EF Core插入含嵌套子实体的ProjectRequest时Translation的ProjectRequestId为0的原因

实体定义 #

public class Entity
{
    public int Id { get; set; }
    public DateTime CreatedOn { get; set; }
    public DateTime? ModifiedOn { get; set; }
}

public class EntityBase : Entity
{
    [ForeignKey("CreatedBy")]
    public int CreatedById { get; set; }
    public User CreatedBy { get; set; }
    [ForeignKey("ModifiedBy")]
    public int? ModifiedById { get; set; }
    public User ModifiedBy { get; set; }
}

public class ProjectRequest : EntityBase
{
    public string RequestTitle { get; set; }
    public string RequestType { get; set; }
    ...
   
    public virtual ICollection<Material> Materials { get; set; }
    public virtual ICollection<Translation> Translations { get; set; }
}

public class Material : EntityBase
{
    [ForeignKey("ProjectRequest")]
    public int ProjectRequestId { get; set; }
    public virtual ProjectRequest ProjectRequest { get; set; }
    ...
    public virtual ICollection<Translation> Translations { get; set; }
}

public class Translation:EntityBase
{
    [ForeignKey("ProjectRequest")]
    public int ProjectRequestId { get; set; }
    public virtual ProjectRequest ProjectRequest { get; set; }
    [ForeignKey("Material")]
    public int MaterialId { get; set; }
    public virtual Material Material { get; set; }
    public string ProductMasterText { get; set; }
    [MaxLength(40)]
    public string ShortDescription { get; set; }
    public string MasterDescriptionLine1 { get; set; }
    public string MasterDescriptionLine2 { get; set; }
    public string MasterDescriptionLine3 { get; set; }
    public string LanguageCode { get; set; }
}

问题现象 #

未通过Fluent API修改实体配置，插入包含嵌套Materials和Translations子实体的ProjectRequest对象时，Translation的ProjectRequestId被设置为0，但MaterialId能正确赋值。

原因分析 #

1. EF的自动外键赋值仅针对直接父级导航属性：Translation是通过Material.Translations集合添加的，属于Material的子实体，EF只会自动识别并填充MaterialId（直接关联的外键）。
2. Translation的ProjectRequestId是非可空int类型，默认值为0，由于没有显式设置Translation.ProjectRequest导航属性，EF不会自动向上追溯关联ProjectRequest，因此该字段保留默认值0。

解决方案 #

• 显式设置导航属性：在将Translation添加到Material.Translations时，同时设置translation.ProjectRequest = projectRequestInstance，EF会自动填充ProjectRequestId。
• 手动赋值外键：直接给translation.ProjectRequestId赋值为对应的ProjectRequest的Id（如果是新增的ProjectRequest，可利用EF的追踪特性，保存时自动关联，无需提前获取Id）。
• 可选调整字段可空性：如果业务允许，可将Translation.ProjectRequestId改为int?，避免默认值0的干扰，但建议优先通过导航属性关联的方式解决。

内容的提问来源于stack exchange，提问作者Shashank