would appear to be the "backbone" of your relationships.
How have you implemented the relationship to T_Attributes_Not_Linked?
To get a portal to filter on a category selected in a field, do this:
Define a field with global storage, Globals::gCategory. Place it on your layout. Format it with a value list of your categories.
Assuming you have this relationship for the portal:
T_Objects::anyfield X All_Attributes::Anyfield
Define this Filter expression (Requires Filemaker 11):
All_Attributes::Category = Globals::gCategory
All Attributes is an occurrence of T_Attributes.
Add the OnObjectModify (pop up menu) or OnObjectSave (drop down list) trigger to gCategory to perform this script:
Refresh Window [Flush Cached Join Results]
To update the portal when you make a selection.
Here's a download link to a fairly new demo file on many to many relationships that I have created. It includes a check box selection method you can probably adapt with your filtered portal to make the needed selections: http://www.4shared.com/file/dZ0bjclw/ManyToManywDemoWExtras.html
Woah ! I am impressed. I was not expecting an answer so soon. Thank you.