EF Core，按 UTC 日期的月份和年份分组

我将所有日期存储为 UTC，并且我需要按月份和年份对实体进行分组，所以我正在这样做：

_dbContext.Tickets.Where(x => x.Date >= from && x.Date <= to).GroupBy(x=> new {
                Year = x.Date.Year,
                Month = x.Date.Month
            }).Select(x=> new {x.Key, Count = x.Count()})

由于日期是 UTC，我在 1/09/2020 00:30 AM 创建了一张票证，但由于它存储为 UTC（我是 +2），因此它将存储为 2020-08-31 22:25。现在，如果我按月分组，我会将此实体分组在错误的月份。有没有什么简单的方法可以不依赖内存来做到这一点？

有几种方法可以引入映射到 sql 函数或原始 sql 片段的 c# 函数。

对于直接映射到同名sql函数的函数，只需要定义方法即可；

public static ReturnType CustomMethod(ArgType arg) => throw new NotImplementedException();

protected override void OnModelCreating(ModelBuilder builder){
    builder.HasDbFunction(typeof(Context).GetMethod(nameof(CustomMethod)));
}

或者使用[DbFunction]该方法的属性。

为了生成像“@date AT TIME ZONE @name”这样的sql片段，你不能只使用一个实例SqlFunctionExpression，还有一些工作要做。

public static DateTimeOffset ToTimeZone(this DateTimeOffset value, string name) => throw new NotImplementedException();

public class SqlFragmentListExpression : SqlExpression
{
    public SqlFragmentListExpression(Type type, RelationalTypeMapping typeMapping, params SqlExpression[] fragments) : base(type, typeMapping)
    {
        Fragments = fragments;
    }

    public SqlExpression[] Fragments { get; }

    public override void Print(ExpressionPrinter expressionPrinter)
    {
        foreach (var f in Fragments)
            f.Print(expressionPrinter);
    }

    protected override Expression VisitChildren(ExpressionVisitor visitor)
    {
        var frags = new SqlExpression[Fragments.Length];
        var changed = false;
        for(var i = 0; i < Fragments.Length; i++)
        {
            frags[i] = (SqlExpression)visitor.Visit(Fragments[i]);
            changed |= frags[i] != Fragments[i];
        }
        return changed ? new SqlFragmentListExpression(Type, TypeMapping, frags) : this;
    }
}

// OnModelCreating
builder                
    .HasDbFunction(typeof(Extensions).GetMethod(nameof(Extensions.ToTimeZone)))
    .HasTranslation(args => {
        var dto = args.ElementAt(0);
        return new SqlFragmentListExpression(dto.Type, dto.TypeMapping,
            dto,
            new SqlFragmentExpression(" AT TIME ZONE "),
            args.ElementAt(1));
    });

然后您可以在查询中使用该方法，例如；

_dbContext.Tickets
    .Where(x => x.Date >= from && x.Date <= to)
    .Select(x => new {
        Date = x.Date.ToTimeZone("Central European Standard Time")
    })
    .GroupBy(x => new {
        Year = x.Date.Year,
        Month = x.Date.Month
    },
    (x, e) => new {
        x.Year,
        x.Month,
        Count = e.Count()
    })

应该将其翻译成sql；

SELECT DATEPART(year, [t].[Date] AT TIME ZONE N'Central European Standard Time') AS [Year],
    DATEPART(month, [t].[Date] AT TIME ZONE N'Central European Standard Time') AS [Month], 
    COUNT(*) AS [Count]
FROM [Tickets] AS [t]
GROUP BY DATEPART(year, [t].[Date] AT TIME ZONE N'Central European Standard Time'),
    DATEPART(month, [t].[Date] AT TIME ZONE N'Central European Standard Time')