将实体框架的多对多关系保存到 MySQL答案

【问题标题】：Saving Many-To-Many Relationship with Entity Framework to MySQL将实体框架的多对多关系保存到 MySQL
【发布时间】：2017-05-17 09:08:56
【问题描述】：

我有 BOM 和 Project 表，它们是多对多的。

以下是我的 BOM

代码

public class BOM
{
    public int Id { get; set; }

    [Required(ErrorMessageResourceType = typeof(Resource), ErrorMessageResourceName = "ThisFieldIsRequired")]
    [StringLength(OwnConstants.StringLengthShort)]
    public string BOMRevision { get; set; }

    [Required(ErrorMessageResourceType = typeof(Resource), ErrorMessageResourceName = "ThisFieldIsRequired")]
    [Index("BOM Code", 1, IsUnique = true)]
    [StringLength(OwnConstants.StringLengthShort)]
    public string BOMCode { get; set; }

    [Required(ErrorMessageResourceType = typeof(Resource), ErrorMessageResourceName = "ThisFieldIsRequired")]
    [StringLength(OwnConstants.StringLengthShort)]
    public string Title { get; set; }

    public virtual ICollection<Project> Projects { get; set; }

    public virtual ICollection<BOMDetail> BOMDetails { get; set; }
}

以下是我的 Project

代码

public class Project
{
    public int Id { get; set; }

    [Required(ErrorMessageResourceType = typeof(Resource), ErrorMessageResourceName = "ThisFieldIsRequired")]
    [Index("Project Code", 1, IsUnique = true)]
    [StringLength(OwnConstants.StringLengthShort)]
    public string ProjectCode { get; set; }

    [Required(ErrorMessageResourceType = typeof(Resource), ErrorMessageResourceName = "ThisFieldIsRequired")]
    [Index("Description", 1, IsUnique = true)]
    [StringLength(OwnConstants.StringLengthShort)]
    public string Description { get; set; }

    [Required(ErrorMessageResourceType = typeof(Resource), ErrorMessageResourceName = "ThisFieldIsRequired")]
    public int? CustomerId { get; set; }

    public virtual Customer Customer { get; set; }

    public virtual ICollection<BOM> BOMs { get; set; }
}

我正在尝试将新条目保存到 BOM。但是，当我尝试添加 Project 时，我得到了 null。下面是我要保存的代码。

    public override void Save()
    {
        using (var db = new ApplicationDbContext())
        {
            var currentID = (Entity as BOM).Id;
            var ExistingBOM = db.BOMs.SingleOrDefault(x => x.BOMCode == BOMCodeTextEdit.Text);
            if (ExistingBOM != null)
            {
                XtraMessageBox.Show("Record Exist", "Error", MessageBoxButtons.OK);
                return;
            }

            if (currentID == 0)
            {                  
                BOM boms = new BOM()
                {
                    BOMRevision = BOMRevisionTextEdit.Text,
                    BOMCode = BOMCodeTextEdit.Text,
                    Title = TitleTextEdit.Text,
                };

                Project proj = new Project();
                int Project_Id = Convert.ToInt32(ProjectsLookUpEdit.EditValue);
                proj = db.Projects.SingleOrDefault(x => x.Id == Project_Id);
                db.Projects.Attach(proj);
                boms.Projects.Add(proj); // <== getting null here
            }

            db.SaveChanges();
            XtraMessageBox.Show("Save Successfully", "Information", MessageBoxButtons.OK);
        }
    }

我错过了什么吗？

更新代码

int Project_Id = Convert.ToInt32(ProjectsLookUpEdit.EditValue);
var proj = db.Projects.Where(x => x.Id == Project_Id).ToList();
boms.Projects.Add(proj);
db.BOM.Add(boms);

【问题讨论】：

尝试boms.Projects=proj; 而不是boms.Projects.Add(proj);
@M.Wiśnicki 我试过了。我收到一个错误Severity Code Description Project File Line Suppression State Error CS0266 Cannot implicitly convert type 'ERP_System.Model.Project' to 'System.Collections.Generic.ICollection<ERP_System.Model.Project>'. An explicit conversion exists (are you missing a cast?)
如果你删除 db.Projects.Attach(proj); 并只使用 Add() 会怎样
@M.Wiśnicki 我得到了同样的空异常错误。
好的，您尝试再次添加存在Project，但您需要添加关系才能添加Bom。因此，使用var projs =db.Projects.Where(x => x.Id == Project_Id).ToList(); 获取项目并将其设置为Bom：boms.Projects=projs。如果您想再次添加项目并将 i 设置为 Bom 在使用 SingleOrDefault(); 后更改 Id

标签： c# mysql entity-framework ef-code-first devexpress

【解决方案1】：

您尝试在需要添加ICollection<T> 的位置添加单个记录，更改您的查询并获取列表而不是单个项目。

var projs =db.Projects.Where(x => x.Id == Project_Id).ToList();

【讨论】：