Add shape 'Arch' for ERD subtype relations

Question

In ERD diagrams subtypes are not drawn directly to the supertype but connect onto an arch first. In this arch one can specify an optional xor. The shape for an arch is missing from PlantUML.

The closest I've gotten is using the following snippet. In there the allow_mixing is turned on, and three options are tested x**.

Having more standard shapes in PlantUML would help a lot already. One of them being halve a circle, or maybe even an invisible 'point' to connect to/from.

@startuml

' need to allow mixing to accomplish something with mixed shapes
allow_mixing

' rename the x* variants into x1 to see effect
() "xor\nsubtypes" as x1
stack "xor" as x_1
label "xor\nsubtypes" as x__1

' super/subtype relations using x1 as intermediate connection
Super <|-- x1
x1 -- SubA
x1 -- SubB
@enduml

Answer 1

The following plantuml script will render an ERD. The "()" rendering a circle, could be replaced by "(|" to render the arch as commonly used in ERD in such cases.

@startuml
skinparam dpi 120
hide circle
hide stereotype
hide empty members

entity "<&link-intact>  ** Memo ** " as memo  {
  <color orange><&key></color> Article <&link-intact>
  }
entity "<&link-intact>  ** Document ** " as document {
  <color orange><&key></color> Article <&link-intact>
  }
entity " ** Article ** " as article  {
  <color orange><&key></color> Article No
    Article type
  }

() "Either" as xor1
xor1 ||--|> article
memo |o-- xor1
document |o-- xor1  

' instead of () the symbol for XOR could be simply a halve circle or arch
' depicted by (|

@enduml