Is it possible to create an extension method that can be used in an sql query? - sql

Question

I have an application that draws elements from a database when a search button is clicked. I need to update it so that I can find elements that have a specific Substring exactly 6 places into the string. For example, I need to find 111-2233-44-555 by looking for 33 at the 6th and 7th places. My first instinct was to create an extension method for the string class so that I could say something like this:
Dim example As String = "111-2233-44-555"
If example.HasYear(33) Then
'Do Something'
End If
And this worked perfectly. Here's the method I made:
Public Module StringExtensionMethods
''' <summary>
''' Finds the year in a competition number of format XXX-XXXX-XX-XXX
''' </summary>
''' <param name="pstrCompNum">The competition number to find the year in</param>
<Extension()>
Public Function HasYear(ByVal pstrCompNum As String, ByVal pstrCompYear As String) As Boolean
Try
Dim testString As String = pstrCompNum
Debug.Print(testString)
Dim testSubstring As String = testString.Substring(6, 2)
If testSubstring.Equals(pstrCompYear) Then
Return True
End If
Return False
Catch ex As Exception
Throw ex
End Try
End Function
End Module
But a problem arises when I try to use this method in an SQL query. Rightfully so, since HasYear() isn't related to SQL in any way whatsoever. Here's the query I want to execute:
Dim o = From c In myContext.Competitions.Include("CodeJusticeBranches").Include("CodeJusticeLocations").Include("CodeCompetitionTypes").Include("CodePositionTypes").Include("CompetitionPositions") _
Where (pstrCompNum Is Nothing OrElse c.comp_number = pstrCompNum) _
And (pstrCompYear Is Nothing OrElse c.comp_number.HasYear(strYear) = True) _
And (pstrCompTypeId Is Nothing OrElse c.CodeCompetitionTypes.code_ct_id = CInt(pstrCompTypeId)) _
And (pstrBranchId Is Nothing OrElse c.CodeJusticeBranches.code_branch_id = CInt(pstrBranchId)) _
And (pstrPosTypeId Is Nothing OrElse c.CodePositionTypes.code_pos_type_id = CInt(pstrPosTypeId)) _
Order By c.comp_number _
Select c
I get the error LINQ to Entities does not recognize the method 'Boolean HasYear (System.String, System.String)' method, and this method cannot be translated into a store expression.
So what I'm looking for is essentially a way to make an extension method that can be used in an SQL query. Any ideas?

Answer 1

Don't include custom functions in your LINQ. It must be convertable to SQL so your extension method doesn't qualify. Just execute your query with supported where clauses, then tack on your custom queries after.
Dim o = (
From c In myContext.Competitions.Include("CodeJusticeBranches").Include("CodeJusticeLocations").Include("CodeCompetitionTypes").Include("CodePositionTypes").Include("CompetitionPositions")
Where (pstrCompNum Is Nothing OrElse c.comp_number = pstrCompNum) _
And (pstrCompTypeId Is Nothing OrElse c.CodeCompetitionTypes.code_ct_id = CInt(pstrCompTypeId)) _
And (pstrBranchId Is Nothing OrElse c.CodeJusticeBranches.code_branch_id = CInt(pstrBranchId)) _
And (pstrPosTypeId Is Nothing OrElse c.CodePositionTypes.code_pos_type_id = CInt(pstrPosTypeId))
Order By c.comp_number Select c).ToList().
Where(Function(c) c.pstrCompYear Is Nothing OrElse c.comp_number.HasYear(strYear))

Answer 2

I like what #djv posted, but I've decided to try out Andrew Morton's suggestion, to just omit the HasYear() altogether. I ended up using this:
Dim k = From c In myContext.Competitions.Include("CodeJusticeBranches").Include("CodeJusticeLocations").Include("CodeCompetitionTypes").Include("CodePositionTypes").Include("CompetitionPositions") _
Where (pstrCompNum Is Nothing OrElse c.comp_number = pstrCompNum) _
And (pstrCompYear Is Nothing OrElse c.comp_number.Substring(6, 2) = pstrCompYear) _
And (pstrCompTypeId Is Nothing OrElse c.CodeCompetitionTypes.code_ct_id = CInt(pstrCompTypeId)) _
And (pstrBranchId Is Nothing OrElse c.CodeJusticeBranches.code_branch_id = CInt(pstrBranchId)) _
And (pstrPosTypeId Is Nothing OrElse c.CodePositionTypes.code_pos_type_id = CInt(pstrPosTypeId)) _
Order By c.comp_number _
Select c
Notice the line And (pstrCompYear Is Nothing OrElse c.comp_number.Substring(6, 2) = pstrCompYear)
This solved my problem very simply.