Alias properties in LINQ to SQL

Question

I'm using a shim property to make sure that the date is always UTC. This in itself is pretty simple but now I want to query on the data. I don't want to expose the underlying property, instead I want queries to use the shim property. What I'm having trouble with is mapping 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;

Attempting to execute the query results in an exception:

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

This makes sense - how could LINQ to SQL know how to treat the Started property - it's not a column or association? But, I was looking for something like a ColumnAliasAttribute that tells SQL to treat properties of Started as Started_ (with underscore).

Is there a way to help LINQ to SQL translate the expression tree to the Started property can be used just like the Started_ property?

Answer 1

There's a code sample showing how to do that (i.e. use client-side properties in queries) on Damien Guard's blog:

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

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

E.g.:

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

Answer 2

Based on @KrstoferA's answer I came up with a reliable solution that hides the fact that the properties are aliased from client code. Since I'm using the repository pattern returning an 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 ) ); 
        }
    }
}