如何从一个表中删除通过其 PK 链接到另一表的记录？

Question

我在毛伊岛应用程序中有一个数据库，有两个表。第一个表设置了这样的类：

public class Groups
{
    [PrimaryKey, AutoIncrement]
    public long Id { get; set; }
    public string? GrpName { get; set; }
}

另一个表是用此类创建的：

public class Sites
{
    [PrimaryKey, AutoIncrement]
    public long Id { get; set; }
    public string? Title { get; set; }
    public string? SiteAddr { get; set; }
    public long GrpId { get; set; }    // The Id number matching the Group Id
    public string? Notes { get; set; }
}

我想要做的是从我已经创建的数据网格中选择一个或多个组，并删除通过 GrpId 字段链接到组表的所有组和站点记录。

我在 LINQ 中进行了研究，认为我可以以与 MySql 类似的方式删除站点记录，但我就是无法理解它。有什么建议吗？

Answer 1

据我了解，如果 UI 操作从视图中删除了

Group

对象，则关系

Site

对象应该从数据库中永久删除（或者可能只是从视图中的可见性中删除）。

---

无论哪种方式，人们都可以在绑定上下文中定义此行为，在其视图中添加和删除关系

Site

对象，作为对

CollectionChanged.Add

集合触发的

CollectionChanged.Remove

Groups

事件的响应。

// <PackageReference Include="sqlite-net-pcl" Version="1.9.172" />
using SQLite;
class MainPageBinding : INotifyPropertyChanged
{
    public MainPageBinding()
    {
        TestCommand = new Command(OnTest);
        Groups.CollectionChanged += (sender, e) =>
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Remove:
                    if (e.OldItems != null)
                    {
                        foreach (Group group in e.OldItems)
                        {
                            InMemoryDatabase.Delete(group);
                            foreach(
                                var site 
                                in InMemoryDatabase.Table<Site>()
                                .Where(_=>_.GrpId == group.Id))
                            {
                                Sites.Remove(site);
                                InMemoryDatabase.Delete(site);
                            }
                        }
                    }
                    break;
                case NotifyCollectionChangedAction.Add:
                    if (e.NewItems != null)
                    {
                        foreach (Group group in e.NewItems)
                        {
                            foreach (
                                Site site in
                                InMemoryDatabase.Table<Site>()
                                .Where(_ => _.GrpId == group.Id))
                            {
                                Sites.Add(site);
                            }
                        }
                    }
                    break;
            }
        };
        // Query the database to populate the CollectionView controls on MainView
        foreach (Group group in InMemoryDatabase.Table<Group>()) Groups.Add(group);
    }

    #region T E S T I N G
    // For testing purposes:
    // Using an in-memory SQLite database instead of .db file
    SQLiteConnection InMemoryDatabase 
    { 
        get
        {
            if(_singleton == null)
            {
                _singleton = new SQLiteConnection(":memory:");
                InMemoryDatabase.CreateTable<Group>();
                InMemoryDatabase.CreateTable<Site>();
                var groupA = new Group { GrpName = "GroupA" };
                InMemoryDatabase.Insert(groupA);
                InMemoryDatabase.Insert(new Site { Title = "GroupA.Site1", GrpId = groupA.Id });
                InMemoryDatabase.Insert(new Site { Title = "GroupA.Site2", GrpId = groupA.Id });

                var groupB = new Group { GrpName = "GroupB" };
                InMemoryDatabase.Insert(groupB);
                InMemoryDatabase.Insert(new Site { Title = "GroupB.Site1", GrpId = groupB.Id });
                InMemoryDatabase.Insert(new Site { Title = "GroupB.Site2", GrpId = groupB.Id });
            }
            return _singleton;
        }
    } 
    SQLiteConnection? _singleton = null;
    #endregion T E S T I N G
}

---

[Table("groups")]
public class Group : IEquatable<Group>
{
    // Consider making this key unique at the point of
    // instantiation rather than the point of insertion.
    [PrimaryKey]
    public string Id { get; set; } = Guid.NewGuid().ToString().ToUpper();
    [Unique]
    public string? GrpName { get; set; }
    public bool Equals(Group? other) =>
        Id == other?.Id;
    public override string ToString() => GrpName ?? "Default";
}

[Table("site")]
public class Site : IEquatable<Site>
{
    [PrimaryKey]
    public string Id { get; set; } = Guid.NewGuid().ToString().ToUpper();
    [Unique]
    public string? Title { get; set; }
    public string? SiteAddr { get; set; }
    public string? GrpId { get; set; }    // The Id number matching the Group Id
    public string? Notes { get; set; }
    public bool Equals(Site? other) =>
        Id == other?.Id;
    public override string ToString() => Title ?? "Default";
}