使用表达式树构造LINQ GroupBy查询

Question

我已经坚持这个问题一个星期,没有找到解决方案.

我有一个像下面的POCO:

public class Journal {
    public int Id { get; set; }
    public string AuthorName { get; set; }
    public string Category { get; set; }
    public DateTime CreatedAt { get; set; }
}

我想知道在特定日期范围内(按月或年分组)期刊的数量由AuthorName或Category计算.

在将查询对象发送到JSON序列化程序之后,然后生成如下所示的JSON数据(仅使用JSON来演示我想要获取的数据,如何将对象序列化为JSON不是我的问题)

data: {
    '201301': {
        'Alex': 10,
        'James': 20
    },
    '201302': {
        'Alex': 1,
        'Jessica': 9
    }
}

要么

data: {
    '2012': {
         'C#': 230
         'VB.NET': 120,
         'LINQ': 97
     },
     '2013': {
         'C#': 115
         'VB.NET': 29,
         'LINQ': 36
     }
}

我所知道的是以"方法方式"编写LINQ查询,如:

IQueryable<Journal> query = db.GroupBy(x=> new 
    {
        Year = key.CreatedAt.Year,
        Month = key.CreatedAt.Month
    }, prj => prj.AuthorName)
    .Select(data => new {
        Key = data.Key.Year * 100 + data.Key.Month, // very ugly code, I know
        Details = data.GroupBy(y => y).Select(z => new { z.Key, Count = z.Count() })
    });

按月或年,AuthorName或Category分组的条件将通过两个字符串类型方法参数传递.我不知道的是如何在GroupBy()方法中使用"Magic String"参数.经过一些谷歌搜索后,我似乎无法通过传递像"AuthorName"这样的魔术字符串来对数据进行分组.我应该做的是构建一个表达式树并将其传递给GroupBy()方法.

任何解决方案或建议都表示赞赏.

Answer 1

哦,这看起来像一个有趣的问题:)

首先,让我们设置我们的虚假来源,因为我没有你的数据库方便:

// SETUP: fake up a data source
var folks = new[]{"Alex", "James", "Jessica"};
var cats = new[]{"C#", "VB.NET", "LINQ"};
var r = new Random();
var entryCount = 100;
var entries = 
    from i in Enumerable.Range(0, entryCount)
    let id = r.Next(0, 999999)
    let person = folks[r.Next(0, folks.Length)]
    let category = cats[r.Next(0, cats.Length)]
    let date = DateTime.Now.AddDays(r.Next(0, 100) - 50)
    select new Journal() { 
        Id = id, 
        AuthorName = person, 
        Category = category, 
        CreatedAt = date };    

好的,现在我们已经有了一组可以使用的数据,让我们看看我们想要什么...我们想要一些像"形状"的东西:

public Expression<Func<Journal, ????>> GetThingToGroupByWith(
    string[] someMagicStringNames, 
    ????)

它具有与(伪代码)大致相同的功能:

GroupBy(x => new { x.magicStringNames })

让我们一次解剖一件.首先,我们如何动态地做到这一点？

x => new { ... }

编译器通常会为我们带来魔力 - 它的作用是定义一个新的Type,我们也可以这样做:

    var sourceType = typeof(Journal);

    // define a dynamic type (read: anonymous type) for our needs
    var dynAsm = AppDomain
        .CurrentDomain
        .DefineDynamicAssembly(
            new AssemblyName(Guid.NewGuid().ToString()), 
            AssemblyBuilderAccess.Run);
    var dynMod = dynAsm
         .DefineDynamicModule(Guid.NewGuid().ToString());
    var typeBuilder = dynMod
         .DefineType(Guid.NewGuid().ToString());
    var properties = groupByNames
        .Select(name => sourceType.GetProperty(name))
        .Cast<MemberInfo>();
    var fields = groupByNames
        .Select(name => sourceType.GetField(name))
        .Cast<MemberInfo>();
    var propFields = properties
        .Concat(fields)
        .Where(pf => pf != null);
    foreach (var propField in propFields)
    {        
        typeBuilder.DefineField(
            propField.Name, 
            propField.MemberType == MemberTypes.Field 
                ? (propField as FieldInfo).FieldType 
                : (propField as PropertyInfo).PropertyType, 
            FieldAttributes.Public);
    }
    var dynamicType = typeBuilder.CreateType();

所以我们在这里做的是定义一个自定义的一次性类型,它为我们传入的每个名称都有一个字段,它与源类型上的(属性或字段)类型相同.太好了!

