在MySQL和Entity Framework 6的Where中使用TimeSpan时出现错误的结果

Question

We have table "Tasks":

Id | ToTime
1  | 23:59:59

And code:

TimeSpan currentTime = DateTime.Now.TimeOfDay;
var count = context.Tasks.Where(p => p.ToTime > currentTime).Count();

The problem:

Theoretically, count should be always equals to 1 (in a daytime). But that not so cause sometimes (in random time) MySQL finds no rows (count = 0).

Why?

Take a look at answer. It took 5 hours to find solution.

Answer 1

Milliseconds

Entity Framework generate SQL like this:

SELECT
`GroupBy1`.`A1` AS `C1`
FROM (SELECT
COUNT(1) AS `A1`
FROM `Tasks` AS `Extent1`
 WHERE `Extent1`.`ToTime` > '0 04:24:49.1438543') AS `GroupBy1`

But MySQL allowing only 6 digits for milliseconds. From MySQL docs :

A trailing fractional seconds part is recognized in the 'D HH:MM:SS.fraction', 'HH:MM:SS.fraction', 'HHMMSS.fraction', and HHMMSS.fraction time formats, where fraction is the fractional part in up to microseconds (6 digits) precision. Although this fractional part is recognized, it is discarded from values stored into TIME columns.

Extra digits appears because of redundancy TimeSpan and EntityFramework do not cuts that extra digits. Yep, looks like a bug. If someone know where to report it - please report or send link in comments.

And what to do?

The solution is quite simple. MySQL does not use milliseconds in Time and just discart it. So we can discart it before request. Final code:

TimeSpan currentTime = DateTime.Now.TimeOfDay;
// Re-creating TimeSpan to discart/cut milliseconds
currentTime = new TimeSpan(currentTime.Hours, currentTime.Minutes, currentTime.Seconds);
var count = context.Tasks.Where(p => p.ToTime > currentTime).Count();

And generated SQL would be always with 6 digits (000000). That resolves the problem.

Bug?

As I said before that look like a bug. So if you know where to report it - please report or send link in comments.