SUMMARY
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Apologies - I looked for a route to submit an enhancement request but didn't see one: using this as the next best thing... 

Umbrello does not presently have a mechanism to express the existence of composite keys, for example when creating a logical association between two data elements via a link table... The link table would hold fields for the foreign keys of each of the two tables being linked... along with a unique, auto-incrementing primary key. To prevent a scenario in which additional connections are made between the same pair of referenced records, a composite key can be created in the link table by combining both of the foreign keys. 

It is currently possible to set a Foreign Key entity constraint to a data element, but it is not possible to combine two columns in to a composite key. 
***


STEPS TO REPRODUCE
1. Create 3 tables
2. In the 3rd table, create Foreign Key fields linking to the Primary Keys of the 1st and 2nd Tables, respectively
3. Try and fail to annotate that the two Foreign Keys in the 3rd [link] table should be treated as a unique [composite] key, to allow the database to prevent creation of duplicate links. 

OBSERVED RESULT


EXPECTED RESULT
It is not possible to express composite keys in Umbrello


SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION