Pass-through query losing changes made to SQL

Question

I have a pass-through query MyPassThru . This is a simplified version of the query's SQL:

SET NOCOUNT ON

DECLARE @FromDate datetime = '1/25/2014'
DECLARE @ThruDate datetime = '3/1/2014'

SELECT *
  FROM MtgDailyTrans M
  WHERE M.TransDate >= @FromDate
    AND M.TransDate <= @ThruDate

I need to make changes to the @FromDate and @ThruDate parameters. I wrote the following function to do this ( source for RegExReplace ):

Private gDb As DAO.Database

Function Db() As DAO.Database
    If gDb Is Nothing Then Set gDb = CurrentDb
    Set Db = gDb
End Function

Sub MyPassThruSetDates(FromDate As Date, ThruDate As Date)
    Const FromPattern As String = "(@FromDate datetime = ')([\d/]+)'"
    Const ThruPattern As String = "(@ThruDate datetime = ')([\d/]+)'"
    Dim qd As DAO.QueryDef
    Set qd = Db.QueryDefs("MyPassThru")
    qd.SQL = RegExReplace(FromPattern, qd.SQL, "$1" & Format(FromDate, "m/d/yyyy") & "'")
    qd.SQL = RegExReplace(ThruPattern, qd.SQL, "$1" & Format(ThruDate, "m/d/yyyy") & "'")
    Set qd = Nothing
End Sub

The problem is that when I make changes to the SQL for MyPassThru and then run the MyPassThruSetDates() procedure, the changes I made to the SQL get rolled back. Why?

Answer 1

The reason this is happening is that the persistent Db object is caching a copy of the QueryDef and its properties (to include the .SQL property). If you call MyPassThruSetDates , then make a change to the SQL for MyPassThru , then call MyPassThruSetDates again, the original SQL overwrites any changes made since the original call to MyPassThruDates .

The solution is to refresh the QueryDefs collection to ensure it is using the most current values:

Sub MyPassThruSetDates(FromDate As Date, ThruDate As Date)
    Const FromPattern As String = "(@FromDate datetime = ')([\d/]+)'"
    Const ThruPattern As String = "(@ThruDate datetime = ')([\d/]+)'"
    Dim qd As DAO.QueryDef
    Db.QueryDefs.Refresh   ' <--- This is the key!!!
    Set qd = Db.QueryDefs("MyPassThru")
    qd.SQL = RegExReplace(FromPattern, qd.SQL, "$1" & Format(FromDate, "m/d/yyyy") & "'")
    qd.SQL = RegExReplace(ThruPattern, qd.SQL, "$1" & Format(ThruDate, "m/d/yyyy") & "'")
    Set qd = Nothing
End Sub

---

For further explanation of why this happens, refer to the following heavily-commented test routine:

Sub TestDbCache()
    Const QName As String = "TempQry"

    Dim qd As DAO.QueryDef, db As DAO.Database

    'First, we create a querydef
    Set db = CurrentDb
    Set qd = db.CreateQueryDef(QName, "SELECT 'original'")
    Debug.Print qd.SQL                           '--> SELECT 'original';

    'Next, we update the querydef's .SQL outside the scope of our db object
    CurrentDb.QueryDefs(QName).SQL = "SELECT 'changed'"

    'The querydef and db objects are unaware of the change to .SQL
    Debug.Print qd.SQL                           '--> SELECT 'original';
    Debug.Print db.QueryDefs(QName).SQL          '--> SELECT 'original';
    Debug.Print CurrentDb.QueryDefs(QName).SQL   '--> SELECT 'changed';

    'Refreshing the collection updates both the db and qd objects
    db.QueryDefs.Refresh
    Debug.Print qd.SQL                           '--> SELECT 'changed';
    Debug.Print db.QueryDefs(QName).SQL          '--> SELECT 'changed';

    'Note that the .SQL is "SELECT 'changed'" when we set the NewDb object
    Dim NewDb As DAO.Database
    Set NewDb = CurrentDb

    'We change the .SQL without refreshing the NewDb's QueryDefs collection
    CurrentDb.QueryDefs(QName).SQL = "SELECT 'changed again'"

    'Since the NewDb object never cached the contents of the query,
    '   it returns the correct current value of .SQL
    Debug.Print NewDb.QueryDefs(QName).SQL       '--> SELECT 'changed again';

    'The other db object has not refreshed its QueryDefs collection,
    '   so it is wrong once again
    Debug.Print qd.SQL                           '--> SELECT 'changed';
    Debug.Print db.QueryDefs(QName).SQL          '--> SELECT 'changed';
End Sub