mm:ss}")]
                        public DateTime? LastUpdateTime { get; set; }
                    
                    }

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學生類

public class Student
                    {
                        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
                        public int Id { get; set; }
                    
                        [Display(Name = "學生姓名")]
                        [Required(ErrorMessage = "學生姓名不能為空")]
                        [StringLength(50, ErrorMessage = "學生姓名最大長度為50")]
                        public string Name { get; set; }
                    
                        [Display(Name = "年齡")]
                        [Required(ErrorMessage = "年齡不能為空")]
                        [Range(minimum: 10, maximum: 100, ErrorMessage = "學生年齡必須在（10 ~ 100）之間")]
                        public int Age { get; set; }
                    
                        public School School { get; set; }
                    
                        [Display(Name = "創建時間")]
                        [DataType(DataType.DateTime), DisplayFormat(DataFormatString = "{0:yyyy-MM-dd HH:mm:ss}")]
                        public DateTime CreateTime { get; set; }
                    
                        [Display(Name = "最後更新時間")]
                        [DataType(DataType.DateTime), DisplayFormat(DataFormatString = "{0:yyyy-MM-dd HH:mm:ss}")]
                        public DateTime? LastUpdateTime { get; set; }
                    }

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配置預設值

protected override void OnModelCreating(ModelBuilder modelBuilder)
                    {
                        modelBuilder.Entity<Models.School>()
                                    .Property(p => p.CreateTime)
                                    .HasDefaultValueSql("getdate()");
                    
                        modelBuilder.Entity<Models.Student>()
                                    .Property(p => p.CreateTime)
                                    .HasDefaultValueSql("getdate()");
                    }

View Code

模型定義好之後，我們需要把模型添加到DbContext

public DbSet<Models.School> Schools{ get; set; }
                    public DbSet<Models.Student> Students { get; set; }

然後需要更新模型到資料庫，執行下麵兩條命令 1. 新增一個遷移

dotnet ef migrations add DatabaseInit

2. 更新到資料庫

dotnet ef migrations add DatabaseInit

查看資料庫，我們可以看到下麵關係圖

在Student表裡面多了一個SchoolId，這個我們是沒有定義，是EF Core生成的陰影屬性，當然我們也可以顯示定義這個欄位

實體類定義我們用到數據註釋和Fluent API約束實體類生成，下麵列取經常用到的

註釋	用途
Key	主鍵
Required	必須
MaxLength	最大長度
NotMapped	不映射到資料庫
ConcurrencyCheck	併發檢查
Timestamp	時間戳欄位

查詢數據

一、聯接查詢

var query = from a in _context.School
                        join b in _context.Student on a.Id equals b.School.Id
                        select new
                        {
                            SchoolName = a.Name,
                            StudentName = b.Name
                        };

View Code

對應生成的Sql

SELECT [s].[Name] AS [SchoolName], [t].[Name] AS [StudentName]
                  FROM [School] AS [s]
                  INNER JOIN (
                      SELECT [s0].[Id], [s0].[Age], [s0].[CreateTime], [s0].[LastUpdateTime], [s0].[Name], [s0].[SchoolId], [s1].[Id] AS [Id0], [s1].[Address], [s1].[CreateTime] AS [CreateTime0], [s1].[LastUpdateTime] AS [LastUpdateTime0], [s1].[Name] AS [Name0]
                      FROM [Student] AS [s0]
                      LEFT JOIN [School] AS [s1] ON [s0].[SchoolId] = [s1].[Id]
                  ) AS [t] ON [s].[Id] = [t].[Id0]

和我們預期有點不一致，預期是兩個表的全連接，為什麼出現這個，原因是Student裡面的導航屬性School，Linq遇到導航屬性是通過連表得到，為了驗證這個，我們不使用陰影屬性，顯示加上SchoolId試試

var query = from a in _context.School
                        join b in _context.Student on a.Id equals b.SchoolId
                        select new
                        {
                            SchoolName = a.Name,
                            StudentName = b.Name
                        };

