Question

我使用的是Xamarin表单，SQLite.net和SQLitenet扩展，我无法弄清楚为什么我希望简单的东西不起作用。

我有两个班级

public class MeasurementInstanceModel
{
    public MeasurementInstanceModel ()
    {
    }

    [PrimaryKey]
    [AutoIncrement]
    public int Id {
        get;
        set;
    }

    [ForeignKey(typeof(MeasurementDefinitionModel))]
    public int MeasurementDefinitionId {
        get;
        set;
    }

    [ManyToOne(CascadeOperations = CascadeOperation.CascadeRead)]
    public MeasurementDefinitionModel Definition {
        get;
        set;
    }

    [ForeignKey(typeof(MeasurementSubjectModel))]
    public int MeasurementSubjectId {
        get;
        set;
    }

    [ManyToOne(CascadeOperations = CascadeOperation.CascadeRead)]
    public MeasurementSubjectModel Subject {
        get;
        set;
    }

    public DateTime DateRecorded {
        get;
        set;
    }

    [OneToMany(CascadeOperations = CascadeOperation.All)]
    public List<MeasurementGroupInstanceModel> MeasurementGroups {
        get;
        set;
    }
}

和

public class MeasurementSubjectModel
{


    [PrimaryKey]
    [AutoIncrement]
    public int Id {
        get;
        set;
    }
    public string Name {
        get;
        set;
    }

    [OneToMany (CascadeOperations = CascadeOperation.All)]
    public List<MeasurementInstanceModel> MeasurementInstances {get;set;}
}

我只是尝试执行以下查询，但它总是失败。

db.Table<MeasurementInstanceModel>().Where(w => w.Subject.Name == avariable);

我得到了这个例外

System.Diagnostics.Debugger.Mono_UnhandledException（ex = {System.Reflection.TargetInvocationException：调用目标已抛出异常.---＆gt; System.NullReferenceException：未将对象引用设置为对象的实例在SQLite.Net.TableQuery 1[MeasureONE.MeasurementInstanceModel].CompileExpr (System.Linq.Expressions.Expression expr, System.Collections.Generic.List 1 queryArgs）[0x00000] in：0 在SQLite.Net.TableQuery 1[MeasureONE.MeasurementInstanceModel].CompileExpr (System.Linq.Expressions.Expression expr, System.Collections.Generic.List 1 queryArgs）[0x00000] in：0 在SQLite.Net.TableQuery 1[MeasureONE.MeasurementInstanceModel].CompileExpr (System.Linq.Expressions.Expression expr, System.Collections.Generic.List 1 queryArgs）[0x00000] in：0 在SQLite.Net.TableQuery 1[MeasureONE.MeasurementInstanceModel].GenerateCommand (System.String selectionList) [0x00000] in <filename unknown>:0 at SQLite.Net.TableQuery 1 [MeasureONE.MeasurementInstanceModel] .GetEnumerator（）[0x00000] in：0 at System.Collections.Generic.List 1[MeasureONE.MeasurementInstanceModel].AddEnumerable (IEnumerable 1 enumerable）[0x00000] in：0 在System.Collections.Generic.List 1[MeasureONE.MeasurementInstanceModel]..ctor (IEnumerable 1集合）[0x00000] in：0 at System.Linq.Enumerable.ToList [MeasurementInstanceModel]（IEnumerable 1 source) [0x00000] in <filename unknown>:0 at MeasureONE.Repository 1 [MeasureONE.MeasurementInstanceModel] .GetAll [DateTime]（System.Linq.Expressions.Expression 1 predicate, System.Linq.Expressions.Expression 1 orderBy，Nullable {{1} } 1 skip，Nullable 1 descending, Nullable 1 [MeasureONE.MeasurementInstanceModel] .GetAllWithChildren [DateTime]（System.Linq.Expressions.Expression 1 count) [0x00094] in /Users/jean-sebastiencote/MeasureONE/MeasureONE/Models/Repository/Repository.cs:48 at MeasureONE.Repository 1 orderBy，Nullable 1 predicate, System.Linq.Expressions.Expression 1 skip，Nullable {{1 1个pred，Nullable 1 descending, Nullable 1个计数）[0x00049] /Users/jean-sebastiencote/MeasureONE/MeasureONE/ViewModels/MeasurementListViewModel.cs:42 在MeasureONE.MeasurementListViewModel.Load（MeasureONE.FilterViewModel过滤器）[0x000cf] /Users/jean-sebastiencote/MeasureONE/MeasureONE/ViewModels/MeasurementListViewModel.cs:34 在MeasureONE.MeasurementListViewModel.m__1（GalaSoft.MvvmLight.Messaging.NotificationMessage`1 msg）[0x00007] in /Users/jean-sebastiencote/MeasureONE/MeasureONE/ViewModels/MeasurementListViewModel.cs:21 at（wrapper managed-to-native）System.Reflection.MonoMethod：InternalInvoke（System.Reflection.MonoMethod，object，object []，System.Exception＆amp;）在System.Reflection.MonoMethod.Invoke（System.Object obj，BindingFlags invokeAttr，System.Reflection.Binder binder，System.Object [] parameters，System.Globalization.CultureInfo culture）[0x00000] in：0 ---内部异常堆栈跟踪结束---

从堆栈跟踪中可以看出，代码中还有一些内容，例如order by。但这一切都很好，只要我没有儿童餐桌上的条件。

Answer 1

SQLite-Net Extensions没有添加任何查询功能（至少到现在为止）。这意味着您无法在查询时访问关系，因为该对象需要JOIN才能执行。这就是您获得NullReferenceException。

的原因

您需要手动执行JOIN。替换此代码：

db.Table<MeasurementInstanceModel>().Where(w => w.Subject.Name == avariable);

有了这个：

var result = conn.Query<MeasurementInstanceModel>(
    "SELECT * " +
    "FROM MeasurementInstanceModel AS it " +
    "JOIN MeasurementSubjectModel AS sb " +
    "ON it.MeasurementSubjectId == sb.Id " +
    "WHERE sb.Name == ?", avariable);

自动创建此类查询非常复杂，并且计划在不久的将来不再支持SQLite-Net Extensions。

使用SQLite-Net Extension关系的另一个选项是使用GetAllWithChildren方法过滤所需的主题，然后浏览关系以获取实例：

var subjects = conn.GetAllWithChildren<MeasurementSubjectModel>(s => s.Name == avariable);
var result = subjects.Select(s => s.MeasurementInstances).Distinct().ToList();

这样您就不必手动键入JOIN，结果完全相同，但是此选项会受N+1 issue的影响，因此可能会受到一些性能损失。

希望它有所帮助。

sqlite.net表子表上的条件

1 个答案: