Question

I’m a bit new to vb.net and used to working in perl, this is what I’d like to do.

I wanted something similar to DBIX::Class::Resultset’s search (from cpan) in my vb.net project, so that I can give my function a hash containing keys and values to search on a table.

Currently it returns a single matching result of type T where I want it to return all results as a data.linq.table(of T)

How should I alter my expression.lambda so that I can say table.Select(Predicate) to get a set of results? After that I think it should be as simple as saying results.intersect(result) instead of Return test.

Any help will be very much appreciated.

Thanks in advance

-Paul

<System.Runtime.CompilerServices.Extension()> _
Public Function Search(Of T As Class)(ByVal context As DataContext, _
                                       ByVal parameters As Hashtable) As T
    Dim table = context.GetTable(Of T)()
    Dim results As Data.Linq.Table(Of T)
    For Each Parameter As DictionaryEntry In parameters
        Dim column As Object = Parameter.Key
        Dim value As String = Parameter.Value
        Dim param = Expression.Parameter(GetType(T), column)
        Dim Predicate = Expression.Lambda(Of Func(Of T, Boolean)) _
                        (Expression.[Call](Expression.Convert(Expression.Property(param, column), _
                            GetType(String)), GetType(String).GetMethod("Contains"), _
                            Expression.Constant(value)), New ParameterExpression() {param})
        Dim test = table.First(Predicate)

        Return test
        ' result.intersect(result)
    Next
    'Return results
End Function

Answer 1

This works assuming you want an “AND” conjunction between predicates

For instance:

    Dim h = New System.Collections.Hashtable
    h.Add("FieldA", "01 5149")
    h.Add("FieldB", "WESTERN")
    Dim t = (New DBDataContext).Search(Of DBrecord)(h)
    Debug.Print(t.Count.ToString)

Would return those records where fieldA matched AND fieldb matched.
If you wanted OR, DiceGuy’s right, use UNION.

Here’s the search…
Note, I used STARTSWITH instead of contains because it’s alot faster for large sets
You can always change it back.

<System.Runtime.CompilerServices.Extension()> _
Public Function Search(Of T As Class)(ByVal context As DataContext, _
                                   ByVal parameters As Hashtable) As IQueryable(Of T)
    Dim table = context.GetTable(Of T)()
    Dim results As IQueryable(Of T) = Nothing
    For Each Parameter As DictionaryEntry In parameters
        Dim column = DirectCast(Parameter.Key, String)
        Dim value As String = DirectCast(Parameter.Value, String)
        Dim param = Expression.Parameter(GetType(T), column)
        Dim Predicate = Expression.Lambda(Of Func(Of T, Boolean)) _
                        (Expression.[Call](Expression.Convert(Expression.Property(param, column), _
                            GetType(String)), GetType(String).GetMethod("StartsWith", New Type() {GetType(String)}), _
                            Expression.Constant(value)), New ParameterExpression() {param})
        Dim r = table.Where(Predicate)
        If results Is Nothing Then
            results = r
        Else
            results = results.Intersect(r)
        End If
    Next
    Return results
End Function