我想在类声明中定义哪些项目是索引，比如:

public class MyClass {
    public int SomeNum { get; set; }
    [THISISANINDEX]
    public string SomeProperty { get; set; }
}

因此与 ensureIndex("SomeProperty") 具有相同的效果

这可能吗？

最佳答案

我认为这是个好主意，但你必须自己做，没有内置支持。如果你有一个访问层，你可以在那里做。你需要一个属性类，像这样；

public enum IndexConstraints
{
    Normal     = 0x00000001, // Ascending, non-indexed
    Descending = 0x00000010,
    Unique     = 0x00000100,
    Sparse     = 0x00001000, // allows nulls in the indexed fields
}

// Applied to a member
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
public class EnsureIndexAttribute : EnsureIndexes
{
    public EnsureIndex(IndexConstraints ic = IndexConstraints.Normal) : base(ic) { }
}

// Applied to a class
[AttributeUsage(AttributeTargets.Class)]
public class EnsureIndexesAttribute : Attribute
{
    public bool Descending { get; private set; }
    public bool Unique { get; private set; }
    public bool Sparse { get; private set; }
    public string[] Keys { get; private set; }

    public EnsureIndexes(params string[] keys) : this(IndexConstraints.Normal, keys) {}
    public EnsureIndexes(IndexConstraints ic, params string[] keys)
    {
        this.Descending = ((ic & IndexConstraints.Descending) != 0);
        this.Unique = ((ic & IndexConstraints.Unique) != 0); ;
        this.Sparse = ((ic & IndexConstraints.Sparse) != 0); ;
        this.Keys = keys;
    }

}//class EnsureIndexes

然后您可以按如下方式在类或成员级别应用属性。我发现与在类级别添加相比，在成员级别添加不太可能与架构不同步。当然，您需要确保获得的是实际元素名称，而不是 C# 成员名称；

[CollectionName("People")]
//[EnsureIndexes("k")]// doing it here would allow for multi-key configs
public class Person 
{
    [BsonElement("k")] // name mapping in the DB schema
    [BsonIgnoreIfNull]
    [EnsureIndex(IndexConstraints.Unique|IndexConstraints.Sparse)] // name is implicit here
    public string userId{ get; protected set; }

// other properties go here
}

然后在您的数据库访问实现(或存储库)中，您需要这样的东西；

    private void AssureIndexesNotInlinable()
    {
                // We can only index a collection if there's at least one element, otherwise it does nothing
                if (this.collection.Count() > 0)
                {

                    // Check for EnsureIndex Attribute
                    var theClass = typeof(T);

                    // Walk the members of the class to see if there are any directly attached index directives
                    foreach (var m in theClass.GetProperties(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.FlattenHierarchy))
                    {
                        List<string> elementNameOverride = new List<string>(1);
                        EnsureIndexes indexAttr = null;

                        // For each members attribs
                        foreach (Attribute attr in m.GetCustomAttributes())
                        {
                            if (attr.GetType() == typeof(EnsureIndex))
                                indexAttr = (EnsureIndex)attr;

                            if (attr.GetType() == typeof(RepoElementAttribute))
                                elementNameOverride.Add(((RepoElementAttribute)attr).ElementName);

                            if ((indexAttr != null) && (elementNameOverride.Count != 0))
                                break;
                        }

                        // Index
                        if (indexAttr != null)
                        {
                            if (elementNameOverride.Count() > 0)
                                EnsureIndexesAsDeclared(indexAttr, elementNameOverride);
                            else
                                EnsureIndexesAsDeclared(indexAttr);
                        }
                    }

                    // Walk the atributes on the class itself. WARNING: We don't validate the member names here, we just create the indexes
                    // so if you create a unique index and don't have a field to match you'll get an exception as you try to add the second
                    // item with a null value on that key
                    foreach (Attribute attr in theClass.GetCustomAttributes(true))
                    {
                        if (attr.GetType() == typeof(EnsureIndexes))
                            EnsureIndexesAsDeclared((EnsureIndexes)attr);

                    }//foreach

                }//if this.collection.count

    }//AssureIndexesNotInlinable()

EnsureIndexes 看起来像这样；

    private void EnsureIndexesAsDeclared(EnsureIndexes attr, List<string> indexFields = null)
    {
        var eia = attr as EnsureIndexes;

        if (indexFields == null)
            indexFields = eia.Keys.ToList();

        // use driver specific methods to actually create this index on the collection
        var db = GetRepositoryManager(); // if you have a repository or some other method of your own 
        db.EnsureIndexes(indexFields, attr.Descending, attr.Unique, attr.Sparse);

    }//EnsureIndexes()

请注意，您将在每次更新后放置它，因为如果您忘记某处，您的索引可能无法创建。因此，重要的是要确保您优化调用，以便在通过所有反射代码之前没有索引可做时它可以快速返回。理想情况下，您只需执行一次，或者至少在每次应用程序启动时执行一次。因此，一种方法是使用静态标志来跟踪您是否已经这样做，并且您需要围绕它进行额外的锁定保护，但过于简单化，它看起来像这样；

    void AssureIndexes()
    {
        if (_requiresIndexing)
            AssureIndexesInit();
    }

因此，这就是您在进行的每个数据库更新中都需要的方法，如果幸运的话，JIT 优化器也会内联该方法。

关于mongodb - 如何将一个class item装饰成一个index，得到和ensureIndex一样的效果？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7823187/