c# - 更新 Entity Framework Core 中的多对多关系

Question

我正在尝试使用 Entity Framework Core 更新 ASP.NET Core MVC Controller 中的多对多关系。我设法让它工作以添加到关系，但不更新(如果我只是打开/保存实体，则会导致重复的键错误)。

如何在有效地更新/插入新关系之前从数据库中删除关系？

public async Task<IActionResult> Edit(int id, [Bind("Id,Name,SalesClerkIds")] Plant plant)
{
        if (id != plant.Id)
        {
            return NotFound();
        }

        if (ModelState.IsValid)
        {
            try
            {
                plant.SalesClerks = new List<PlantSalesClerk>();

                if (plant.SalesClerkIds != null)
                {
                    foreach (var scId in plant.SalesClerkIds)
                    {
                        plant.SalesClerks.Add(new PlantSalesClerk()
                        {
                            Plant = plant,
                            User = _context.Users.FirstOrDefault(u => u.Id == scId)
                        });
                    }
                }

                _context.Update(plant);

                await _context.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!PlantExists(plant.Id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }
            return RedirectToAction(nameof(Index));
        }

        return View(plant);
  }

Answer 1

编写您的 Edit post 方法，如下所示:

public async Task<IActionResult> Edit(int id, [Bind("Id,Name,SalesClerkIds")] Plant plant)
{
    if (id != plant.Id)
    {
        return NotFound();
    }

    if (ModelState.IsValid)
    {
        try
        {
            Plant plantToBeUpdated = await _context.Plants.Include(p => p.SalesClerks).FirstOrDefaultAsync(p => p.Id == id);

            if (plantToBeUpdated != null)
            {
                 plantToBeUpdated.SalesClerks.Clear(); // Here you have to clear the existing children before adding the new

                 if (plant.SalesClerkIds.Count > 0)
                 {
                      foreach (var scId in plant.SalesClerkIds)
                      {
                         plantToBeUpdated.SalesClerks.Add(new PlantSalesClerk()
                         {
                            PlantId = plantToBeUpdated.Id,
                            UserId = scId
                         });
                     }
                 }

                 plantToBeUpdated.Name = plant.Name;

                // Map other properties here if any

                _context.Plants.Update(plantToBeUpdated);

                await _context.SaveChangesAsync();
           }

        }
        catch (DbUpdateConcurrencyException)
        {
            if (!PlantExists(plant.Id))
            {
                return NotFound();
            }
            else
            {
                throw;
            }
        }
        return RedirectToAction(nameof(Index));
    }

    return View(plant);
}

注意:我没有看到您的模型类和编辑 View 。我已经根据您的代码假设了一切。因此可能需要进行一些调整，但这就是 EF core 中使用子级更新模型的概念。