Can't Get Mdx Parameter To Refresh SQL Data Set

Hi, I have an OLAP data source that includes a date field "DateLongName" (has format e.g. May 08, 2008) , on which I checked the box when setting up the data set to indicate it is a parameter. This of course creates the attendant MDX data set that can be used for available values so that the user can pick from the various dates to choose the date they want to filter data on. So far so good.

Also in this report I have a relational data source that populates a chart that I would like to update when the user chooses the date from the above OLAP data source.

My thoughts were to create another MDX data source (called DayConstraint) like

SELECT {} ON COLUMNS, (STRTOMEMBER(@DimDateDateLongName,CONSTRAINED)) ON ROWS FROM [DailySalesCube]

Then, for my relational source, it would just be Select * from table where date=@DayConstraint

Nice in theory, but the DayConstraint variable doesn't change when the user changes the value in the olap picklist. So, the "DateLongName" has a default of previous day, so if it was May 8th, both the "DateLongName" and "DayConstraint" show May 7th. But, when you pick a different date (from the available values when the report has ran), say May 6th, the "DateLongName" changes, but the "DayConstraint" doesn't update, so of course my relational data set doesn't update.

If I pull in the DayConstraint data into a text box to look at the value (=First(Fields!Date_Long_Name.Value, "DayConstraint")), the data set is updating correctly when one changes the values in the OLAP pick list. Wish there was some way in my relational query to do something like Select * from table where date=Fields!Date_Long_Name.Value, "DayConstraint", but can't see anyway to make this work.

I am starting to think you can't use cascading parameters like this - can anyone confirm this, and if I am trying to do the impossible, any ideas on how this could be done in another fashion? Have been beating my head against the desk for a couple days on this, so any help would be very much appreciated.

Thanks!