Model database relationships together in Mermaid

How it works

1. Open Shared Mermaid in the Room, then choose the ER starter template to get a working erDiagram to edit.
2. List the entities and their key attributes, then declare the relationships and their cardinality between them.
3. Review the model in Room voice or chat, correcting entities and relationships as the discussion settles them.
4. Copy the erDiagram source into your schema documentation, migration notes or design document.

What you can do

• Write Mermaid erDiagram source with a preview that redraws as you type.
• Declare entities, their attributes and their keys.
• Express one-to-one, one-to-many and many-to-many relationships with cardinality.
• Review a proposed schema with the people who will build against it.
• Copy the finished erDiagram source into technical documentation.

Why use it in a Free4Chat Room

Schema disagreements are usually about cardinality and ownership, and those are exactly the details that get lost when a model is described rather than drawn. Keeping the source open in the Room while people talk means the argument about whether an order has one line item or many, or whether a customer can exist without an order, ends with the diagram already updated. The ER template gives you a small worked example of the syntax to edit.

Limits & privacy

It renders Mermaid, not a database client. There is no connection to a live database, no schema import, no reverse engineering, no migration generation and no visual drag-and-drop entity editing—the model is the text you write. Only the source text is shared between participants and each browser renders it locally. The source is capped at 12 KiB and there is no document history or file download, so copy the source into your documentation before the Room ends.

Frequently asked questions

What is an ER diagram for?

Describing the entities a system stores, the attributes each one holds, and how the entities relate to each other. It is the diagram for a schema review: agreeing what a table represents, which key identifies it, and whether a relationship is one-to-one, one-to-many or many-to-many before anybody writes a migration.

Does it connect to my database or generate migrations?

No. There is no database connection, no schema import, no reverse engineering and no migration or DDL generation. Your browser never talks to a database—the model is the Mermaid text in the editor and nothing else. Free4Chat is a temporary Room, not a modelling tool that knows your infrastructure.

Can I drag entities around to lay out the diagram?

No. This is a text editor with a live preview, not a visual schema designer. Entity boxes, attributes and relationship lines all come from the erDiagram source, and the layout is produced by Mermaid's own renderer.

How does it express how many rows can relate to each other?

Through Mermaid's cardinality markers on the relationship line—for example one-to-many or many-to-many—written directly in the source. Getting that notation right is the point of reviewing the model together, because the cardinality is the part of a schema that is expensive to change later.

Can we review a schema together in real time?

Yes. Everyone in the Room edits the same erDiagram source and sees the same rendered model. Your preview updates on each keystroke and the Room hears about your edit once your typing settles, so a review can proceed with one person typing while everybody else reads the same picture.

Is this the same App as the general Mermaid editor?

It is the same runtime and the same shared editor, approached from the schema-modelling task. Both launch the identical tool; this page covers the ER workflow and its template, and the main Mermaid editor page covers diagramming more generally.