The efcore-demo-dotnetcore3.1's intro from weidongnian

在mac下使用vscode，基于dotnetcore3.1 webapp创建的项目,数据层使用efcore+sqlite

0 安装 dotnet sdk 3.1

1，安装efcore cli

dotnet tool intall --global dotnet-ef

2，创建目录和解决方案,创建 webapp

mkdir EFCoreStudy

dotnet new sln -n EFCore

dotnet new webapp -o ContosoUniversity --no-https

3，将项目加到解决方案里

dotnet sln add ContosoUniversity/ContosoUniversity.csproj

4，修改布局面 _layout.cshtml

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>@ViewData["Title"] - Contoso University</title>
    <link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.css" />
    <link rel="stylesheet" href="~/css/site.css" />
</head>
<body>
    <header>
        <nav class="navbar navbar-expand-sm navbar-toggleable-sm navbar-light bg-white border-bottom box-shadow mb-3">
            <div class="container">
                <a class="navbar-brand" asp-area="" asp-page="/Index">Contoso University</a>
                <button class="navbar-toggler" type="button" data-toggle="collapse" data-target=".navbar-collapse" aria-controls="navbarSupportedContent"
                        aria-expanded="false" aria-label="Toggle navigation">
                    <span class="navbar-toggler-icon"></span>
                </button>
                <div class="navbar-collapse collapse d-sm-inline-flex flex-sm-row-reverse">
                    <ul class="navbar-nav flex-grow-1">
                        <li class="nav-item">
                            <a class="nav-link text-dark" asp-area="" asp-page="/About">About</a>
                        </li>
                        <li class="nav-item">
                            <a class="nav-link text-dark" asp-area="" asp-page="/Students/Index">Students</a>
                        </li>
                        <li class="nav-item">
                            <a class="nav-link text-dark" asp-area="" asp-page="/Courses/Index">Courses</a>
                        </li>
                        <li class="nav-item">
                            <a class="nav-link text-dark" asp-area="" asp-page="/Instructors/Index">Instructors</a>
                        </li>
                        <li class="nav-item">
                            <a class="nav-link text-dark" asp-area="" asp-page="/Departments/Index">Departments</a>
                        </li>
                    </ul>
                </div>
            </div>
        </nav>
    </header>
    <div class="container">
        <main role="main" class="pb-3">
            @RenderBody()
        </main>
    </div>

    <footer class="border-top footer text-muted">
        <div class="container">
            &copy; 2019 - Contoso University - <a asp-area="" asp-page="/Privacy">Privacy</a>
        </div>
    </footer>

    <script src="~/lib/jquery/dist/jquery.js"></script>
    <script src="~/lib/bootstrap/dist/js/bootstrap.bundle.js"></script>
    <script src="~/js/site.js" asp-append-version="true"></script>

    @RenderSection("Scripts", required: false)
</body>
</html>

5，修改主页 index.cshtml

@page
@model IndexModel
@{
    ViewData["Title"] = "Home page";
}

<div class="row mb-auto">
    <div class="col-md-4">
        <div class="row no-gutters border mb-4">
            <div class="col p-4 mb-4 ">
                <p class="card-text">
                    Contoso University is a sample application that
                    demonstrates how to use Entity Framework Core in an
                    ASP.NET Core Razor Pages web app.
                </p>
            </div>
        </div>
    </div>
    <div class="col-md-4">
        <div class="row no-gutters border mb-4">
            <div class="col p-4 d-flex flex-column position-static">
                <p class="card-text mb-auto">
                    You can build the application by following the steps in a series of tutorials.
                </p>
                <p>
                    <a href="https://docs.microsoft.com/aspnet/core/data/ef-rp/intro" class="stretched-link">See the tutorial</a>
                </p>
            </div>
        </div>
    </div>
    <div class="col-md-4">
        <div class="row no-gutters border mb-4">
            <div class="col p-4 d-flex flex-column">
                <p class="card-text mb-auto">
                    You can download the completed project from GitHub.
                </p>
                <p>
                    <a href="https://github.com/dotnet/AspNetCore.Docs/tree/master/aspnetcore/data/ef-rp/intro/samples" class="stretched-link">See project source code</a>
                </p>
            </div>
        </div>
    </div>
</div>

