---

name: data-modeler description: Creates entity model documents with Mermeaid.js ER diagrams and attribute tables.

Data Modeler

Instructions

Create an entity model document with ER diagram and attribute tables.

DO NOT

• Add attributes/columns to the Mermaid diagram
• Write prose descriptions like "Key attributes: name, email..."
• Create a "Relationships" table
• Skip the attribute tables

Document Structure

# Entity Model

## Entity Relationship Diagram

```mermaid
erDiagram
    ROOM_TYPE ||--o{ ROOM : "categorizes"
    GUEST ||--o{ RESERVATION : "makes"

ENTITY_NAME

One sentence describing the entity.

Attribute	Description	Data Type	Length/Precision	Validation Rules
id	...	Long	19	Primary Key, Sequence
...	...	...	...	...

Required Format for Each Entity

Every entity MUST have:

1. A ### heading with ENTITY_NAME
2. One sentence description
3. An attribute table with exactly 5 columns

Example Entity

ROOM_TYPE

Defines categories of rooms with shared characteristics.

Attribute	Description	Data Type	Length/Precision	Validation Rules
id	Unique identifier	Long	19	Primary Key, Sequence
name	Name of the room type	String	50	Not Null, Unique
description	Detailed description	String	500	Optional
capacity	Maximum number of guests	Integer	10	Not Null, Min: 1, Max: 10
price	Price per night in CHF	Decimal	10,2	Not Null, Min: 0

Mermaid Diagram Rules

• Show entity names and relationships ONLY
• NO attributes inside entity blocks
• Use relationship syntax: ENTITY_A ||--o{ ENTITY_B : "relationship"

Validation Rules Reference

Use these values in the "Validation Rules" column (never leave empty):

Attribute Type	Validation Rules Value
Primary key	Primary Key, Sequence
Required field	Not Null
Unique field	Not Null, Unique
Foreign key	Not Null, Foreign Key (TABLE.id)
Optional field	Optional
With range	Not Null, Min: X, Max: Y
With values	Not Null, Values: A, B, C
Email	Not Null, Format: Email

Data Types Reference

Data Type	Length/Precision	Usage
Long	19	IDs, foreign keys
String	varies (50-500)	Text fields
Integer	10	Whole numbers
Decimal	10,2	Currency, percentages
Boolean	1	True/false flags
Date	-	Date only
DateTime	-	Date and time

Multi-Column Constraints

If validation spans multiple columns, add after the table:

Constraints: Check-out date must be after check-in date.

Workflow

1. Read the requirements document
2. Use TodoWrite to create a task for each entity
3. Write the document header and ER diagram (relationships only)
4. For each entity:
   • Write ### heading
   • Write one sentence description
   • Write attribute table with 5 columns
   • Add constraints if needed
   • Mark todo complete
5. Verify every entity has an attribute table