Unanswered: Expression field in query won't show up in table
This is probably very simple, but I can't get this to work...
I have a form (TRDXCal_frm) based on a query (TRDXCal_Qry), which holds a table called (TRDX_Calibration_tbl). In this query I have a field expression I use to calculate a date: (NextCalDate) which adds the fields (TRDXCalIntervalNumber) and (TRDXLastCalDate).(TRDXCalIntervalNumber) is a number that represents months, and (TRDXLastCalDate) is a date field.
You can't "as is." Select queries don't update their underlying tables. And even though there are other types (Update, for example) the queries underlying forms are generally SELECT queries.
You'll probably want to do something slightly different. Try adding an OnOpen or OnLoad event to your form that reads the two fields, concatenates them (the same way your query did) and then writes them to TRDXNextCalDate. You'll need to change the ControlSource property of the field on your form that showed the new date to point to the TRDXNextCalDate field (which is initially empty) instead of to your expression field from your query.
That approach only makes sense if, by definition, opening the form is meant to initiate the creation of this next date. It does not make sense if this form is used for other reasons (managing data, viewing existing records, etc.). If that's the case, add a button and put the code for updating the field in its onClick event.