6，运行

dotnet run

7，进入项目，创建Modles目录，编写实体类

cd ContosoUniversity

mkdir Models

//Student

using System;
using System.Collections.Generic;

namespace ContosoUniversity.Models
{
    public class Student
    {
        public int ID { get; set; }
        public string LastName { get; set; }
        public string FirstMidName { get; set; }
        public DateTime EnrollmentDate { get; set; }

        /// <summary>
        /// 导航属性，外键表，一对多
        /// </summary>
        /// <value></value>
        public ICollection<Enrollment> Enrollments { get; set; }
    }
}

namespace ContosoUniversity.Models
{
    public enum Grade
    {
        A, B, C, D, F
    }

    /// <summary>
    /// 报名表
    /// </summary>
    public class Enrollment
    {
        public int EnrollmentID { get; set; }
        public int CourseID { get; set; }
        public int StudentID { get; set; }
        public Grade? Grade { get; set; }

        public Course Course { get; set; }
        public Student Student { get; set; }
    }
}

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;

namespace ContosoUniversity.Models
{
    /// <summary>
    /// 课程表
    /// </summary>
    public class Course
    {
        /// <summary>
        /// 主键，DatabaseGenerated 特性指定主键，而无需靠数据库生成
        /// </summary>
        /// <value></value>
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int CourseID { get; set; }
        public string Title { get; set; }
        public int Credits { get; set; }

        public ICollection<Enrollment> Enrollments { get; set; }
    }
}

8，添加 nuget 包

dotnet add package Microsoft.EntityFrameworkCore.SQLite
dotnet add package Microsoft.EntityFrameworkCore.SqlServer
dotnet add package Microsoft.EntityFrameworkCore.Design
dotnet add package Microsoft.EntityFrameworkCore.Tools
dotnet add package Microsoft.VisualStudio.Web.CodeGeneration.Design
dotnet add package Microsoft.Extensions.Logging.Debug

9，运行以下命令安装 aspnet-codegenerator 基架工具

dotnet tool install --global dotnet-aspnet-codegenerator

10.1 在 Windows 上运行以下命令，搭建“学生”页的基架。

dotnet aspnet-codegenerator razorpage -m Student -dc ContosoUniversity.Data.SchoolContext -udl -outDir Pages/Students --referenceScriptLibraries

10.2 在 macOS 或 Linux 上运行以下命令，搭建“学生”页的基架。

dotnet aspnet-codegenerator razorpage -m Student -dc ContosoUniversity.Data.SchoolContext -udl -outDir Pages/Students --referenceScriptLibraries

11，如果对上述步骤有疑问，请生成项目并重试基架搭建步骤。基架流程：在“Pages/Students”文件夹中创建 Razor 页面： Create.cshtml 和 Create.cshtml.cs Delete.cshtml 和 Delete.cshtml.cs Details.cshtml 和 Details.cshtml.cs Edit.cshtml 和 Edit.cshtml.cs Index.cshtml 和 Index.cshtml.cs 创建 Data/SchoolContext.cs 。将上下文添加到 Startup.cs 中的依赖项注入。将数据库连接字符串添加到 appsettings.json 。

12，修改数据库链接 Data Source=CU.db

13，完善SchoolContext代码 using Microsoft.EntityFrameworkCore; using ContosoUniversity.Models;

namespace ContosoUniversity.Data
{
    public class SchoolContext : DbContext
    {
        public SchoolContext (DbContextOptions<SchoolContext> options)
            : base(options)
        {
        }

        /// <summary>
        /// 实体集通常对应数据库表,由于实体集包含多个实体，因此 DBSet 属性应为复数名称
        /// </summary>
        /// <value></value>
        public DbSet<Student> Students { get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }
        public DbSet<Course> Courses { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            //建立实体与表的对应
            modelBuilder.Entity<Course>().ToTable("Course");
            modelBuilder.Entity<Enrollment>().ToTable("Enrollment");
            modelBuilder.Entity<Student>().ToTable("Student");
        }
    }
}

14，修改Startup.cs文件，确保使用sqllite数据库

services.AddDbContext<SchoolContext> (options =>
options.UseSqlite (Configuration.GetConnectionString ("SchoolContext")));

