Mermaid NestedGraph does not create transitions between outer states that do not have labels

[oEscal]

Describe the bug
When I create a HierarchicalGraphMachine and try to generate the graph diagram with the mermaid engine, the resultant Mermaid code does not specify the transitions between actions that do not have labels.

Minimal working example
This is the same example as in diagrams, but I have removed the transitions labels:

from transitions.extensions.diagrams import HierarchicalGraphMachine

states = ['A', 'B', {'name': 'C',
                     'final': True,
                     'parallel': [{'name': '1', 'children': ['a', {"name": "b", "final": True}],
                                   'initial': 'a',
                                   'transitions': [['go', 'a', 'b']]},
                                  {'name': '2', 'children': ['a', {"name": "b", "final": True}],
                                   'initial': 'a',
                                   'transitions': [['go', 'a', 'b']]}]}]
transitions = [['', 'C', 'A'], ["", "A", "B"], ["", "B", "C"]]
m = HierarchicalGraphMachine(states=states, transitions=transitions, initial="A", show_conditions=True,
                             title="Mermaid", graph_engine="mermaid", auto_transitions=False)

print(m.get_graph().draw(None))

Expected behavior
The produced Mermaid code should represent the transitions between A, B, and C. However, the code leads to the following graph:

The expected graph is:

[aleneum]

Hi @oEscal,

a transition is usually associated with a 'trigger' or event that will initiate the transition. That's why empty triggers/labels is a case that I haven't considered yet. How come you want to have triggers/events without names in your machine?

[oEscal]

Thank you for your response @aleneum
I was thinking about completion transitions, where the transition happens right after the completion of the previous state's behavior. Therefore, since the trigger is implicit, we should not be forced to define it explicitly.

[aleneum]

Could you show me a minimal example of how you intend to achieve this with transitions? Just so that I get a feel for completion transition. The referenced documentation is rather brief, do you know how completion transitions are usually depicted?

[oEscal]

Maybe here there is a good graphical example: https://d-nb.info/1175470252/34. As the text explains, the transition from AmountEntry to Giving Money is a completion transition and, therefore, does not have any trigger in the figure (nor any label). If you want a Mermaid code example, I have provided one in my original submission above (expected graph).

[aleneum]

I created a branch dev-#706-completion since diagram output is just one issue. This should work:

from transitions.extensions.diagrams import HierarchicalGraphMachine

states = ['A', 'B', {'name': 'C',
                     'final': True,
                     'parallel': [{'name': '1', 'children': ['a', {"name": "b", "final": True}],
                                   'initial': 'a',
                                   'transitions': [['go', 'a', 'b']]},
                                  {'name': '2', 'children': ['a', {"name": "b", "final": True}],
                                   'initial': 'a',
                                   'transitions': [['go', 'a', 'b']]}]}]
transitions = [["start", 'C', 'A'], ["", "A", "B"], ["", "B", "C"]]

# Remove default styling since mermaid can adapt to light and dark themes
del HierarchicalGraphMachine.style_attributes["node"]["default"]
del HierarchicalGraphMachine.style_attributes["node"]["parallel"]

m = HierarchicalGraphMachine(states=states, transitions=transitions, initial="C", show_conditions=True,
                             title="Mermaid", graph_engine="mermaid", auto_transitions=False)

m.start()
print(m.get_graph().draw(None))

I slightly changed your example since having all transitions as completion transitions would cause an infinite loop if I understand the concept correctly.

Output

Mermaid version

---
Mermaid
---
stateDiagram-v2
  direction LR
  classDef s_inactive fill:white,color:black
  classDef s_active color:red,fill:darksalmon
  classDef s_previous color:blue,fill:azure
  
  state "A" as A
  state "B" as B
  state "C" as C
  C --> [*]
  state C {
    state "1" as C_1
    state C_1 {
      [*] --> C_1_a
      state "a" as C_1_a
      state "b" as C_1_b
      C_1_b --> [*]
    }
    --
    state "2" as C_2
    state C_2 {
      [*] --> C_2_a
      state "a" as C_2_a
      state "b" as C_2_b
      C_2_b --> [*]
    }
  }
  class B s_previous
  class C_1_a, C_2_a s_active
  class C_1_b, C_2_b, C, A s_default
  class C_2, C_1 s_parallel
  
  C --> A: start
  A --> B
  B --> C
  C_1 --> C_1_a
  C_2 --> C_2_a
  C_1_a --> C_1_b: go
  C_2_a --> C_2_b: go
  [*] --> C

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