Merefolk

An Obsidian plugin that renders Mermaid entity-relationship diagrams as beautiful, editable Excalidraw drawings — not the rasterized image fallback you get from @excalidraw/mermaid-to-excalidraw (which does not natively support ER diagrams).

Website: merefolk.amberlogica.com · Support: Buy me a coffee ☕

Write a normal ```mermaid erDiagram block. Merefolk replaces it inline with a hand-drawn Excalidraw render — crow's-foot cardinality, clean orthogonal routing, non-crowding labels — and lets you open it in a full Excalidraw canvas to edit, with your changes saved back and shown in the note.

Install

[!NOTE] Merefolk isn't in the Obsidian community store yet. Until it is, install it via BRAT or build it from source — see Development.

Merefolk is desktop-only and requires Excalidraw 0.18+ to be installed and enabled (for crow's-foot arrowheads).

Usage

1. In any note, write a fenced ```mermaid block containing an erDiagram:

   ```mermaid
   erDiagram
       CUSTOMER ||--o{ ORDER : places
       ORDER ||--|{ LINE_ITEM : contains
       CUSTOMER {
           string name
           string email PK
       }
   ```
   

2. Switch to Reading view or Live Preview. Merefolk replaces the block inline with a hand-drawn Excalidraw diagram — crow's-foot cardinality, clean orthogonal routing, and real table rows. It scales to fit the note and repaints when you switch light/dark theme.

3. Hover the diagram and use the button group:

   • Open — launches the diagram in a full, editable Excalidraw canvas.
   • Copy — copies the rendered SVG.

4. Edit in the canvas. Your changes save to a companion .excalidraw scene, and the inline render prefers that saved scene from then on.

5. Changed your mind? On an edited diagram, use Re-sync from Mermaid to discard the saved scene and regenerate from the source block.

[!NOTE] The Mermaid text is the source of truth until your first edit — after that, the saved scene wins.

Three entry points open a diagram: the inline Open button, the command "Convert Mermaid ER diagram to Excalidraw", and the ribbon icon (Merefolk: ER → Excalidraw).

Settings

Open Settings → Merefolk.

[!NOTE] Changing the scene folder does not move existing scenes.

Features

• Native crow's-foot cardinality per side (||, |o, o{, }o → Excalidraw 0.18 crow's-foot arrowheads).
• ELK layout with orthogonal, box-avoiding arrow routing and edge-label placement (labels don't crowd the lines).
• Real table rendering — each entity is a table: centered header, divider, aligned [keys] [type] [name] columns (one element per cell), and subtle row separators.
• Theme-aware colors (light/dark), with live repaint on theme switch.
• Clean defaults — Comic Shanns (mono, easy to read), small text, "architect" (clean) roughness.
• Edit + persist — Open into a live Excalidraw canvas; edits save to a .excalidraw scene and the inline render prefers it. A Re-sync button discards edits and regenerates from the source.
• Robust to malformed input — unrecognized lines are flagged with a ⚠ badge (never silently dropped); a totally invalid block falls back to Mermaid's own error; non-ER Mermaid is left untouched.

Architecture

Mermaid `erDiagram`
  └─ src/mermaid/parseER.ts     parse -> ERModel (+ skipped lines)
     └─ src/layout/elkLayout.ts   ELK layered layout -> positions, edge routes, label positions
        └─ src/excalidraw/emit.ts   -> native Excalidraw elements (theme colors, crow's-foot)
           ├─ src/excalidraw/toSvg.ts   exportToSvg -> inline render (Feature 2)
           └─ src/view/*               React canvas in an ItemView -> edit + persist (Feature 1)

Scene persistence: src/scene/ (stable %% merefolk:<id> block id + .excalidraw files under .obsidian/merefolk/scenes/).

Development

Merefolk isn't in the community store yet. To run it from source:

npm install
npm run dev      # esbuild watch -> main.js

Symlink (or junction) this repo into a test vault's plugins folder so Obsidian loads it, then enable Merefolk under Settings → Community plugins:

# Windows (no admin needed): junction the repo into the vault
New-Item -ItemType Junction -Path "C:\path\to\Vault\.obsidian\plugins\merefolk" -Target (Get-Location)

npm run build    # type-check + production bundle

Open test/sample-er.md (which also contains malformed/edge-case fixtures) to exercise every path.

Hard-won implementation notes

[!IMPORTANT] Requires Excalidraw 0.18+ — crow's-foot arrowheads (crowfoot_one / crowfoot_many / crowfoot_one_or_many) don't exist before it.

1. 0.18 build wiring — 0.18 ships conditional exports and its CSS subpath has no default condition, so esbuild.config.mjs pins conditions: ["production"]. 0.18 does NOT auto-inject its stylesheet, so it's imported and bundled as a string via the .css -> text loader, then injected at runtime (src/globals.d.ts declares *.css).
2. Bound arrows need explicit x/y + points — otherwise convertToExcalidrawElements derives NaN geometry, which cascades to a NaN zoom and a call-stack overflow. Arrows use ELK's route points.
3. elbowed doesn't route in a static export — elbow routing is interaction-time only. We use ELK's orthogonal edge.sections for the arrow points instead.
4. Load scenes via initialData on a fresh mount, never setState from the excalidrawAPI render-phase callback (infinite render loop).
5. Theme: colors are baked per active theme (a fixed dark stroke is invisible on a dark background); inline renders re-derive on the css-change event.
6. Bundle: aliased out Excalidraw's built-in @excalidraw/mermaid-to-excalidraw dialog dep (cytoscape/katex, ~6 MB) via src/stubs/ — it's only lazily imported and we never open that dialog.

Roadmap

• Settings tab for the scene-folder location.
• "Re-sync from Mermaid" escape hatch — a per-diagram button (shown only on edited scenes) that discards edits and regenerates from the source.
• Render each attribute as its own row — a real table (centered header + divider + aligned [keys] [type] [name] columns), one element per cell.
• Swap the hand-written parser for Mermaid's own ER parser (full syntax fidelity, at the cost of the mermaid dep).
• Bundle Excalidraw assets locally for offline vaults.
• Unit tests for parseER / elkLayout (Vitest).

Support

If Merefolk is useful to you, you can support development:

• ☕ Buy me a coffee
• 🌐 merefolk.amberlogica.com

License

MIT © Josh Quiachon / Amberlogica