Mermaid ER Server

An open-source Model Context Protocol (MCP) server that parses Mermaid ER diagrams, creates PostgreSQL database tables, and exposes automatic REST/GraphQL CRUD APIs.

Features

• 🔍 Parse Mermaid ER Diagrams - Extract entities, attributes, and relationships
• 🗄️ Auto-Create PostgreSQL Tables - Generate DDL and execute against your database
• 🚀 REST API Generation - Automatic CRUD endpoints for all entities
• 📊 GraphQL API Generation - Type-safe queries and mutations
• 🔑 Key Detection - Identifies PK, FK, and UK constraints
• ⚙️ Configurable - Environment variables for database and API settings

Installation

git clone https://github.com/yourusername/mcp-mermaid-er-server.git
cd mcp-mermaid-er-server
npm install
npm run build

Configuration

Create a .env file (copy from .env.example):

# Mermaid Source
MERMAID_DIAGRAM_PATH=./examples/sample-er.mmd

# Database (PostgreSQL)
DB_HOST=localhost
DB_PORT=5432
DB_NAME=mydb
DB_USER=postgres
DB_PASSWORD=password

# API Server
API_TYPE=rest       # rest, graphql, or both
API_PORT=3000
API_HOST=0.0.0.0

Claude Desktop Integration

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "mermaid-er": {
      "command": "node",
      "args": ["/path/to/mcp-tools/dist/index.js"],
      "env": {
        "MERMAID_DIAGRAM_PATH": "/path/to/your/diagram.mmd",
        "DB_HOST": "localhost",
        "DB_NAME": "mydb",
        "DB_USER": "postgres",
        "DB_PASSWORD": "password"
      }
    }
  }
}

Available MCP Tools (12 Total)

Parsing Tools

Tool	Description
parse_er_diagram	Parse ER diagram, return full schema
list_entities	List all entity names
get_entity_details	Get attributes for an entity
get_relationships	Get all relationships
validate_diagram	Validate diagram syntax

Database Tools

Tool	Description
test_connection	Test PostgreSQL connection
generate_sql	Generate DDL without executing
create_schema	Create tables in database
drop_schema	Drop all tables (requires confirmation)

API Tools

Tool	Description
start_api_server	Start REST/GraphQL server
stop_api_server	Stop the API server
get_api_endpoints	List all endpoints

Workflow Example

1. Parse your ER diagram → parse_er_diagram
2. Review the SQL → generate_sql
3. Create database tables → create_schema
4. Start the API server → start_api_server
5. Use the auto-generated endpoints!

API Endpoints

REST (when API_TYPE=rest or both)

GET    /api/{entity}       - List all records
GET    /api/{entity}/:id   - Get by ID
POST   /api/{entity}       - Create record
PUT    /api/{entity}/:id   - Update record
DELETE /api/{entity}/:id   - Delete record

GraphQL (when API_TYPE=graphql or both)

# Queries
query { customers { customer_id email } }
query { customer(id: "1") { email } }

# Mutations
mutation { createCustomer(input: { email: "[email protected]" }) { customer_id } }
mutation { updateCustomer(id: "1", input: { email: "[email protected]" }) { email } }
mutation { deleteCustomer(id: "1") { customer_id } }

Supported ER Syntax

erDiagram
    CUSTOMER {
        int customer_id PK "Primary key"
        string email UK "Unique email"
        string name
    }
    
    ORDER {
        int order_id PK
        int customer_id FK
        datetime order_date
    }
    
    CUSTOMER ||--o{ ORDER : "places"

Development

npm run dev      # Run with ts-node
npm run build    # Compile TypeScript
npm test         # Run tests

License

MIT License - see LICENSE