ER diagrams that render on GitHub

The diagram that lives in the repo

Most ER diagrams die in a drawing tool: exported once as a PNG, pasted into a doc, stale by the second migration. The fix is a diagram that is text — Mermaid's erDiagram renders natively on GitHub, in most editors and docs sites, and diffs like any other file.

The ER designer is a free, no-login canvas that outputs exactly that. Draw entities with typed columns; drag relations between them; download data-model.mmd or copy it straight into a README.

Relations that keep the model honest

Cardinality is where hand-drawn diagrams cheat, so the canvas doesn't let you. Draw a 1–N relation and the foreign key actually appears on the child, typed to match the parent's primary key. Make it 1–1 and the key is also unique. Ask for N–N and you get what a relational model actually needs: the join entity, with a key of its own and a foreign key to each side.

Because the export reads relations back off those columns — not off a separate list of arrows — the diagram and the schema it implies can never disagree. The Mermaid carries the full picture: typed attributes, PK/FK/UK markers, crow's-foot notation.

Deliberately not a migration

The export is a diagram, not DDL — and that's a feature. The same model can be written into Postgres, Prisma, Drizzle or Mongo; emitting one of those would be choosing for you. A .mmd hands the model over in the one form every stack can be written from.

Your diagram persists in your browser between visits. In the Nodlume workspace, the same board goes further: the entities ground generated TypeScript types, API routes and test cases for a real project. Sketching the structure too? The structure painter is the route-tree half of the same idea.