让我解释一下，我有2个表格：燃料和车辆。1辆车可以有更多加油口，一辆加油车有1辆车。因此，这是正常的1-M关系。
但是，当我尝试插入新的Fuel并选择已经存在的Vehicle时，这是一个例外：
Duplicate entry '5' for key 'vehicle.PRIMARY'
就像当我尝试创建一个新的Fuel条目，而不是将Fuel条目链接到现有的Vehicle时，它尝试创建一个新的。实际上，如果我使用后 map，而不是使用现有车辆，而是放置新 map，它会同时创建“燃料”和“车辆”条目。
这是我实际使用的代码：Fuel类：
public class Fuel
{
[Key]
public int id { get; set; }
public DateTime date { get; set; }
public float cost { get; set; }
[ForeignKey("vehicleId")]
public virtual Vehicle vehicle { get; set; }
public int vehicleId { get; set; }
public float startKm { get; set; }
public float endKm { get; set; }
public float liter { get; set; }
public float average { get; set; }
}
车辆类别：
public class Vehicle
{
[Key]
public int id { get; set; }
[Required, MaxLength(7), MinLength(7)]
public string plate { get; set; }
[JsonIgnore, InverseProperty("vehicle")]
public ICollection<Fuel> Fuels { get; set; } = new HashSet<Fuel>();
}
DatabaseContext：
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<JourneyAddress>()
.HasKey(bc => new { bc.journeyId, bc.addressId });
modelBuilder.Entity<JourneyAddress>()
.HasOne(bc => bc.journey)
.WithMany(b => b.journeyAddress)
.HasForeignKey(bc => bc.journeyId)
.OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<JourneyAddress>()
.HasOne(bc => bc.address)
.WithMany(c => c.JourneyAddress)
.HasForeignKey(bc => bc.addressId)
.OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<Fuel>()
.HasOne(c => c.vehicle)
.WithMany(e => e.Fuels)
.HasForeignKey(c => c.vehicleId)
.OnDelete(DeleteBehavior.Cascade);
}
控制器：
public async Task<ActionResult<Fuel>> PostFuel(Fuel fuel)
{
_repo.Add(fuel);
var save = await _repo.SaveAsync(fuel);
return CreatedAtAction("GetFuel", new { id = fuel.id }, fuel);
}
我已经通过使用此功能而不是旧功能解决了该问题：
Controller.cs
public async Task<ActionResult<Fuel>> PostFuel(Fuel fuel)
{
var vehicle = _context.Vehicle.SingleOrDefault(t => t.id == fuel.vehicleId);
fuel.vehicle = vehicle;
_repo.Add(fuel);
var save = await _repo.SaveAsync(fuel);
return CreatedAtAction("GetFuel", new { id = fuel.id }, fuel);
}