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對應生成的Sql

SELECT [s].[Name] AS [SchoolName], [s0].[Name] AS [StudentName]
            FROM [School] AS [s]
            INNER JOIN [Student] AS [s0] ON [s].[Id] = [s0].[SchoolId]

這次生成的Sql就很簡潔，跟預期一樣，所以如果使用聯接查詢，最好是避免使用陰影屬性

兩個Sql的執行計劃

二、GroupBy查詢

var query = from a in _context.School
                    join b in _context.Student on a.Id equals b.SchoolId
                    group a by a.Name into t
                    where t.Count() > 0
                    orderby t.Key
                    select new
                    {
                        t.Key,
                        Count = t.Count(),
                    };

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對應生成的Sql

SELECT [s].[Name] AS [Key], COUNT(*) AS [Count]
        FROM [School] AS [s]
        INNER JOIN [Student] AS [s0] ON [s].[Id] = [s0].[SchoolId]
        GROUP BY [s].[Name]
        HAVING COUNT(*) > 0
        ORDER BY [s].[Name]

EF Core 支持的聚合運算符如下所示

平均值
計數
LongCount
最大值
最小值
Sum

三、左連接

var query = from a in _context.School
                    join b in _context.Student on a.Id equals b.SchoolId into t1
                    from t in t1.DefaultIfEmpty()
                    select new
                    {
                        SchoolName = a.Name,
                        StudentName = t.Name
                    };
        
        var list = query.AsNoTracking().ToList();

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對應生成的Sql

SELECT [s].[Name] AS [SchoolName], [s0].[Name] AS [StudentName]
        FROM [School] AS [s]
        LEFT JOIN [Student] AS [s0] ON [s].[Id] = [s0].[SchoolId]

四、小結

全聯接時避免使用導航屬性連表

預設情況是跟蹤查詢，這表示可以更改這些實體實例，然後通過 SaveChanges() 持久化這些更改，

如果只需要讀取，不需要修改可以指定非跟蹤查詢AsNoTracking

非跟蹤查詢可以在每個查詢後面指定，還可以在上下文實例級別更改預設跟蹤行為

context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;

保存數據

一、關聯數據

_context.School.Add(new Models.School
            {
                Name = "暨南大學",
                Address = "廣州市黃埔大道西601號",
                Students = new System.Collections.Generic.List<Models.Student>()
                {
                    new Models.Student
                    {
                        Name= "黃偉",
                        Age = 21,
                    },
                },
            });
    
    _context.SaveChanges();

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同時在School，Student表保存數據，自動維護Student表的SchoolId欄位數據

二、級聯刪除

var school = _context.School.Include(m => m.Students).FirstOrDefault(m => m.Name == "濟南大學");
    
    _context.School.Remove(school);
    
    _context.SaveChanges();

View Code 對應生成的Sql

--1. 讀取濟南大學和他所有學生
    SELECT [t].[Id], [t].[Address], [t].[CreateTime], [t].[LastUpdateTime], [t].[Name], [s0].[Id], [s0].[Age], [s0].[CreateTime], [s0].[LastUpdateTime], [s0].[Name], [s0].[SchoolId]
    FROM (
        SELECT TOP(1) [s].[Id], [s].[Address], [s].[CreateTime], [s].[LastUpdateTime], [s].[Name]
        FROM [School] AS [s]
        WHERE [s].[Name] = N'濟南大學'
    ) AS [t]
    LEFT JOIN [Student] AS [s0] ON [t].[Id] = [s0].[SchoolId]
    ORDER BY [t].[Id], [s0].[Id]
    
    --2. 迴圈每個學生刪除
    SET NOCOUNT ON;
    DELETE FROM [Student]
    WHERE [Id] = @p0;
    SELECT @@ROWCOUNT;
    SET NOCOUNT ON;
    DELETE FROM [Student]
    WHERE [Id] = @p0;
    SELECT @@ROWCOUNT;
    SET NOCOUNT ON;
    DELETE FROM [Student]
    WHERE [Id] = @p0;
    SELECT @@ROWCOUNT;
    
