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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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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