15，如果没有数据库，请更新 Program.cs 以创建数据库如果有上下文的数据库，则 EnsureCreated 方法不执行任何操作。如果没有数据库，则它将创建数据库和架构。 EnsureCreated 启用以下工作流来处理数据模型更改：删除数据库。任何现有数据丢失。更改数据模型。例如，添加 EmailAddress 字段。运行应用。 EnsureCreated 创建具有新架构的数据库

    private static void CreateDbIfNotExists(IHost host)
    {
        using (var scope = host.Services.CreateScope())
        {
            var services = scope.ServiceProvider;

            try
            {
                var context = services.GetRequiredService<SchoolContext>();
                context.Database.EnsureCreated();
            }
            catch (Exception ex)
            {
                var logger = services.GetRequiredService<ILogger<Program>>();
                logger.LogError(ex, "An error occurred creating the DB.");
            }
        }
    }

16，运行并测试数据新增修改

dotnet run

17，设定数据库种子，创建 Data/DbInitializer.cs 停止运行，删除数据库，修改Program.cs

// context.Database.EnsureCreated();
DbInitializer.Initialize(context);

18，异步代码 Web 服务器的可用线程是有限的，而在高负载情况下的可能所有线程都被占用。当发生这种情况的时候，服务器就无法处理新请求，直到线程被释放。使用同步代码时，可能会出现多个线程被占用但不能执行任何操作的情况，因为它们正在等待 I/O 完成。使用异步代码时，当进程正在等待 I/O 完成，服务器可以将其线程释放用于处理其他请求。因此，使用异步代码可以更有效地利用服务器资源，并且服务器可以无延迟地处理更多流量。

but EF Core 上下文并非线程安全：请勿尝试并行执行多个操作!!

public async Task OnGetAsync()
{
    Students = await _context.Students.ToListAsync();
}

19，完善注册功能 Pages/Students/Details.cshtml.cs

//Student = await _context.Students.FirstOrDefaultAsync(m => m.ID == id);
//Include 和 ThenInclude 方法使上下文加载 Student.Enrollments 导航属性，
//并在每个注册中加载 Enrollment.Course 导航属性。

Student = await _context.Students
.Include (s => s.Enrollments)
.ThenInclude (e => e.Course)
.AsNoTracking ()//AsNoTracking 方法将会提升性能
.FirstOrDefaultAsync (m => m.ID == id);

20，修改Pages/Students/Details.cshtml页

    <dt class="col-sm-2">
        @Html.DisplayNameFor(model => model.Student.Enrollments)
    </dt>
    <dd class="col-sm-10">
        <table class="table">
            <tr>
                <th>Course Title</th>
                <th>Grade</th>
            </tr>
            @foreach (var item in Model.Student.Enrollments)
            {
                <tr>
                    <td>
                        @Html.DisplayFor(modelItem => item.Course.Title)
                    </td>
                    <td>
                        @Html.DisplayFor(modelItem => item.Grade)
                    </td>
                </tr>
            }
        </table>
    </dd>

21，更新“创建”页并 dotnet run

var emptyStudent = new Student();

if (await TryUpdateModelAsync<Student>(
    emptyStudent,
    "student",   // Prefix for form value.
    s => s.FirstMidName, s => s.LastName, s => s.EnrollmentDate))
{
    _context.Students.Add(emptyStudent);
    await _context.SaveChangesAsync();
    return RedirectToPage("./Index");
}

22，创建 Student 视图模型

public class StudentVM
{
    public int ID { get; set; }
    public string LastName { get; set; }
    public string FirstMidName { get; set; }
    public DateTime EnrollmentDate { get; set; }
}

23�，修改Student/Create.cshtml.cs

[BindProperty]
public StudentVM StudentVM { get; set; }

public async Task<IActionResult> OnPostAsync()
{
    if (!ModelState.IsValid)
    {
        return Page();
    }

    var entry = _context.Add(new Student());
    entry.CurrentValues.SetValues(StudentVM);
    await _context.SaveChangesAsync();
    return RedirectToPage("./Index");
}

24，更新Pages/Students/Edit.cshtml.cs页

public async Task<IActionResult> OnGetAsync(int? id)
{
    if (id == null)
    {
        return NotFound();
    }