    --3. 刪除學校
    SET NOCOUNT ON;
    DELETE FROM [School]
    WHERE [Id] = @p1;
    SELECT @@ROWCOUNT;

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級聯刪除要用Include把子項也包含到實體

三、使用事務

預設情況下，如果資料庫提供程式支持事務，則會在事務中應用對 SaveChanges() 的單一調用中的所有更改。如果其中有任何更改失敗，則會回滾事務且所有更改都不會應用到資料庫。這意味著，SaveChanges() 可保證完全成功，或在出現錯誤時不修改資料庫。

對於大多數應用程式，此預設行為已足夠。如果應用程式要求被視為有必要，則應該僅手動控制事務中間調用多次SaveChanges()也不會直接保存到資料庫，最後transaction.Commit()

using (var transaction = _context.Database.BeginTransaction())
    {
        var school = _context.School.Add(new Models.School
        {
            Name = "濟南大學",
            Address = "山東省濟南市南辛莊西路336號",
        });
        _context.SaveChanges();
    
        System.Threading.Thread.Sleep(2000);  //for testing
        _context.Student.Add(new Models.Student
        {
            Name = "張三",
            Age = 29,
            School = school.Entity
        });
        _context.SaveChanges();
    
        transaction.Commit();
    }

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下麵是Sql Server Profiler

註意兩次RPC:Completed時間，每次調用SaveChanges提交到資料庫執行，外麵包一層事務，所以事務裡面要儘可能的控制操作最少，時間最少

四、併發衝突

EF Core實現的是樂觀併發，有關樂觀併發和悲觀併發這裡就不展開。

EF處理併發分兩種情況，單個屬性併發檢查和時間戳（又叫行版本），單個屬性只保證單個欄位併發修改，時間戳是保證整條數據的併發修改

我們在Student的Age加上[ConcurrencyCheck]，在School加上行版本

[ConcurrencyCheck]
public int Age { get; set; }

[Timestamp]
public byte[] RowVersion { get; set; }

1. 模擬Age併發衝突

var student = _context.Student.Single(m => m.Id == 1);
student.Age = 32;

#region 模擬另外一個用戶修改了Age

var task = Task.Run(() =>
{
    var options = HttpContext.RequestServices.GetService<DbContextOptions<Data.EFCoreDbContext>>();
    using (var context = new Data.EFCoreDbContext(options))
    {
        var student = context.Student.Single(m => m.Id == 1);
        student.Age = 23;
        context.SaveChanges();
    }
});
task.Wait();

#endregion

try
{
    _context.SaveChanges();
}
catch (DbUpdateConcurrencyException ex)
{
    _logger.LogError(ex, "database update error");
}

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2. 資料庫數據

可以看到是Task裡面的更新成功了

3. 異常信息

Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded

異常信息描述很明確,就是資料庫操作期望1行被影響,實際是0行,數據可能被修改或刪除自從實體載入後

4. SQL

exec sp_executesql N'SET NOCOUNT ON;
UPDATE [Student] SET [Age] = @p0
WHERE [Id] = @p1 AND [Age] = @p2;
SELECT @@ROWCOUNT;

',N'@p1 int,@p0 int,@p2 int',@p1=1,@p0=23,@p2=25

exec sp_executesql N'SET NOCOUNT ON;
UPDATE [Student] SET [Age] = @p0
WHERE [Id] = @p1 AND [Age] = @p2;
SELECT @@ROWCOUNT;

',N'@p1 int,@p0 int,@p2 int',@p1=1,@p0=32,@p2=25

View Code

加上併發檢查的欄位會在where條件後面加上原始值，Timestamp也是一樣道理，只是Timestamp是每次(插入/更新)資料庫會更新這個欄位,數字遞增的形式。

5. 解決併發衝突

要解決上面衝突，先要介紹EF Core裡面三組數值

原始值：實體從資料庫載入時的值　 (例子：Age = 25)

當前值：實體當前的值　　　　　　 (例子：Age = 32)

資料庫值：當前資料庫中的值　　　 (例子：Age = 23)