Question

I have a “manager” class with a number of sub-classes. I find one particular method being duplicated in all or almost all of the sub-classes, so I want to generalize it. In one case, it looks like this:

                        var Results =
                        from j in Job.All( )
                        where guids.Contains( j.JobID )
                        orderby j.JobNumber
                        select j;

I am using SubSonic, so Job.All( ) is a static method that returns an IQueryable<Job>. guids is an IEnumerable<Guid> which holds a list of keys into the Job table. My final result is a SQL query with WHERE Job.JobID IN (x, y, z).

What I’m looking for is a way to call a method like this where I pass in guids, “Job”, “JobID” and “JobNumber” so that I can plug them into the appropriate places.

There is a limit to the number of parameters a SQL IN clause can handle, so my general method would also check guids.Count and code a little bit different query when it exceeded a certain number (say, 2000). This code is also in all the sub-classes.

Note: I have no objection to using the lambda-style notation. This is my first cut, which has problems with “T.All( )”, and item.???:

    const int MAX_ITEMS = 2000;
    public List<T> GetFromList<T>( List<Guid> _IDs ) 
        where T : class, IActiveRecord
    {
        List<T> rc;
        if ( MAX_ITEMS > _IDs.Count )
        {
            var Results =
                from item in T.All( )
                where _IDs.Contains( item.??? )
                orderby item.???
                select item;
            rc = Results.ToList<T>( );
        }
        else // too many for IN clause
        {
            var Results =
                from id in _IDs
                join item in T.All( ) on id equals item.???
                orderby item.???
                select item;
            rc = Results.ToList<T>( );
        }
        return rc;
    }

Answer 1

const int MAX_ITEMS = 2000; 
public List<T> GetFromList<T>( List<Guid> _IDs, T _db, 
              Expression<Func<T, Guid>> GetID, 
              Expression<Func<IQueryable<T>> GetAll  )  
    where T : class, IActiveRecord 
{ 
    List<T> rc; 
    if ( MAX_ITEMS > _IDs.Count ) 
    { 
        var Results = 
            from item in GetAll( ) 
            where _IDs.Contains( GetID(item)) 
            orderby GetID(item) 
            select item; 
        rc = Results.ToList<T>( ); 
    } 
    else // too many for IN clause 
    { 
        var Results = 
            from id in _IDs 
            join item in T.All( ) on id equals GetID(item) 
            orderby GetID(item)
            select item; 
        rc = Results.ToList<T>( ); 
    } 
    return rc; 
}

called by:

     GetFromList(ids, db, item=>item.JobID, ()=> Job.All());