属性外键不能设置为null

Question

我有4个表

public class StokGroup:BaseModel
{
     public string GroupName { get; set; }
     public List<GroupFeature> GroupFeatures { get; set; }
     public List<StokCard> StokCards { get; set; }
}
public class GroupFeature : BaseModel
{
     public int StokGroupId { get; set; } 
     public StokGroup StokGroup { get; set; }
     public string FeatureName { get; set; }
     public List<StokCardAdditionalFeatureValue> StokCardAdditionalFeatureValues { get; set; }

}
public class StokCard : BaseModel
{
     public string IPMCode { get; set; }
     public int StokGroupId { get; set; }
     public StokGroup StokGroup { get; set; }
     .......................................
}
public class StokCardAdditionalFeatureValue:BaseModel
{
    public int StokCardId { get; set; }
    public StokCard StokCard { get; set; }
    public int GroupFeatureId { get; set; }
    public GroupFeature GroupFeature { get; set; }
    public string Value { get; set; }

}

，我的basemodel是具有主键ID的类，所有类都从基本模型继承

public class BaseModel
{
     [Key] 
      public int Id { get; set; }
        
}

，我也有：

modelBuilder.Entity<StokCard>()
.HasOne(o => o.StokGroup)
.WithMany(m => m.StokCards)
.OnDelete(DeleteBehavior.NoAction);

modelBuilder.Entity<GroupFeature>()
.HasOne(o => o.StokGroup)
.WithMany(m => m.GroupFeatures);

modelBuilder.Entity<StokCardAdditionalFeatureValue>()
.HasOne(o => o.StokCard)
.WithMany(m => m.StokCardAdditionalFeatureValues)
.OnDelete(DeleteBehavior.NoAction);

modelBuilder.Entity<StokCardAdditionalFeatureValue>()
.HasOne(o => o.GroupFeature)
.WithMany(m => m.StokCardAdditionalFeatureValues);

现在问题是在删除stokgroup时自动删除Stokcard，因为这种关系是因为这种关系 我该如何解决这个问题？

我将null设置为stokgroupid在Stokcards桌子上，但迁移失败了，

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提前致谢