EF5中的动态谓词构建

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问题描述

好吧

我必须建立一个谓词，以从EF5 AND 过滤器中的已知实体中选择未知数量的列，而其中未知数量的列 ONE 总是子集合中的名称.

I have to build a predicate to select an unknown number of columns from a known entity in EF5 AND filter by an unknown number of columns ONE of which will always be a name from a child collection.

所以这就是我要结束的

var q = db.Set<Entity>()
          .Where(e => e.Code.Contains("q") || e.Translations.FirstOrDefault(t => t.Culture.ID == "whatever").Description.Contains("q"))
          .Select(e => new
                       {
                           e.ID,
                           e.Code,
                           Name = e.Translations.FirstOrDefault(t => t.Culture.ID == "whatever").Description
                       });

但是我不确定如何构建表达式的谓词

but I'm not sure how to approach building the predicate for the expression

e.Translations.FirstOrDefault(t => t.Culture.ID == "whatever").Description.Contains("q")

要选择的列和要过滤的字段以字符串数组的形式提供，在这种情况下，过滤器始终包含.

The columns to select and the fields to filter on are supplied as an array of strings, and in this instance the filter is always contains.

我熟悉构建谓词，这不是问题，更多的是，我以前从未研究过子集合，而且一天总的来说是"WTF":-)

I'm familiar with building predicates, that's not the issue, it's more I've never had to look into child collections before and I'm at a total "WTF" point of the day :-)

任何朝正确方向的推动都是最合适的.

Any and all pushes in the right direction would be most appriciated.

One approach might be to use DynamicLinq. See http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

如果您向下看第三个查询，您应该可以执行以下操作:

If you look at the third query down, you should be able to do something like:

 e.Translations
.Where("Culture.ID = 'whatever'") //Conditions defined as string as per example
.FirstOrDefault() //etc.

关于select an unknown number of columns from a known entity in EF5 AND filter by an unknown number of columns，可以在使用字符串之前使用构建字符串所需的任何表达式.对于包含"子句，它有点复杂-请参阅此处描述的方法-

Regarding select an unknown number of columns from a known entity in EF5 AND filter by an unknown number of columns, you could use whatever expression is required to build your string prior to using it. For the 'contains' clause it is a bit more complex - please see the methods described here - How to write String.Contains in Dynamic Linq

由于我没有访问EF5的权限，因此我尚未进行实际测试，但似乎DynamicLinq存在基于EF5的问题(因此应该可以使用)-

I haven't actually tested this as I don't have access to EF5, but it looks like there are EF5 based questions for DynamicLinq (so it should work) - https://stackoverflow.com/questions/9929396/entity-framework-5-0-performance-with-dyanmic-linq

此外，我应该指出，我更愿意为此目的使用PredicateBuilder，但我相信您将无法不事先知道字段名称/使用某种形式的反射(请参阅).也值得肯定这个答案( https://stackoverflow.com/a/2497335/201648 )方法.

Also, I should point out that I would prefer to use PredicateBuilder for this purpose, but I believe you will be unable to without knowing the fieldnames in advance/using some sort of reflection (see Using Dynamic Column Names in a Linq Query). It is also worth acknowledging this answer (https://stackoverflow.com/a/2497335/201648) which suggests the above approach.

此外，也可以使用lambda设置字段的条件规则，如下所述: http://www.codeproject.com/Articles/28580/LINQ-and-Dynamic-Predicate-Construction-at-Runtime

Also, it is possible to setup conditional rules for fields using lambdas as described here: http://www.codeproject.com/Articles/28580/LINQ-and-Dynamic-Predicate-Construction-at-Runtime

// use tryparse to make sure we don't run a bogus query.
if (cbxUseEmployeeID.Checked &&
    int.TryParse(filterEmployeeId.Text, out emplId) &&
    emplId > 0) {

    // here's how simple it is to add a condition to the query.
    // Still not executing yet, just building a tree.
    predicate = predicate.And(e => e.EmployeeID == emplId);
}

据我所知，这仍然需要您知道可能的字段集(我仍然认为它很酷，值得一提).

This would still require you to know the possible set of fields as far as I can see (I still think its cool and worth a mention).

这篇关于EF5中的动态谓词构建的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！

DynamicLinq

EF5中的动态谓词构建

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