c# - MySQL C# 查找某个最大距离内的 Lat Lon 条目

Question

我正在尝试查找所有具有经纬度且距我的位置在一定英里范围内的数据库条目。

我的答案基于这个 stackoverflow 答案:

MySQL Great Circle Distance (Haversine formula)

这是我所拥有的:

                IQueryable<myTable> busLst = (from b in db.myTable
                                             where (3959 * Math.Acos(Math.Cos(radians(latLonRequest.lat)) * Math.Cos(radians(b.lat))
                                            * Math.Cos(radians(b.lon) - radians(latLonRequest.lon)) + Math.Sin(radians(latLonRequest.lat)) *
                                            Math.Sin(radians(b.lat)))) < latLonRequest.MaxDistance
                                             select b
                                            );

我收到以下错误:

"errorCode": "NotSupportedException",
    "message": "LINQ to Entities does not recognize the method 'Double Acos(Double)' method, and this method cannot be translated into a store expression.",
    "stackTrace": "[GetByLatLonRequest: 6/24/2013 6:57:14 PM]:\n[REQUEST: {lat:3,lon:3,maxDistance:10,measureSystem:N}]\nSystem.NotSupportedException: LINQ to Entities does not recognize the method 'Double Acos(Double)' method, and this method cannot be translated into a store expression.\r\n   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.DefaultTranslator.Translate(ExpressionConverter parent,

Answer 1

这意味着 linq 不知道如何将您的函数转换为 SQL。您要做的就是先提取数据，然后对其运行逻辑:

IQueryable<myTable> busLst = (from b in db.myTable.AsEnumerable()
                                             where (3959 * Math.Acos(Math.Cos(radians(latLonRequest.lat)) * Math.Cos(radians(b.lat))
                                            * Math.Cos(radians(b.lon) - radians(latLonRequest.lon)) + Math.Sin(radians(latLonRequest.lat)) *
                                            Math.Sin(radians(b.lat)))) < latLonRequest.MaxDistance
                                             select b
                                            );

这将在运行数学函数之前枚举结果。

更好的选择是构建谓词:http://www.albahari.com/nutshell/predicatebuilder.aspx