Question

Hi Geniuses,

I have a Call entity with Start and End properties (of type DateTime?).

I want to issue a query to get the average difference between Call.Start and Call.End (i.e. the average length of a call):

Query.All<call>().Where(call => call.Start.HasValue && call.End.HasValue).Select(call => call.End.Value.Ticks - call.Start.Value.Ticks).Average();

Unfortunately, DO cannot translate the DateTime.Ticks property. I tried to write my own LINQ translator according to: http://dataobjectsdotnet.googlecode.com ... ationToSQL

But I can't seem to figure it out. Could you please provide any advice?

Btw, I've already thought about having a persistent property that tracks the difference between Start and End, but the above is a very simplified example. There are many "DateTime difference" calculations I need to make with many types and properties, so I can't add duration properties for each calculation.

Thanks!

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Answer 1

psulek wrote:

Add property to you entity Call like this:

public class Call: Entity
{
...
    public long AverageCallLength
    {
        get
        {
            return End.Ticks - Start.Ticks;
        }
    }
}

And create new static class with linq translator method like this:

[CompilerContainer(typeof(Expression))]
public static class LinqExtensions
{
    [Compiler(typeof(CallEntity), "AverageCallLength", TargetKind.PropertyGet)]
    public static Expression AverageCallLength(Expression sourceExpression)
    {
        // sourceExpression -> CallEntity
        Expression<Func<CallEntity, long>> expression = (entity) => entity.End.Ticks - entity.Start.Ticks;
        return expression.BindParameters(sourceExpression);
    }
}

Now you can use property AverageCallLength like this:

long callLen = Query.All<CallEntity>().Select(call => call.AverageCallLength).Single();

Alex (Xtensive) wrote:

This really wont work, since you rewrite Call.get_AverageCallLength to an expression utilizing DateTime.Ticks, which can't be translated :roll:

So to use this, you must rewrite it so that DateTime.Ticks isn't used, i.e. express the result using some other translatable expressions for DateTime. I'll ask the guys if this is possible.

Btw, you can define a rewriter directly for DateTime.Ticks property:

[CompilerContainer(typeof(Expression))]
public static class LinqExtensions
{
    [Compiler(typeof(DateTime), "Ticks", TargetKind.PropertyGet)]
    public static Expression AverageCallLength(Expression sourceExpression)
    {
        // sourceExpression -> CallEntity
        Expression<Func<DateTime, long>> expression = d => (d - new DateTime(1900,1,1)).TotalMilliseconds * (long) 10000; // May be this will work
        return expression.BindParameters(sourceExpression);
    }
}

psulek wrote:

Alex (Xtensive) wrote:

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