    Student = await _context.Students.FindAsync(id);

    if (Student == null)
    {
        return NotFound();
    }
    return Page();
}

public async Task<IActionResult> OnPostAsync(int id)
{
    var studentToUpdate = await _context.Students.FindAsync(id);

    if (studentToUpdate == null)
    {
        return NotFound();
    }

    if (await TryUpdateModelAsync<Student>(
        studentToUpdate,
        "student",
        s => s.FirstMidName, s => s.LastName, s => s.EnrollmentDate))
    {
        await _context.SaveChangesAsync();
        return RedirectToPage("./Index");
    }

    return Page();
}

25，更新“删除”页 Pages/Students/Delete.cshtml.cs

using ContosoUniversity.Models;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.EntityFrameworkCore;
using System.Threading.Tasks;

namespace ContosoUniversity.Pages.Students
{
    public class DeleteModel : PageModel
    {
        private readonly ContosoUniversity.Data.SchoolContext _context;

        public DeleteModel(ContosoUniversity.Data.SchoolContext context)
        {
            _context = context;
        }

        [BindProperty]
        public Student Student { get; set; }
        public string ErrorMessage { get; set; }

        public async Task<IActionResult> OnGetAsync(int? id, bool? saveChangesError = false)
        {
            if (id == null)
            {
                return NotFound();
            }

            Student = await _context.Students
                .AsNoTracking()
                .FirstOrDefaultAsync(m => m.ID == id);

            if (Student == null)
            {
                return NotFound();
            }

            if (saveChangesError.GetValueOrDefault())
            {
                ErrorMessage = "Delete failed. Try again";
            }

            return Page();
        }

        public async Task<IActionResult> OnPostAsync(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var student = await _context.Students.FindAsync(id);

            if (student == null)
            {
                return NotFound();
            }

            try
            {
                _context.Students.Remove(student);
                await _context.SaveChangesAsync();
                return RedirectToPage("./Index");
            }
            catch (DbUpdateException /* ex */)
            {
                //Log the error (uncomment ex variable name and write a log.)
                return RedirectToAction("./Delete",
                                    new { id, saveChangesError = true });
            }
        }
    }
}

26，向“删除”Razor 页面添加错误消息 (Pages/Students/Delete.cshtml)

@Model.ErrorMessage

27，添加排序 Pages/Students/Index.cshtml.cs

    public string NameSort { get; set; }
    public string DateSort { get; set; }
    public string CurrentFilter { get; set; }
    public string CurrentSort { get; set; }

    public IList<Student> Students { get; set; }

    public async Task OnGetAsync(string sortOrder)
    {
        NameSort = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";
        DateSort = sortOrder == "Date" ? "date_desc" : "Date";

        IQueryable<Student> studentsIQ = from s in _context.Students
                                        select s;

        switch (sortOrder)
        {
            case "name_desc":
                studentsIQ = studentsIQ.OrderByDescending(s => s.LastName);
                break;
            case "Date":
                studentsIQ = studentsIQ.OrderBy(s => s.EnrollmentDate);
                break;
            case "date_desc":
                studentsIQ = studentsIQ.OrderByDescending(s => s.EnrollmentDate);
                break;
            default:
                studentsIQ = studentsIQ.OrderBy(s => s.LastName);
                break;
        }

        Students = await studentsIQ.AsNoTracking().ToListAsync();
    }

28，向“学生索引”页添加列标题超链接，使用以下代码替换 Students/Index.cshtml 中的代码。

@page
@model ContosoUniversity.Pages.Students.IndexModel

@{
    ViewData["Title"] = "Students";
}

<h2>Students</h2>
<p>
    <a asp-page="Create">Create New</a>
</p>

<table class="table">
    <thead>
        <tr>
            <th>
                <a asp-page="./Index" asp-route-sortOrder="@Model.NameSort">
                    @Html.DisplayNameFor(model => model.Students[0].LastName)
                </a>
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Students[0].FirstMidName)
            </th>
            <th>
                <a asp-page="./Index" asp-route-sortOrder="@Model.DateSort">
                    @Html.DisplayNameFor(model => model.Students[0].EnrollmentDate)
                </a>
            </th>
            <th></th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model.Students)
        {
            <tr>
                <td>
                    @Html.DisplayFor(modelItem => item.LastName)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.FirstMidName)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.EnrollmentDate)
                </td>
                <td>
                    <a asp-page="./Edit" asp-route-id="@item.ID">Edit</a> |
                    <a asp-page="./Details" asp-route-id="@item.ID">Details</a> |
                    <a asp-page="./Delete" asp-route-id="@item.ID">Delete</a>
                </td>
            </tr>
        }
    </tbody>
</table>

