sparlaman said:
I've read through some of the posts here and I just can't quite understand
how to do this. I have 3 tables: teachers, subjects, &
subjects_to_teachers.
I have relationships established for a many to many relationship btwn
subjects & teachers. I have the base form which shows teacher info. How do
I
create a control on this form that allows the user to select all subjects
that apply to a teacher? I would like to use check boxes where the user
checks all that apply but I'm also interested in knowing how to use a
multiselect list box to do the same.
I've used checkboxes on a subform, but found it cumbersome and fragile.
I've used a multiselect list box effectively, like this:
*** Storing Multiple Selections From A List Box ***
The best way to store multiple items in a single field is not to do it
at all. Instead, use multiple records in a related table to represent
these items. It's a principle of relational database design that a
single field (column) holds only one datum.
An Access subform is designed to display and edit multiple records from
a related table (these records being related to the record currently
displayed on the main form), and does it with no code at all. A list
box isn't designed to do this, but for small "pick-lists" I do like the
multiselect list box. However, you need to use code to read the related
records for each new main record and select the appropriate items in the
list box, and then whenever the list box is updated you need to use code
to update the set of records in the related table.
Here's code from a sample form that represents "family members", with a
list box named "lstHobbies" that represents, for each family member,
that person's hobbies from the list of all possible hobbies.
'----- start of code -----
Option Compare Database
Option Explicit
Private Sub ClearHobbySelections()
Dim intI As Integer
With Me.lstHobbies
For intI = (.ItemsSelected.Count - 1) To 0 Step -1
.Selected(.ItemsSelected(intI)) = False
Next intI
End With
End Sub
Private Sub Form_Current()
Dim rs As DAO.Recordset
Dim intI As Integer
' Clear all currently selected hobbies.
ClearHobbySelections
If Not Me.NewRecord Then
Set rs = CurrentDb.OpenRecordset( _
"SELECT HobbyID FROM tblFamilyMembersHobbies " & _
"WHERE MemberID=" & Me.MemberID)
' Select the hobbies currently on record for this MemberID.
With Me.lstHobbies
Do Until rs.EOF
For intI = 0 To (.ListCount - 1)
If .ItemData(intI) = CStr(rs!HobbyID) Then
.Selected(intI) = True
Exit For
End If
Next intI
rs.MoveNext
Loop
rs.Close
Set rs = Nothing
End With
End If
End Sub
Private Sub lstHobbies_AfterUpdate()
On Error GoTo Err_lstHobbies_AfterUpdate
Dim db As DAO.Database
Dim ws As DAO.Workspace
Dim strSQL As String
Dim blnInTransaction As Boolean
Dim varItem As Variant
' Make sure the current member record has been saved.
If Me.Dirty Then Me.Dirty = False
Set ws = Workspaces(0)
Set db = ws.Databases(0)
ws.BeginTrans
blnInTransaction = True
' Delete all hobbies now on record.
strSQL = "DELETE FROM tblFamilyMembersHobbies " & _
"WHERE Memberid = " & Me.MemberID
db.Execute strSQL, dbFailOnError
' Add each hobby selected in the list box.
With Me.lstHobbies
For Each varItem In .ItemsSelected
strSQL = _
"INSERT INTO tblFamilyMembersHobbies " & _
"(MemberID, HobbyID) VALUES (" & _
Me.MemberID & ", " & .ItemData(varItem) & ")"
db.Execute strSQL, dbFailOnError
Next varItem
End With
ws.CommitTrans
blnInTransaction = False
Exit_lstHobbies_AfterUpdate:
Set db = Nothing
Set ws = Nothing
Exit Sub
Err_lstHobbies_AfterUpdate:
MsgBox "Error " & Err.Number & ": " & Err.Description, _
vbExclamation, "Unable to Update"
If blnInTransaction Then
ws.Rollback
blnInTransaction = False
End If
Resume Exit_lstHobbies_AfterUpdate
End Sub
Private Sub lstHobbies_BeforeUpdate(Cancel As Integer)
Dim intI As Integer
' Don't allow hobbies to be updated before a MemberID has
' been generated.
If IsNull(Me.MemberID) Then
MsgBox "Please enter other information for this family " & _
"member before choosing hobbies.", , _
"Define Member First"
Cancel = True
Me.lstHobbies.Undo
' Clear the user's selection.
ClearHobbySelections
End If
End Sub
'----- end of code -----
As you see, there's a fair amount of code involved, because we're using
the list box to do something it wasn't built to do, but it works quite
nicely.
