Question

我在mongoDB中保存了以下json：

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {
        "ID": "1753242",
        "TYPE": "8003"
      }
    },
    {
      "type": "Feature",
      "properties": {
        "ID": "4823034",
        "TYPE": "7005"
      }
    }
  ]
}

当我想搜索特定的TYPE时，我可以这样做：

db.geo.find({"features.properties.TYPE":"8003"})

我的问题是，这个查询返回整个json而不仅仅是TYPE＆＃34; 8003＆＃34;的元素。有没有人知道，如何使用TYPE＆＃34; 8003＆＃34;通过查询？

Answer 1

使用Mongo db 3.2发行版，您可以使用新的$filter聚合运算符在投影期间过滤数组，其中包括数组中的所有匹配项

db.test.aggregate([
{$match: {'features': {$elemMatch : {"properties.TYPE": '8003' }}},
{$project: {
    features: {$filter: {
        input: '$features',
        as: 'feature',
        cond: {$eq: ['$$feature.properties.TYPE', '8003']}
    }}
}}
]);

如果您只想要结果的first元素，则可以使用位置$运算符，如下所示：

db.geo.find( { "features.properties.TYPE":"8003"}, { "features.$": 1 } )

Answer 2

$ elemMatch 运算符仅返回与查询结果中 $ elemMatch 条件匹配的第一个元素。

请尝试执行以下查询

  public class FileFetchManager
{
    public const string DEFAULT_FILE_TYPE = "txt";

    public static readonly string[] IMAGE_TYPES = { "bmp", "jpg", "jpeg", "png", "gif", "tiff", "tif" };

    public static string GetFileFetchUrl(string TableName, string FieldName, string FileNameFieldName, string PrimaryKeyValue)
    {
        return GetFileFetchUrl(TableName, FieldName, FileNameFieldName, PrimaryKeyValue, null);
    }

    public static string GetFileFetchUrl(string TableName, string FieldName, string FileNameFieldName, string PrimaryKeyValue, int? thumbnailImageSize)
    {
        return QueryStringCoder.CodeUrl("~/UserControl/JcoDataGrid/DBFileFetch.aspx?t=" + TableName + "&f=" + FieldName + "&fnf=" + FileNameFieldName + "&i=" + PrimaryKeyValue + (thumbnailImageSize == null ? "" : "&s=" + thumbnailImageSize));
    }

    public static string GetFileFetchUrl(string fileBytesSessionName, string fileName)
    {
        return GetFileFetchUrl(fileBytesSessionName, fileName, null);
    }

    public static string GetFileFetchUrl(string fileBytesSessionName, string fileName, int? thumbnailImageSize)
    {
        return QueryStringCoder.CodeUrl("~/UserControl/JcoDataGrid/MemoryFileFetch.aspx?sn=" + fileBytesSessionName + "&fn=" + fileName + (thumbnailImageSize == null ? "" : "&s=" + thumbnailImageSize));
    }

    public static string GetFileType(string fileName)
    {
        if (fileName.IsNullOrWhiteSpace())
            return DEFAULT_FILE_TYPE;
        return fileName.Substring(fileName.IndexOf(".") + 1);
    }

    public static bool IsImage(string fileName)
    {
        string fileType = fileName.Substring(fileName.LastIndexOf(".") + 1);
        bool isImage = FileFetchManager.IMAGE_TYPES.Contains(fileType, StringComparison.OrdinalIgnoreCase, true);
        return isImage;
    }

    public static byte[] GetThumbnailImage(string fileName, byte[] imageBytes, int size)
    {
        if (size <= 0)
            throw new Exception("ErrorCode : 1390/10/23-22:04 : " + QueryStringCoder.Code("اندازه درخواستي جهت تامبنيل تصوير(" + size + ") مجاز نمي باشد!"));
        ImageFormat imageType = ConvertFileExtensionToImageFormat(GetFileType(fileName));
        MemoryStream ms = new MemoryStream(imageBytes);
        ms.Position = 0;
        System.Drawing.Image img = System.Drawing.Image.FromStream(ms);
        int width;
        int height;
        if (img.Height > img.Width)
        {
            height = size.ToInt();
            width = img.Width * size.ToInt() / img.Height;
        }
        else //if (img.height <= img.width)
        {
            width = size.ToInt();
            height = img.Height * size.ToInt() / img.Width;
        }
        if (imageType == ImageFormat.Png || imageType == ImageFormat.Gif)
        {                
            System.Drawing.Image.GetThumbnailImageAbort myCallback = new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback);
            img = img.GetThumbnailImage(width, height, myCallback, IntPtr.Zero);
            imageType = ImageFormat.Png;
        }
        else
            img = ImageResizer.ConstrainProportions(img, height, Dimensions.Height);
        MemoryStream thumbnailImageStream = new MemoryStream();
        thumbnailImageStream.Position = 0;
        img.Save(thumbnailImageStream, imageType);
        return thumbnailImageStream.ToArray();
    }

    public static bool ThumbnailCallback()
    {
        return false;
    }

    public static ImageFormat ConvertFileExtensionToImageFormat(string fileExtension)
    {
        switch (fileExtension.ToLower())
        {
            case "jpeg":
            case "jpg":
                return ImageFormat.Jpeg;
            case "bitmap":
            case "bmp":
                return ImageFormat.Bmp;
            case "gif":
                return ImageFormat.Gif;
            case "icon":
            case "ico":
                return ImageFormat.Icon;
            case "png":
                return ImageFormat.Png;
            case "tiff":
            case "tif":
                return ImageFormat.Tiff;
            default:
                throw new NotImplementedException();
            //break;
        }
    }

    public static void SetResposeContentType(string fileName)
    {
        string fileType = fileName.Substring(fileName.LastIndexOf(".") + 1);
        if (FileFetchManager.IMAGE_TYPES.Contains(fileType, StringComparison.OrdinalIgnoreCase, true))
            HttpContext.Current.Response.ContentType = "image/" + FileFetchManager.ConvertFileExtensionToImageFormat(fileType).ToString();
        else
            HttpContext.Current.Response.ContentType = "application/" + fileType;
    }
}

请参阅 $ elemMatch 运营商的文档，如下面提到的网址所述

https://docs.mongodb.com/manual/reference/operator/projection/elemMatch/

MongoDB：在集合中搜索字符串并仅返回匹配的集合项

2 个答案: