如何在运行时创建一个表达式，为在GROUPBY（）与实体框架使用？ [英] How to create an expression at runtime for use in GroupBy() with Entity Framework?

问题描述

我建立这将动态地能够从实体框架组查询结果的新的扩展方法。

I am building a new extension method which will dynamically be able to group query results from Entity Framework.

我已经能够建立动态，其中表达式中使用。LinqKit，但是这似乎是一个不同的动物

I have been able to build dynamic "where" expressions using LinqKit, but this seems to be a different animal.

新的扩展方法预期用法：

Intended usage of new extension method:

var results = entities.GroupBy("someFieldName").ToList();

扩展方法定义：

Extension method definition:

    public static IQueryable<IGrouping<object, TEntity>> GroupBy<TEntity>(
        this IQueryable<TEntity> source,
        string fieldName) 
        where TEntity : class, IDataEntity
    {
        if (string.IsNullOrEmpty(fieldName))
        {
            return new List<IGrouping<object, TEntity>>().AsQueryable();
        }

        var parameter = Expression.Parameter(typeof(TEntity), "x");
        var fieldXExpression = Expression.Property(parameter, fieldName);
        var lambda = Expression.Lambda<Func<TEntity, object>>(
            Expression.Convert(fieldXExpression, typeof(object)), // throws error when using EF
            parameter);
        return source.AsExpandable().GroupBy(lambda);
    }

我需要使用 Expression.Convert（.. 。），因为当我使用linqTOobjects测试，当列是代码失败 INT32 。所以，我需要手动转换列对象和它的工作太棒了。

I need to use Expression.Convert(...) because when I was testing using linqTOobjects, the code failed when the column was a int32. So I needed to manually convert the column to object and it worked great.

现在我正在使用EF实体测试它，我想这EF试图翻译。转换成等价的SQL，当然我知道不存在

Now that I'm testing it with EF entities, I guess that EF is trying to translate the convert to equivalent SQL, which of course I know doesn't exist.

错误：

System.NotSupportedException: Unable to cast the type 'System.String' to type 'System.Object'. LINQ to Entities only supports casting EDM primitive or enumeration types.

有没有在运行时产生适合于GROUPBY表达的方式也与EF兼容？

Is there a way to generate an expression suitable for GroupBy at runtime that is also compatible with EF?

感谢您的任何指导。

推荐答案

我能够做到什么我需要通过使用动态的LINQ。我创建了一个扩展方法调用ListGroupKeys：

I was able to accomplish what I needed by using Dynamic Linq. I created an extension method called ListGroupKeys:

    public static List<object> ListGroupKeys<TEntity>(this IQueryable<TEntity> source, string fieldName)
        where TEntity : class, IDataEntity
    {
        var results = source.GroupBy(fieldName, "it").Select("new (KEY as Group)");

        var list = new List<object>();
        foreach (var result in results)
        {
            var type = result.GetType();
            var prop = type.GetProperty("Group");

            list.Add(prop.GetValue(result));
        }

        return list;
    }

这让我拿到小组第一键值在后续查询使用。

This allowed me to get the group key values for use in subsequent queries.

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