pátek 21. března 2014

EF: intColumn.ToString() doesn't work in LINQ to Entities, SqlFunctions.StringConvert outside

As you may already know LINQ to Entities will not let you do something like
  "Blah blah blah " + t.charColumn + " (" + t.intColumn.ToString() + ")"
in a query. What quite a few do not know is that you can instead use 
  "Blah blah blah " + t.charColumn + " (" + System.Data.Objects.SqlClient.SqlFunctions.StringConvert(t.intColumn) + ")"

there is a catch though ... this will ONLY work inside a query! So if you happen to have an entity already, you can't use this ... it would probably be too hard to implement, much harder than throwing an exception ...

Why do I care? Generated code. Suppose you have quite a few lookup tables (countries, types of this, types of that, regions, ... anything really) and you need to populate the pulldowns. Sometimes it's enough to display one property, sometimes you want to do something a bit more complicated ... like include the ID in the displayed text. To simplify things for the developers you extend the EF designer so that they could specify that an entity may be turned into a dropdown and what's the property/expression to display. And the tweaked .tt file generates the interface implementation and the extension methods that turn an IQueryable of entities to an IEnumerable<SelectListItems> etc. etc. etc. With the extension methods written so that we do not fetch the whole entities, but rather Select() just what we need. 
And here comes the catch of course. The same code to compute the text to display is used in
 query.Select(e => new {Value = e.Id, Text = HERE}).AsEnumerable().Select(s => new SelectListItem{Id = s.Id, Text = s.Text})
and in
 string IListItem.Text { { get { return HERE } }
where the later is used when I already have an entity and need to display the text as in the drop down.
And seems if I need to include the ID in the text there is no solution that'd work both in LINQ to Entities and outside.
Or am I wrong?

Yes, I am wrong. There is a solution. The same one I used to allow me to use LIKE in LINQ to Entities queries. 

If I add this into the .edmx into <edmx:Runtime><edmx:ConceptualModels><Schema> :
<Function Name="Int32_SqlToString" ReturnType="Edm.String" ef4ex:RenameTo="SqlToString">
  <Parameter Name="val" Type="Edm.Int32" />
   Cast(val as Edm.String)
   return val.ToString();

[System.Data.Objects.DataClasses.EdmFunction( "Xxxx.Models", "Int32_SqlToString")]
 public static String SqlToString(this Int32 val) {
  return val.ToString();

into a static class (it's actually generated using the data from the ef4ex tags and attributes), then I can use
The only problem is that in a project where I'd need two different models based on two .edmx files I'd have to define the function twice and then could only import one or the other namespace and there's be nothing preventing me from using the wrong .SqlToString() triggering a run time error.

To solve that I would have to be allowed to put several [System.Data.Objects.DataClasses.EdmFunction()] attributes to one extension method and the LINQ to Entities would have to select the right EntitySQL implementation based on the namespaceName.
It's a shame there is (as far as I know) no way to attach the EdmFunction mapping to a method written by someone else in a way the LINQ to Entities provider would understand and thus give it a hint how to convert the method into the SQL :-(

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