LINQ Lambda，分组与列表 [英] LINQ Lambda, Group by with list

问题描述

我有一些麻烦找到正确的语法来完成以下操作：

I'm having some trouble with finding the right syntax to accomplish the following:

可能使用LINQ（Lambda表达式）到.GroupBy数据，而不是使用通常的.Sum（）或.Count（）我想得到的数据是Int的列表。

Is it possible with LINQ (Lambda Expression) to .GroupBy data and instead of using the usual .Sum() or .Count() I want the resulting data to be a List of Int.

我定义了我自己的类名为：Filter_IDs。它的构造函数需要两个参数：

I defined my own class named: Filter_IDs. Its constructor needs two parameters:

public int? type; // Represents the object_type column from my database
public List<int?> objects; // Represents the object_id column from my database

我要将数据从我的数据库加载到此对象。以下LINQ查询应产生Filter_IDs列表：

I want to load data from my database into this object. The following LINQ query should result in a List of Filter_IDs:

以下LINQ查询应产生Filter_ID列表：

The following LINQ query should result in a List of Filter_IDs:

List<Filter_IDs> filterids = ef.filterLine
        .GroupBy(fl => fl.objectType)
    .Select(fl => new Filter_IDs { type = fl.Key, objects = fl.Select(x => x.object_id).ToList() })
    .ToList();

使用此查询不会产生构建错误，但在RunTime上提供了NotSupportedException。

Using this query gives no building error but gives an 'NotSupportedException' on RunTime.

数据库看起来像这样，可以更好地理解数据：

The database looks like this to give you a better understanding of the data:

http://d.pr/i/mnhq+ （droplr图片）

http://d.pr/i/mnhq+ (droplr image)

提前感谢，
Gerben

Thanks in advance, Gerben

推荐答案

我认为问题是DB不能在select中调用ToList，创建一个新的Filter_ID。

I think the problem is the DB is not able to call ToList in the select, nor to create a new Filter_ID.

尝试这样：

List<Filter_IDs> filterids = ef.filterLine.Select(o => new { objectType = o.objectType, object_id=o.object_id})
    .GroupBy(fl => fl.objectType).ToList()
    .Select(fl => new Filter_IDs { type = fl.Key, objects = fl.Select(x => x.object_id).ToList() })
    .ToList();

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