29，使用以下代码替换 Students/Index.cshtml.cs 中的代码，以添加筛选

using ContosoUniversity.Data;
using ContosoUniversity.Models;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace ContosoUniversity.Pages.Students
{
    public class IndexModel : PageModel
    {
        private readonly SchoolContext _context;

        public IndexModel(SchoolContext context)
        {
            _context = context;
        }

        public string NameSort { get; set; }
        public string DateSort { get; set; }
        public string CurrentFilter { get; set; }
        public string CurrentSort { get; set; }

        public IList<Student> Students { get; set; }

        public async Task OnGetAsync(string sortOrder, string searchString)
        {
            NameSort = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";
            DateSort = sortOrder == "Date" ? "date_desc" : "Date";

            CurrentFilter = searchString;
            
            IQueryable<Student> studentsIQ = from s in _context.Students
                                            select s;
            if (!String.IsNullOrEmpty(searchString))
            {
                studentsIQ = studentsIQ.Where(s => s.LastName.Contains(searchString)
                                    || s.FirstMidName.Contains(searchString));
            }

            switch (sortOrder)
            {
                case "name_desc":
                    studentsIQ = studentsIQ.OrderByDescending(s => s.LastName);
                    break;
                case "Date":
                    studentsIQ = studentsIQ.OrderBy(s => s.EnrollmentDate);
                    break;
                case "date_desc":
                    studentsIQ = studentsIQ.OrderByDescending(s => s.EnrollmentDate);
                    break;
                default:
                    studentsIQ = studentsIQ.OrderBy(s => s.LastName);
                    break;
            }

            Students = await studentsIQ.AsNoTracking().ToListAsync();
        }
    }
}

30，更新 Razor 页面，替换 Pages/Students/Index.cshtml 中的代码，以创建“搜索”按钮和各种 chrome

31，添加分页本部分将创建一个 PaginatedList 类来支持分页。 PaginatedList 类使用 Skip 和 Take 语句在服务器上筛选数据，而不是检索所有表格行

32，创建 PaginatedList 类

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;

namespace ContosoUniversity
{
    public class PaginatedList<T> : List<T>
    {
        public int PageIndex { get; private set; }
        public int TotalPages { get; private set; }

        public PaginatedList(List<T> items, int count, int pageIndex, int pageSize)
        {
            PageIndex = pageIndex;
            TotalPages = (int)Math.Ceiling(count / (double)pageSize);

            this.AddRange(items);
        }

        public bool HasPreviousPage
        {
            get
            {
                return (PageIndex > 1);
            }
        }

        public bool HasNextPage
        {
            get
            {
                return (PageIndex < TotalPages);
            }
        }

        public static async Task<PaginatedList<T>> CreateAsync(
            IQueryable<T> source, int pageIndex, int pageSize)
        {
            var count = await source.CountAsync();
            var items = await source.Skip(
                (pageIndex - 1) * pageSize)
                .Take(pageSize).ToListAsync();
            return new PaginatedList<T>(items, count, pageIndex, pageSize);
        }
    }
}

32，删除数据库,操作之前先按dotnet-ef工具，dotnet tool install --global dotnet-ef

dotnet ef database drop --force

32，创建初始数据迁移

dotnet ef migrations add IniticalCreate

dotnet ef database update

但在window的 visual studio 下请如下执行:

Add-Migrations IniticalCreate

Update-Database

33，新增更多实体，通过指定格式设置、验证和数据库映射规则来自定义数据模型

![Image 新增更多实体](https://docs.microsoft.com/zh-cn/aspnet/core/data/ef-rp/complex-data-model/_static/diagram.png?view=aspnetcore-3.1)

34，ef core 会针对不可为空的FK和多对多关系，启用级联删除,某一情况下要禁用级联删除

modelBuilder.Entity<Department>()
.HasOne(d => d.Administrator)
.WithMany()
.OnDelete(DeleteBehavior.Restrict)

