If you set up two calculation fields in System WorkPlan, one can list all the IDs from Maintenance to list all available tasks and one can list all IDs from System Task to list all selected tasks. These can then be used in a relationship to yet another occurrence of Maintenance to produce the desired portal. No portal filter is used in this example:
List ( System Task::Maintenance Task ID )
List ( Maintenance::Maintenance Task ID )
System Workplan::cTaskListByType = Maintenance Tasks | available::Maintenance Task ID AND
System Workplan::cSelectedTaskList ≠ Maintenance Tasks | available::Maintenance Task ID
A portal to Maintenance Tasks | available will then show only those tasks not yet selected. This same relationship, BTW, can be used to produce a value list of maintenance tasks that only lists available tasks and that dwindles with each new selection.
On another note, it looks like you have a problem with the field options in Maintenance Task ID in the System Task table. I should see "crows feet" on the end of the relationship line that I don't see. This suggests that this field is either defined with an auto-enter serial number or unique values validation field option and eithe such setting must be removed for this field in this table in order for these relationships to work.
Thanks a lot!
It is a pleasure to meet people so prepared!
I have some problem to understand what it mean 'crows feed'. If I remove the auto enter option how can I give an unique value to the task id?
The Maintenance task ID must be unique in the maintenance table. It cannot be unique in the System Task table as there will be multiple records, all linked to different System Workplan records that all have the same Maintenance Task ID as they all link to the same record in the Maintenance table.