Querying distinct values - breeze

Question

i am trying to get this to work for hours now. I tried the examples and read documentation but i just cant figure it out.
I want to query distinct values from the DB with a name field.
I think this should work but it does not. The method Distinct is not found
[HttpGet]
public IQueryable<MvlOP> MvlOpsPerson(long mvlId)
{
System.Data.Entity.Infrastructure.DbQuery<MvlOP> query = ContextProvider.Context.MvlOps;
query = query.Include("StatusOP");
return query.Where(op => op.MvlId == mvlId).Distinct(new PropertyComparer<MvlOP>("Id_P"));
}
I get the following error:
ExceptionMessage=LINQ to Entities does not recognize the method 'System.Linq.IQueryable`1[MAHN.Model.MvlOP] Distinct[MvlOP](System.Linq.IQueryable`1[MAHN.Model.MvlOP], System.Collections.Generic.IEqualityComparer`1[MAHN.Model.MvlOP])' method, and this method cannot be translated into a store expression.
ExceptionType=System.NotSupportedException
So this is wrong. As far as i understand it breeze does not offer querying distinct values. Querying all and filtering is not an option. Any help on how this can be done much appreciated.

Answer 1

I post this so someone who might need it can use it. Maybe this can be improved in some way. (Inspired by Breeze DocCode (thanks for the Partial Class Api Method in the Northwind Controller :) )
Querying distinct values can be done this way:
To be able to use the Distinct(IEqualityComparer) method the query has to be in Memory as an IEnumerable. An EqualityComparer cannot be translated into an SQL-Statement.
So the where clause applies and the resulting records are then filtered by the Comparer.
return query.AsQueryable(): To be able to use skip/take and inlineCount the query has to be an IQueryable<T>. Therefor the method AsQueryable().
//The API Method ----------
[HttpGet]
public IQueryable<MvlOPPersonPartial> MvlOpsPerson(long mvlId)
{
var query = (from op in ContextProvider.Context.MvlOps
where op.MvlId == mvlId
select new MvlOPPersonPartial()
{
MvlId = op.MvlId,
Kundenname = op.Kundenname,
Kundennummer = op.Kundennummer,
Id_P = op.Id_P
})
.AsEnumerable()
.Distinct(new PropertyComparer<MvlOPPersonPartial>("Id_P"));
return query.AsQueryable();
}
public class MvlOp
{
...
public string Kostenstelle { get; set; }
public string Betreuer_Id { get; set; }
public decimal? Id_P { get; set; }
public string Kundenname { get; set; }
public string Kundennummer { get; set; }
public string Person_Typ1 { get; set; }
...
}
//The partial class needed for distinct values ------------
//MvlOP cannot be constructed in an LINQ to Entities query
public class MvlOPPersonPartial
{
public long MvlId { get; set; }
public string Kundenname { get; set; }
public string Kundennummer { get; set; }
public decimal? Id_P { get; set; }
}
//A generic comparer ---------------
public class PropertyComparer<T> : IEqualityComparer<T>
{
private PropertyInfo _PropertyInfo;
/// <summary>
/// Creates a new instance of PropertyComparer.
/// </summary>
/// <param name="propertyName">The name of the property on type T
/// to perform the comparison on.</param>
public PropertyComparer(string propertyName)
{
//store a reference to the property info object for use during the comparison
_PropertyInfo = typeof(T).GetProperty(propertyName,
BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.Public);
if (_PropertyInfo == null)
{
throw new ArgumentException(string.Format("{0} is not a property of type {1}.", propertyName, typeof(T)));
}
}
#region IEqualityComparer<T> Members
public bool Equals(T x, T y)
{
//get the current value of the comparison property of x and of y
object xValue = _PropertyInfo.GetValue(x, null);
object yValue = _PropertyInfo.GetValue(y, null);
//if the xValue is null then we consider them equal if and only if yValue is null
if (xValue == null)
return yValue == null;
//use the default comparer for whatever type the comparison property is.
return xValue.Equals(yValue);
}
public int GetHashCode(T obj)
{
//get the value of the comparison property out of obj
object propertyValue = _PropertyInfo.GetValue(obj, null);
if (propertyValue == null)
return 0;
else
return propertyValue.GetHashCode();
}
#endregion
}

Answer 2

I think that the problem is that the Entity Framework (EF) cannot use your PropertyComparer. EF only supports Distinct without a comparer.