35，多对多关系,一个学生会有多个注册课程信息

![Image 学生会与课程的关联](https://docs.microsoft.com/zh-cn/aspnet/core/data/ef-rp/complex-data-model/_static/student-course.png?view=aspnetcore-3.1)

![Image 实体关系图](https://docs.microsoft.com/zh-cn/aspnet/core/data/ef-rp/complex-data-model/_static/diagram.png?view=aspnetcore-3.1)

36，EF Core 可采用多种方式将相关数据加载到实体的导航属性中：预先加载、显式加载和延迟加载

https://docs.microsoft.com/zh-cn/aspnet/core/data/ef-rp/read-related-data?view=aspnetcore-3.1&tabs=visual-studio

37，搭建“课程”页的基架

dotnet aspnet-codegenerator razorpage -m Course -dc SchoolContext -udl -outDir Pages/Courses --referenceScriptLibraries

38，悲观并发和开放式并发(存储优先方案，sql server中表有一个数据类型rowversion)

    /// <summary>
    /// 并发字段
    /// </summary>
    /// <value></value>
    [Timestamp]
    public byte[] RowVersion { get; set; }

40，保存学院方法,并检查是否并发冲突

    public async Task<IActionResult> OnPostAsync(int id)
    {
        if (!ModelState.IsValid)
        {
            return Page();
        }

        var departmentToUpdate = await _context.Departments
            .Include(i => i.Administrator)
            .FirstOrDefaultAsync(m => m.DepartmentID == id);

        if (departmentToUpdate == null)
        {
            return HandleDeletedDepartment();
        }

        _context.Entry(departmentToUpdate)
            .Property("RowVersion").OriginalValue = Department.RowVersion;

        if (await TryUpdateModelAsync<Department>(
            departmentToUpdate,
            "Department",
            s => s.Name, s => s.StartDate, s => s.Budget, s => s.InstructorID))
        {
            try
            {
                await _context.SaveChangesAsync();
                return RedirectToPage("./Index");
            }
            catch (DbUpdateConcurrencyException ex)
            {
                var exceptionEntry = ex.Entries.Single();
                var clientValues = (Department)exceptionEntry.Entity;
                var databaseEntry = exceptionEntry.GetDatabaseValues();
                if (databaseEntry == null)
                {
                    ModelState.AddModelError(string.Empty, "Unable to save. " +
                        "The department was deleted by another user.");
                    return Page();
                }

                var dbValues = (Department)databaseEntry.ToObject();
                await setDbErrorMessage(dbValues, clientValues, _context);

                // Save the current RowVersion so next postback
                // matches unless an new concurrency issue happens.
                Department.RowVersion = (byte[])dbValues.RowVersion;
                // Clear the model error for the next postback.
                ModelState.Remove("Department.RowVersion");
            }
        }

        InstructorNameSL = new SelectList(_context.Instructors,
            "ID", "FullName", departmentToUpdate.InstructorID);

        return Page();
    }

    private async Task setDbErrorMessage(Department dbValues,
            Department clientValues, SchoolContext context)
    {

        if (dbValues.Name != clientValues.Name)
        {
            ModelState.AddModelError("Department.Name",
                $"Current value: {dbValues.Name}");
        }
        if (dbValues.Budget != clientValues.Budget)
        {
            ModelState.AddModelError("Department.Budget",
                $"Current value: {dbValues.Budget:c}");
        }
        if (dbValues.StartDate != clientValues.StartDate)
        {
            ModelState.AddModelError("Department.StartDate",
                $"Current value: {dbValues.StartDate:d}");
        }
        if (dbValues.InstructorID != clientValues.InstructorID)
        {
            Instructor dbInstructor = await _context.Instructors
               .FindAsync(dbValues.InstructorID);
            ModelState.AddModelError("Department.InstructorID",
                $"Current value: {dbInstructor?.FullName}");
        }

        ModelState.AddModelError(string.Empty,
            "The record you attempted to edit "
          + "was modified by another user after you. The "
          + "edit operation was canceled and the current values in the database "
          + "have been displayed. If you still want to edit this record, click "
          + "the Save button again.");
    }

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efcore-demo-dotnetcore3.1's Introduction

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