如何选择 Azure 存储表中缺少字段的记录？

Question

I'm attempting to process records that were inserted into Azure Table Storage prior to the addition of a new attribute. The LINQ support is limited and I'm struggling to get this to work.

How would one use LINQ (or another method) to filter an Azure Table to just records missing an attribute for a given entity?

Sample Code

I'm writing it in an Azure Function, and the row count here returns 0, when attempting a default value for the field.

#r "Microsoft.WindowsAzure.Storage"

using System.Net;
using Microsoft.WindowsAzure.Storage.Table;

public static HttpResponseMessage Run(HttpRequestMessage req, IQueryable<ts_webhit> inTable, TraceWriter log)
{
    IEnumerable<ts_webhit> recs = (from r in inTable
                                   where "2016-11-21 12:45" == r.PartitionKey
                                   &&    ""                 == r.Category  //The filter I need to run
                                   select r);
    log.Info($"{recs.Count()}");
    return req.CreateResponse(HttpStatusCode.OK, recs.ToList());
}

public class ts_webhit : TableEntity
{

    public string Category { get; set; } = ""; //Attempting a default value

}

Some candidate filters that didn't work

• r.Category is nothing generates a compilation error
• String.IsNullOrEmpty(r.Category) returns that it's not supported
• r.Category == null returns a bad request
• !r.Category.HasValue generates a compilation error as it's a string

Answer 1

如果该属性在写入表存储时实体上不存在，则该列将不存在于表中，因此您在查询中进行的任何比较 - 包括与空字符串的比较 - 都将失败，您将得到一个空回复。