This was asked and answered before, but long ago and a very early version of SDDM.
https://community.oracle.com/tech/developers/discussion/2592898/how-to-add-a-foreign-to-an-existing-arc
I am using SDDM stand-alone 22.2.0.165 (64-bit on Windows 10 64-bit).
In the relational model, I have selected in all combinations between the general table (that the foreign keys point to), the foreign key(s), the Arc, but the "Add Foreign to Arc" icon remains elusive/does not light up/become enabled.
The discriminator column is defined/populated on the foreign keys, with its relevant discriminator value.
On the Arc (General) the discriminator column is also selected.
When I click on only the Arc, both the Add and Remove Foreign key to/from Arc icons enable. As soon as anything else is selected with it (shift or ctrl click) it disables again.
So I have to delete the arc every time and re-create it.
Is there any way to get the "Add Foreign Key to Arc" to work (and by implications its inverse, "Remove...")?