Enhancment: Mermaid style uses classDiagram, not erDiagram

[e12e]

We just looked at an alternative gem that generates Mermaid ER diagrams:rails-mermaid_erd - and noticed it generates far more typical ER diagrams.

The main difference seems to be that rails-erd uses the Mermaid type classDiagram, not erDiagram -- quite possibly this matches better with the old code for dot-diagrams?

At any rate, there seems to be potential to improve Mermaid output considerably, going from:

classDiagram
	direction RL
	class `Course`
	`Course` : +string tag
	`Course` : +string name
	class `CourseType`
	`CourseType` : +string name
	`CourseType` --> `Course`

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to:

erDiagram
  Course {
        integer id PK
        string tag  
        string name  
        integer course_type_id FK
        datetime created_at  
        datetime updated_at  
    }
CourseType {
        integer id PK
        string name  
        datetime created_at  
        datetime updated_at  
    }
Course }o--|| CourseType : "Course of type CourseType"

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erDiagrams are documented at:

https://mermaid.js.org/syntax/entityRelationshipDiagram.html