如何在 LiteDB 中进行级联包含

Question

这里是关于如何在 LiteDB 中存储交叉引用实体的示例。 LiteDB 非常好地存储交叉引用的实体，但是当我尝试返回 find/load 实体时出现问题。我的目标不仅是请求的实体，而且是引用的实体。 LiteDB webpage 上有快速教程部分 "DbRef for cross references" 如何实现它。 LiteDB 有 "Include" 选项（在 "FindAll" 之前调用），它说明必须加载哪些引用的实体。我试图在此代码示例中实现它但没有结果，即代码引发异常（"D_Ref"）意味着未加载 "D_Ref" 引用：

namespace _01_simple {
using System;
using LiteDB;

public class A {
    public int Id { set; get; }
    public string Name { set; get; }
    public B B_Ref { set; get; }
}
public class B {
    public int Id { set; get; }
    public string Name { set; get; }
    public C C_Ref { set; get; }
}
public class C {
    public int Id { set; get; }
    public string Name { set; get; }
    public D D_Ref { set; get; }
}
public class D {
    public int Id { set; get; }
    public string Name { set; get; }
}

class Program {
    static void Main(string[] args) {
        test_01();
    }

    static string NameInDb<T>() {
        var name = typeof(T).Name + "s";
        return name;
    }

    static void test_01() {
        if (System.IO.File.Exists(@"MyData.db"))
            System.IO.File.Delete(@"MyData.db");

        using (var db = new LiteDatabase(@"MyData.db")) {
            var As = db.GetCollection<A>(NameInDb<A>());
            var Bs = db.GetCollection<B>(NameInDb<B>());
            var Cs = db.GetCollection<C>(NameInDb<C>());
            var Ds = db.GetCollection<D>(NameInDb<D>());

            LiteDB.BsonMapper.Global.Entity<A>().DbRef(x => x.B_Ref, NameInDb<B>());
            LiteDB.BsonMapper.Global.Entity<B>().DbRef(x => x.C_Ref, NameInDb<C>());
            LiteDB.BsonMapper.Global.Entity<C>().DbRef(x => x.D_Ref, NameInDb<D>());

            var d = new D { Name = "I am D." };
            var c = new C { Name = "I am C.", D_Ref = d };
            var b = new B { Name = "I am B.", C_Ref = c };
            var a = new A { Name = "I am A.", B_Ref = b };

            Ds.Insert(d);
            Cs.Insert(c);
            Bs.Insert(b);
            As.Insert(a);
        }

        using (var db = new LiteDatabase(@"MyData.db")) {
            var As = db.GetCollection<A>(NameInDb<A>());

            var all_a = As
                .Include(x => x.B_Ref)
                .FindAll();
            foreach (var a in all_a) {
                if (a.B_Ref == null)
                    throw new Exception("B_Ref");
                if (a.B_Ref.C_Ref == null)
                    throw new Exception("C_Ref");
                if (a.B_Ref.C_Ref.D_Ref == null)
                    throw new Exception("D_Ref");
            }
        }
    }
}}

Answer 1

经过小型研究后，我通过添加额外的 "Include" parameterize by "x => x.B_Ref.C_Ref" lambda 解决了这个问题，其中 x.B_Ref.C_Ref 是引用层次结构中的路径：

var all_a = As
    .Include(x => x.B_Ref)
    .Include(x => x.B_Ref.C_Ref)
    .FindAll();

这是完整的例子

namespace _01_simple {
using System;
using LiteDB;

public class A {
    public int Id { set; get; }
    public string Name { set; get; }
    public B B_Ref { set; get; }
}
public class B {
    public int Id { set; get; }
    public string Name { set; get; }
    public C C_Ref { set; get; }
}
public class C {
    public int Id { set; get; }
    public string Name { set; get; }
    public D D_Ref { set; get; }
}
public class D {
    public int Id { set; get; }
    public string Name { set; get; }
}

class Program {
    static void Main(string[] args) {
        test_01();
    }

    static string NameInDb<T>() {
        var name = typeof(T).Name + "s";
        return name;
    }

    static void test_01() {
        if (System.IO.File.Exists(@"MyData.db"))
            System.IO.File.Delete(@"MyData.db");

        using (var db = new LiteDatabase(@"MyData.db")) {
            var As = db.GetCollection<A>(NameInDb<A>());
            var Bs = db.GetCollection<B>(NameInDb<B>());
            var Cs = db.GetCollection<C>(NameInDb<C>());
            var Ds = db.GetCollection<D>(NameInDb<D>());

            LiteDB.BsonMapper.Global.Entity<A>().DbRef(x => x.B_Ref, NameInDb<B>());
            LiteDB.BsonMapper.Global.Entity<B>().DbRef(x => x.C_Ref, NameInDb<C>());
            LiteDB.BsonMapper.Global.Entity<C>().DbRef(x => x.D_Ref, NameInDb<D>());

            var d = new D { Name = "I am D." };
            var c = new C { Name = "I am C.", D_Ref = d };
            var b = new B { Name = "I am B.", C_Ref = c };
            var a = new A { Name = "I am A.", B_Ref = b };

            Ds.Insert(d);
            Cs.Insert(c);
            Bs.Insert(b);
            As.Insert(a);
        }

        using (var db = new LiteDatabase(@"MyData.db")) {
            var As = db.GetCollection<A>(NameInDb<A>());

            var all_a = As
                .Include(x => x.B_Ref)
                .Include(x => x.B_Ref.C_Ref)
                .Include(x => x.B_Ref.C_Ref.D_Ref)
                .FindAll();
            foreach (var a in all_a) {
                if (a.B_Ref == null)
                    throw new Exception("B_Ref");
                if (a.B_Ref.C_Ref == null)
                    throw new Exception("C_Ref");
                if (a.B_Ref.C_Ref.D_Ref == null)
                    throw new Exception("D_Ref");
            }
        }
    }
}}

我希望它能节省一些人的时间。

更新：LiteDB 作者说不支持级联包含。但它计划在下一个版本中（参见 issue）。考虑一下，假设 B_Ref 是 B 的精简版，那么就没有强制更深包含的机制。

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