现在我们如何为LINQ提供它想要的东西？

首先,让我们为我们将返回的func设置一个"输入":

// Create and return an expression that maps T => dynamic type
var sourceItem = Expression.Parameter(sourceType, "item");

我们知道我们需要"新建"我们的新动态类型之一......

Expression.New(dynamicType.GetConstructor(Type.EmptyTypes))

我们需要使用该参数中的值对其进行初始化...

Expression.MemberInit(
    Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)),
    bindings), 

但是我们要用bindings什么呢？嗯...好吧,我们想要一些东西绑定到源类型中的相应属性/字段,但是将它们重新映射到我们的dynamicType字段......

    var bindings = dynamicType
        .GetFields()
        .Select(p => 
            Expression.Bind(
                 p, 
                 Expression.PropertyOrField(
                     sourceItem, 
                     p.Name)))
        .OfType<MemberBinding>()
        .ToArray();

Oof ......看起来很讨厌,但我们还没有完成 - 所以我们需要为Func我们通过Expression树创建一个返回类型...当有疑问时,请使用object!

Expression.Convert( expr, typeof(object))

最后,我们将它绑定到我们的"输入参数",通过Lambda整个堆栈:

    // Create and return an expression that maps T => dynamic type
    var sourceItem = Expression.Parameter(sourceType, "item");
    var bindings = dynamicType
        .GetFields()
        .Select(p => Expression.Bind(p, Expression.PropertyOrField(sourceItem, p.Name)))
        .OfType<MemberBinding>()
        .ToArray();

    var fetcher = Expression.Lambda<Func<T, object>>(
        Expression.Convert(
            Expression.MemberInit(
                Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)),
                bindings), 
            typeof(object)),
        sourceItem);                

为了便于使用,让我们将整个混乱包装为扩展方法,所以现在我们已经:

public static class Ext
{
    // Science Fact: the "Grouper" (as in the Fish) is classified as:
    //   Perciformes Serranidae Epinephelinae
    public static Expression<Func<T, object>> Epinephelinae<T>(
         this IEnumerable<T> source, 
         string [] groupByNames)
    {
        var sourceType = typeof(T);
    // define a dynamic type (read: anonymous type) for our needs
    var dynAsm = AppDomain
        .CurrentDomain
        .DefineDynamicAssembly(
            new AssemblyName(Guid.NewGuid().ToString()), 
            AssemblyBuilderAccess.Run);
    var dynMod = dynAsm
         .DefineDynamicModule(Guid.NewGuid().ToString());
    var typeBuilder = dynMod
         .DefineType(Guid.NewGuid().ToString());
    var properties = groupByNames
        .Select(name => sourceType.GetProperty(name))
        .Cast<MemberInfo>();
    var fields = groupByNames
        .Select(name => sourceType.GetField(name))
        .Cast<MemberInfo>();
    var propFields = properties
        .Concat(fields)
        .Where(pf => pf != null);
    foreach (var propField in propFields)
    {        
        typeBuilder.DefineField(
            propField.Name, 
            propField.MemberType == MemberTypes.Field 
                ? (propField as FieldInfo).FieldType 
                : (propField as PropertyInfo).PropertyType, 
            FieldAttributes.Public);
    }
    var dynamicType = typeBuilder.CreateType();

        // Create and return an expression that maps T => dynamic type
        var sourceItem = Expression.Parameter(sourceType, "item");
        var bindings = dynamicType
            .GetFields()
            .Select(p => Expression.Bind(
                    p, 
                    Expression.PropertyOrField(sourceItem, p.Name)))
            .OfType<MemberBinding>()
            .ToArray();

        var fetcher = Expression.Lambda<Func<T, object>>(
            Expression.Convert(
                Expression.MemberInit(
                    Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)),
                    bindings), 
                typeof(object)),
            sourceItem);                
        return fetcher;
    }
}

现在,使用它:

// What you had originally (hand-tooled query)
var db = entries.AsQueryable();
var query = db.GroupBy(x => new 
    {
        Year = x.CreatedAt.Year,
        Month = x.CreatedAt.Month
    }, prj => prj.AuthorName)
    .Select(data => new {
        Key = data.Key.Year * 100 + data.Key.Month, // very ugly code, I know
        Details = data.GroupBy(y => y).Select(z => new { z.Key, Count = z.Count() })
    });    

var func = db.Epinephelinae(new[]{"CreatedAt", "AuthorName"});
var dquery = db.GroupBy(func, prj => prj.AuthorName);

这个解决方案缺乏"嵌套语句"的灵活性,比如"CreatedDate.Month",但是有了一点想象力,你可以扩展这个想法来处理任何自由形式的查询.