Alias ​​Properties in LINQ to SQL

I am using the shim property to make sure the date is always UTC. This in itself is pretty simple, but now I want to query for data. I don't want to expose the underlying property, instead I want the queries to use the shim property. I am having problems displaying the shim property. For example:

public partial class Activity
{
    public DateTime Started
    {
        // Started_ is defined in the DBML file
        get{ return Started_.ToUniversalTime(); }
        set{ Started_ = value.ToUniversalTime(); }
    }
}


var activities = from a in Repository.Of<Activity>()
                 where a.Started > DateTime.UtcNow.AddHours( - 3 )
                 select a;

      

An attempt to execute the request results in an exception:

System.NotSupportedException: The member 'Activity.Started' has no supported 
translation to SQL.

      

This makes sense - how does LINQ to SQL handle the Started property - is it not a column or an association? But I was looking for something like the ColumnAliasAttribute that tells SQL to handle the Started as Started_ (underscored) properties.

Is there a way to help LINQ to SQL convert an expression tree to a Started property that can be used in the same way as the Started_ property?

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3 answers


Here is a sample code showing how to do this (for example, use client-side properties in requests) on the Damien Guard blog:

http://damieng.com/blog/2009/06/24/client-side-properties-and-any-remote-linq-provider

However, I don't think DateTime.ToUniversalTime will translate to SQL anyway, so you might need to write some db-side logic for UTC translations anyway. In this case, it may be easier to set UTC date / time as the computed db-side column and include it in your L2S classes.



eg:.

create table utc_test (utc_test_id int not null identity,
  local_time datetime not null,
  utc_offset_minutes int not null,
  utc_time as dateadd(minute, 0-utc_offset_minutes, local_time),
  constraint pk_utc_test primary key (utc_test_id));  

insert into utc_test (local_time, utc_offset_minutes) values ('2009-09-10 09:34', 420); 
insert into utc_test (local_time, utc_offset_minutes) values ('2009-09-09 22:34', -240);   

select * from utc_test

      

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Building on @ KrstoferA's answer , I came up with a robust solution that hides the fact that properties are flattened from client code. Since I am using a repository pattern that returns IQueryable [T] for specific tables, I can simply wrap the IQueryable [T] result provided by the underlying data context and then translate the expression before the underlying provider compiles it.

Here's the code:



public class TranslationQueryWrapper<T> : IQueryable<T>
{
    private readonly IQueryable<T> _source;

    public TranslationQueryWrapper( IQueryable<T> source )
    {
        if( source == null ) throw new ArgumentNullException( "source" );
        _source = source;
    }

    // Basic composition, forwards to wrapped source.
    public Expression Expression { get { return _source.Expression; } }
    public Type ElementType { get { return _source.ElementType; } }
    public IEnumerator<T> GetEnumerator() { return _source.GetEnumerator(); }
    IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }

    // Intercept calls to the provider so we can translate first.
    public IQueryProvider Provider 
    { 
        get { return new WrappedQueryProvider(_source.Provider); } 
    }

    // Another wrapper around the provider
    private class WrappedQueryProvider : IQueryProvider
    {
        private readonly IQueryProvider _provider;

        public WrappedQueryProvider( IQueryProvider provider ) { 
            _provider = provider; 
        }

        // More composition
        public object Execute( Expression expression ) { 
            return Execute( expression ); }
        public TResult Execute<TResult>( Expression expression ) { 
            return _provider.Execute<TResult>( expression ); }
        public IQueryable CreateQuery( Expression expression ) { 
            return CreateQuery( expression ); }

        // Magic happens here
        public IQueryable<TElement> CreateQuery<TElement>( 
            Expression expression ) 
        { 
            return _provider
                .CreateQuery<TElement>( 
                    ExpressiveExtensions.WithTranslations( expression ) ); 
        }
    }
}

      

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Another example can't hurt, I think. In my Template class, I have a Seconds field that I convert to TimeStamp relative to UTC time. This statement also has CASE (a? B: c).

    private static readonly CompiledExpression<Template, DateTime> TimeStampExpression =
        DefaultTranslationOf<Template>.Property(e => e.TimeStamp).Is(template => 
            (template.StartPeriod == (int)StartPeriodEnum.Sliding) ? DateTime.UtcNow.AddSeconds(-template.Seconds ?? 0) :
            (template.StartPeriod == (int)StartPeriodEnum.Today) ? DateTime.UtcNow.Date :
            (template.StartPeriod == (int)StartPeriodEnum.ThisWeek) ? DateTime.UtcNow.Date.AddDays(-(int)DateTime.UtcNow.DayOfWeek) :   // Sunday = 0
            (template.StartPeriod == (int)StartPeriodEnum.ThisMonth) ? new DateTime(DateTime.UtcNow.Year, DateTime.UtcNow.Month, 1, 0, 0, 0, DateTimeKind.Utc) :
            (template.StartPeriod == (int)StartPeriodEnum.ThisYear) ? new DateTime(DateTime.UtcNow.Year, 1, 1, 0, 0, 0, DateTimeKind.Utc) :
            DateTime.UtcNow     // no matches
        );

    public DateTime TimeStamp
    {
        get { return TimeStampExpression.Evaluate(this); }
    }

      

My query to initialize history table based on (Event.TimeStamp> = Template.TimeStamp):

    foreach (var vgh in (from template in Templates
                        from machineGroup in MachineGroups
                        let q = (from event in Events
                                 join vg in MachineGroupings on event.MachineId equals vg.MachineId
                                 where vg.MachineGroupId == machineGroup.MachineGroupId
                                 where event.TimeStamp >= template.TimeStamp
                                 orderby (template.Highest ? event.Amount : event.EventId) descending
                                 select _makeMachineGroupHistory(event.EventId, template.TemplateId, machineGroup.MachineGroupId))
                        select q.Take(template.MaxResults)).WithTranslations())
        MachineGroupHistories.InsertAllOnSubmit(vgh);

      

A certain maximum number of events is required for each combination of group templates.

Either way, this trick sped up the request by a factor of four or so.

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