在DbContext上构建过滤器 [英] Building filters on DbContext

问题描述

我有一个ASP .Net Core 2.1 Web Api.我想知道是否存在优雅的解决方案"，用于基于查询字符串在DbContext上构造过滤器?所以...说我有一个[可选]查询字符串:

I have an ASP .Net Core 2.1 Web Api. I was wondering if there is an "elegant solution" for constructing filters on my DbContext based on query strings? So... say I have one [optional] query string:

    // GET: api/Accounts
    [HttpGet]
    public IEnumerable<File> GetAccount([FromQuery] bool? isActive)
    {
        if (isActive.HasValue)
            return _context.Accounts.Where(a => a.IsActive == isActive.Value);
        else
            return _context.Accounts;
    }

足够简单...但是说我有很多(可选)查询字符串:

Simple enough... But say I have a number of (optional) query strings:

    // GET: api/Accounts
    [HttpGet]
    public IEnumerable<File> GetAccount([FromQuery] bool? isActive, [FromQuery] string type, [FromQuery] int? agentId, [FromQuery] bool? someOtherFilter)
    {

    }

如您所见，构建过滤器现在变得更加困难，因为可以组合使用过滤器，具体取决于提供的过滤器.我可以检查第一个查询字符串是否具有值，如果有，执行过滤器并将结果保存到temp变量中.然后，我可以检查下一个查询字符串(如果有一个值)，然后对temp变量执行该过滤器，依此类推.但这听起来会很慢...还有其他建议吗?谢谢...

As you can see, constructing the filters now gets harder, because there can be a combination of filters, depending which ones have been supplied. I could check if the first query string has a value, if it does, perform the filter and save the results into a temp variable. Then I could check the next query string, if that has a value, then perform that filter on the temp variable, and so on and so on. But that sounds like it's going to be slow... Any other suggestions? Thanks...

推荐答案

您可以使用if/else检查所有可选值:

You can check all your optional values with if/else:

[HttpGet]
public IEnumerable<File> GetAccount([FromQuery] bool? isActive, [FromQuery] string type, [FromQuery] int? agentId, [FromQuery] bool? someOtherFilter)
{
    var accounts = context.Accounts;
    if(isActive.HasValue) 
        accounts.Where(a => a.IsActive == isActive.Value);

    if(!string.IsNullOrEmpty(type)) 
        accounts.Where(a => a.Type == type);

    if(agentId.HasValue) 
        accounts.Where(a => a.AgentId == agentId.Value);

    . . .

    if(someOtherFilter.HasValue) 
        accounts.Where(a => a.SomeOtherFilter == someOtherFilter.Value);
}

或在linq to sql中，您可以在Where()方法内检查null:

or in linq to sql you can check for null inside your Where() method:

public IEnumerable<File> GetAccount([FromQuery] bool? isActive, [FromQuery] string type, [FromQuery] int? agentId, [FromQuery] bool? someOtherFilter) =>
    context.Accounts.Where(a => 
        (!isActive.HasValue || a.IsActive == isActive.Value) && 
        (string.IsNullOrEmpty(type) || a.Type == type)       &&
        (!agentId.HasValue || a.AgentId== agentId.Value)     &&
        . . .
        (!someOtherFilter.HasValue || a.SomeOtherFilter== someOtherFilter.Value);

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