Question

我正在编写一个在realm.xamarin linq查询中需要'WHERE IN'like子句的查询。下面是我试过的查询：

var IdsToMatch = observations.Select(x => x.Id)
var results = from d in realm.All<Observations>() where IdsToMatch.Any(p => p == d.Id) select d;

也尝试使用Contains：

var results = from d in realm.All<Observations>() where IdsToMatch.Contains(d.Id) select d;

但这会引发错误：

The method 'Contains' is not supported
The method 'Any' is not supported

这是什么解决方案？

编辑：

这是错误的堆栈跟踪：

UNHANDLED EXCEPTION： 06-07 15：04：55.097 I / MonoDroid（24526）：System.NotSupportedException：不支持方法'Any' 06-07 15：04：55.097 I / MonoDroid（24526）：at Realms.RealmResultsVisitor.VisitMethodCall（System.Linq.Expressions.MethodCallExpression m）[0x00596] in：0 06-07 15：04：55.097 I / MonoDroid（24526）：在Realms.ExpressionVisitor.Visit（System.Linq.Expressions.Expression exp）[0x000ec] in：0 06-07 15：04：55.097 I / MonoDroid（24526）：at Realms.RealmResultsVisitor.VisitMethodCall（System.Linq.Expressions.MethodCallExpression m）[0x0006a] in：0 06-07 15：04：55.097 I / MonoDroid（24526）：在Realms.ExpressionVisitor.Visit（System.Linq.Expressions.Expression exp）[0x000ec] in：0 06-07 15：04：55.097 I / MonoDroid（24526）：at Realms.RealmResults 1[T].CreateResultsHandle () [0x00037] in <filename unknown>:0 06-07 15:04:55.097 I/MonoDroid(24526): at Realms.RealmResults 1 [T] .get_ResultsHandle（）[0x0000d] in：0 06-07 15：04：55.097 I / MonoDroid（24526）：at Realms.RealmResults 1[T].GetEnumerator () [0x00000] in <filename unknown>:0 06-07 15:04:55.097 I/MonoDroid(24526): at System.Collections.Generic.List 1 [T] .. ctor（IEnumerable 1 collection) [0x00073] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/collections/generic/list.cs:104 06-07 15:04:55.098 I/MonoDroid(24526): at System.Linq.Enumerable.ToList[TSource] (IEnumerable 1 source）[0x00011]在/ Users / builder /数据/通道/ 3053 / a94a03b5 /源/单声道/外部/ referencesource / System.Core程序/系统/ LINQ的/ Enumerable.cs：835 06-07 15：04：55.098 I / MonoDroid（24526）：在d：\ rwagh \ mediq \ Code \ VHS中的VHS.MobileApp.Mediq.DataAccess.ObservationDatabase +＆lt;＆gt; c__DisplayClass13.b__8（）[0x00147]。 MobileApp.Mediq.DataAccess \ ObservationDatabase.cs：65 06-07 15：04：55.098 I / MonoDroid（24526）：在/ Users / builder / data / lanes / 3053中的System.Threading.Tasks.Task 1[TResult].InnerInvoke () [0x00012] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/threading/Tasks/Future.cs:686 06-07 15:04:55.098 I/MonoDroid(24526): at System.Threading.Tasks.Task.Execute () [0x00016] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/threading/Tasks/Task.cs:2523 06-07 15:04:55.098 I/MonoDroid(24526): --- End of stack trace from previous location where exception was thrown --- 06-07 15:04:55.098 I/MonoDroid(24526): at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:143 06-07 15:04:55.098 I/MonoDroid(24526): at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00047] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:201 06-07 15:04:55.098 I/MonoDroid(24526): at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x0002e] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:170 06-07 15:04:55.098 I/MonoDroid(24526): at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x0000b] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:142 06-07 15:04:55.098 I/MonoDroid(24526): at System.Runtime.CompilerServices.TaskAwaiter 1 [TResult] .GetResult（）[0x00000] /a94a03b5/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:372

编辑2：

var IdsToMatch = observations.Select(x => x.Id)
var results = from d in realm.All<Observations>() where IdsToMatch.Any(p => p == d.Id) select d;

尝试：var IdsToMatch = observations.Select(x => x.Id).AsQueryable() 按https://github.com/realm/realm-dotnet/blob/7187390529201ba843cd105fc6e3e11acb0c6217/Realm.Shared/linq/RealmResultsVisitor.cs#L121表示

internal override Expression VisitMethodCall(MethodCallExpression m)
        {
            if (m.Method.DeclaringType == typeof(Queryable)) { ...

但这也不起作用。

Answer 1

好的，这很有趣。答案（现在）是建立一个LINQ Expression Tree。

Realm支持LINQ - 我们解析表达式树（好的，有些位丢失，但基础知识在那里，我们会定期改进 - 我们将添加包含）。

因此，只要您需要在运行时构建完全任意的查询表达式，就可以使用LINQ的强大功能。

以下示例可能看起来有点复杂但重点是它使用标准表达式树，因此您可以使用构建此类树的任何其他代码。

class HasIntKey : RealmObject {
    public int UserKey { get; set; }
}


private void FindingManyMatches()
{
    using (var theRealm = Realm.GetInstance ("TestingManyKeys.realm")) {
        // create some sample data
        theRealm.Write (() => {
            for (int i = 1; i < 10000; i++) {
                var obj = theRealm.CreateObject<HasIntKey> ();
                obj.UserKey = i;
            }
        });
        var allInts = theRealm.All<HasIntKey>();
        Console.WriteLine ($"Created {allInts.Count()} objects");

        var idsToMatch = new[] {42,1003,400,57, 6009};

        // technique for how to search for many matches
        // use Expression Trees to dynamically build the LINQ statement
        // see https://msdn.microsoft.com/en-us/library/mt654267.aspx
        ParameterExpression pe = Expression.Parameter (typeof(HasIntKey), "p");

        // building an expression like:
        // allInts.Where (p => p.UserKey == idsToMatch[0] || p.UserKey == idsToMatch[1]...);
        Expression chainedByOr = null;
        // left side of the == will be the same for each clause we add in the loop
        Expression left = Expression.Property(pe, typeof(HasIntKey).GetProperty("UserKey"));  
        foreach (int anId in idsToMatch) {
            // Create an expression tree that represents the expression 'p.UserKey == idsToMatch[n]'.
            Expression right = Expression.Constant(anId);
            Expression anotherEqual = Expression.Equal(left, right);
            if (chainedByOr == null)
                chainedByOr = anotherEqual;
            else 
                chainedByOr = Expression.OrElse (chainedByOr, anotherEqual);
        }
        MethodCallExpression whereCallExpression = Expression.Call(
            typeof(Queryable),
            "Where",
            new Type[] { allInts.ElementType },
            allInts.Expression,
            Expression.Lambda<Func<HasIntKey, bool>>(chainedByOr, new ParameterExpression[] { pe }));

        // Create an executable query from the expression tree.
        IQueryable<HasIntKey> results = allInts.Provider.CreateQuery<HasIntKey>(whereCallExpression);

        // now we have our results, display some details!
        Console.WriteLine ($"Found {results.Count()} objects");

        // Enumerate the results.
        foreach (HasIntKey anObj in results)
            Console.WriteLine($"Found key {anObj.UserKey}");
    }
}

realm.xamarin抛出错误：不支持方法'Contains'。同样适用于'任意'

1 个答案: