【ASP.NET Core學習】Entity Framework Core

時間 2019-11-11

標籤 ASP.NET Core學習 entity framework core 欄目 ASP 简体版

原文原文鏈接

這裏介紹在ASP.NET Core中使用EF Core，這裏數據庫選的是Sql Serverhtml

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

如何使用Sql Servergit

1. 安裝dotnet-ef（已經安裝忽略）

dotnet tool install --global dotnet-ef

2. 添加包Microsoft.EntityFrameworkCore.Designgithub

dotnet add package Microsoft.EntityFrameworkCore.Design

3. 添加包Microsoft.EntityFrameworkCore.SqlServersql

dotnet add package Microsoft.EntityFrameworkCore.SqlServer

4. 添加DbContext數據庫

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

View Code

5.在ConfigureServices注入DbContext併發

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

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通過上面5步，咱們就能夠在項目中使用數據庫，在須要的地方注入DbContext便可app

添加模型ide

咱們就以學校 -> 學生這樣的模型（一對多）爲例，字段也儘可能簡潔，這裏不是展現設計，以展現操做EF Core爲主，因此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 = "學校名稱不能爲空")]
                        [StringLength(100, ErrorMessage = "學校名稱最大長度爲100")]
                        public string Name { get; set; }
                    
                        [Display(Name = "學校地址")]
                        [Required(ErrorMessage = "學校地址不能爲空")]
                        [StringLength(200, ErrorMessage = "學校地址最大長度爲200")]
                        public string Address { get; set; }
                    
                        public List<Student> Students { get; set; }
                    
                        [Display(Name = "建立時間")]
                        [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ui

public class Student
                    {
                        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
                        public int Id { get; set; }
                    
                        [Display(Name = "學生姓名")]
                        [Required(ErrorMessage = "學生姓名不能爲空")]
                        [StringLength(50, ErrorMessage = "學生姓名最大長度爲50")]
                        public string Name { get; set; }
                    
                        [Display(Name = "年齡")]
                        [Required(ErrorMessage = "年齡不能爲空")]
                        [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 = "建立時間")]
                        [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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配置默認值spa

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

模型定義好以後，咱們須要把模型添加到DbContext

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

而後須要更新模型到數據庫，執行下面兩條命令

1. 新增一個遷移

dotnet ef migrations add DatabaseInit

2. 更新到數據庫

dotnet ef migrations add DatabaseInit

查看數據庫，咱們能夠看到下面關係圖

在Student表裏面多了一個SchoolId，這個咱們是沒有定義，是EF Core生成的陰影屬性，固然咱們也能夠顯示定義這個字段

實體類定義咱們用到數據註釋和Fluent API約束實體類生成，下面列取常常用到的

註釋	用途
Key	主鍵
Required	必須
MaxLength	最大長度
NotMapped	不映射到數據庫
ConcurrencyCheck	併發檢查
Timestamp	時間戳字段

查詢數據

1、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]

和咱們預期有點不一致，預期是兩個表的全鏈接，爲何出現這個，緣由是Student裏面的導航屬性School，Linq遇到導航屬性是經過連表獲得，爲了驗證這個，咱們不使用陰影屬性，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]

此次生成的Sql就很簡潔，跟預期同樣，因此若是使用聯接查詢，最好是避免使用陰影屬性

兩個Sql的執行計劃

2、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 支持的聚合運算符以下所示

平均值
計數
LongCount
最大值
最小值
Sum

3、左鏈接

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]

4、小結

全聯接時避免使用導航屬性連表

默認狀況是跟蹤查詢，這表示能夠更改這些實體實例，而後經過 SaveChanges() 持久化這些更改，

若是隻須要讀取，不須要修改能夠指定非跟蹤查詢AsNoTracking

非跟蹤查詢能夠在每一個查詢後面指定，還能夠在上下文實例級別更改默認跟蹤行爲

context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;

保存數據

1、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字段數據

2、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();

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對應生成的Sql

--1. 讀取濟南大學和他全部學生
    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. 循環每一個學生刪除
    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

3、使用事務

默認狀況下，若是數據庫提供程序支持事務，則會在事務中應用對 SaveChanges() 的單一調用中的全部更改。若是其中有任何更改失敗，則會回滾事務且全部更改都不會應用到數據庫。這意味着，SaveChanges() 可保證徹底成功，或在出現錯誤時不修改數據庫。

對於大多數應用程序，此默認行爲已足夠。若是應用程序要求被視爲有必要，則應該僅手動控制事務中間調用屢次SaveChanges()也不會直接保存到數據庫，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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下面是Sql Server Profiler

注意兩次RPC:Completed時間，每次調用SaveChanges提交到數據庫執行，外面包一層事務，因此事務裏面要儘量的控制操做最少，時間最少

4、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裏就不展開。

EF處理併發分兩種狀況，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字段併發修改，時間戳是保證整條數據的併發修改

咱們在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. 數據庫數據

能夠看到是Task裏面的更新成功了

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,就是數據庫操做指望1行被影響,實際是0行,數據可能被修改或刪除自從實體加載後

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

加上併發檢查的字段會在where條件後面加上原始值，Timestamp也是同樣道理，只是Timestamp是每次(插入/更新)數據庫會更新這個字段,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裏面三組數值

原始值：實體從數據庫加載時的值　 (例子：Age = 25)

當前值：實體當前的值　　　　　　 (例子：Age = 32)

數據庫值：當前數據庫中的值　　　 (例子：Age = 23)

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

#region 模擬另一個用戶修改了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

var (trySave, isSave) = (0, false);

while (!isSave && trySave++ < 3)
{
    try
    {
        _context.SaveChanges();
    }
    catch (DbUpdateConcurrencyException ex)
    {
        _logger.LogError(ex, "database update error");

        foreach (var entry in ex.Entries)
        {
            if (entry.Entity is Models.Student)
            {
                var currentValues = entry.CurrentValues;
                var databaseValues = entry.GetDatabaseValues();

                foreach (var property in currentValues.Properties)
                {
                    var currentValue = currentValues[property];
                    var databaseValue = databaseValues[property];

                    //這裏選擇保存哪一個值，這裏簡單選擇當前（30）保存到數據庫，實際可能還需處理，如餘額，就須要數據庫當前餘額 - 當前數值
                    currentValues[property] = currentValue;
                }
                // 刷新原始值
                entry.OriginalValues.SetValues(databaseValues);
            }
        }
    }
}