Entity Framework Core：使用导航属性而不是连接

Posted 2023-03-31

【中文标题】Entity Framework Core：使用导航属性而不是连接

【英文标题】：Entity Framework Core: using navigation properties instead of joins

【发布时间】：2021-09-15 16:19:12

【问题描述】：

最近我将Navigation Properties 添加到没有任何FK 的EF Core 项目中，以便在尽可能多的查询中使用Include 而不是Join，但我发现无论是sintax几乎所有查询都不适合我的需求（需要拆分为多个查询），或者我遗漏了一些东西，所以我举个例子，我想知道，使用Navigation Properties 你会如何获取特定 dayType 的所有 Stops 及其 Line 列表？：

Stop
stopId

Point
pointId
stopId?
routeId

Route
routeId
serviceDetailId

ServiceDetail
serviceDetailId
serviceHeaderId

ServiceHeaderDay
serviceHeaderDayId
serviceHeaderId
dayType

ServiceHeader
serviceHeaderId
lineId

Line
lineId

使用Join 的当前工作查询，我想使用Include 进行翻译：

var query = (await context.Stop
    .Join(
        context.Point,
        (stop) => stop.stopId,
        (point) => point.stopId,
        (stop, point) => new  Stop = stop, Point = point )
    .Join(
        context.Route,
        (join) => join.Point.routeId,
        (route) => route.routeId,
        (join, route) => new  Stop = join.Stop, Route = route )
    .Join(
        context.ServiceDetail,
        (join) => join.Route.routeId,
        (serviceDetail) => serviceDetail.routeId,
        (join, serviceDetail) => new  Stop = join.Stop, ServiceDetail = serviceDetail )
    .Join(
        context.ServiceHeader,
        (join) => join.ServiceDetail.serviceHeaderId,
        (serviceHeader) => serviceHeader.serviceHeaderId,
        (join, serviceHeader) => new  Stop = join.Stop, ServiceHeader = serviceHeader )
    .Join(
        context.ServiceHeaderDay,
        (join) => join.ServiceHeader.serviceHeaderId,
        (serviceHeaderDay) => serviceHeaderDay.serviceHeaderId,
        (join, serviceHeaderDay) => new  Stop = join.Stop, ServiceHeader = join.ServiceHeader, ServiceHeaderDay = serviceHeaderDay )
    .Join(
        context.Line,
        (join) => join.ServiceHeader.lineId,
        (line) => line.lineId,
        (join, line) => new  Stop = join.Stop, ServiceHeaderDay = join.ServiceHeaderDay, Line = line )
    .Where(e => e.ServiceHeaderDay.DayType == "L")
    .Select(e => new  Stop = e.Stop, Line = e.Line )
    .Distinct();
    .ToListAsync())
    // The query ends here, this next step is just grouping by Stops and inserting each Line list into them.
    .GroupBy(e => e.Stop.stopId)
    .Select(e =>
            
        var stop = e.First().Stop;
        stop.Lines = e.Select(e => e.Line).ToList();
        return stop;
    )

使用Include 进行的失败尝试之一：

context.Stop
    .Include(e => e.Points)
    .ThenInclude(e => e.Route)
    .ThenInclude(e => e.ServiceDetail)
    .ThenInclude(e => e.ServiceHeader)
    .ThenInclude(e => e.ServiceHeaderDay
        Where(e => e.DayType = "L")
    // Now I need to Include Line from ServiceHeader, but this is a of type ServiceHeaderDay 
    // and I think you can't use anonymous objects to carry all the tables you just include 
    // so I found people repeating the includes like this:
    .Include(e => e.Point)
    .ThenInclude(e => e.Route)
    .ThenInclude(e => e.ServiceDetail)
    .ThenInclude(e => e.ServiceHeader)
    .ThenInclude(e => e.Line)
    // This doesn't seem to work, but also how would be the select to get the Stops with all 
    // the lines for each Stop here?
    .Select(e => ?)

【参考方案1】：

如果我正确理解您的问题，您的查询可以大大简化。 Include 通常不是用于查询，而是用于加载相关数据进行修改。

var query = context.Stop
    .Where(s => s.Points.Any(p => p.Route.ServiceDetail.ServiceHeader.ServiceHeaderDay.DayType = 'L'))
    .Select(s => new 
    
        Stop = s,
        Lines = s.Points.Where(p => p.Route.ServiceDetail.ServiceHeader.ServiceHeaderDay.DayType = 'L')
            .Select(p => p.Route.ServiceDetail.ServiceHeader.Line)
            .ToList()
    );