---

name: spec-writing-database description: Database schema specification using Mermaid ER diagrams, table structures, constraints, and indexes for multi-tenant applications.

Database Schema Specification Patterns

Use when designing database schemas with Mermaid ER diagrams, tables, constraints, and indexes for multi-tenant applications. IMPORTANT: Specify WHAT needs to be built (data structure, relationships, business rules), NOT HOW to build it (no Entity Framework entity classes or fluent configuration code).

When to use

• Creating ER diagrams for new features or schema documentation
• Defining table structures with columns, data types, defaults, and constraints
• Specifying CHECK, UNIQUE, and FOREIGN KEY constraints for data integrity
• Designing indexes for tenant-filtered queries, uniqueness, and performance
• Planning additive or breaking schema migrations

Core principles

• Use Mermaid erDiagram syntax with PK/FK/UK annotations and field descriptions
• All domain entities include TenantId FK; unique constraints scoped as (TenantId, BusinessKey)
• Indexes lead with TenantId; use INCLUDE for covering indexes; filter WHERE for active records
• Integer PKs for performance; GUID alternate keys for external APIs; audit timestamps (CreatedAt, UpdatedAt)
• CHECK constraints enforce business rules; FK cascade rules respect relationships (RESTRICT, CASCADE)
• Breaking changes require multi-step migrations with application deployments between steps

Resources

• Mermaid ER diagrams: patterns/mermaid-er-diagrams.md
• Table structures: patterns/table-structures.md
• Constraints: patterns/constraints.md
• Indexes: patterns/indexes.md
• Design principles: patterns/design-principles.md
• Schema migrations: patterns/schema-migrations.md

Default locations

• Specifications: docs/specs/{feature-name}.md (Database Schema section)
• Mermaid diagrams embedded in markdown specs
• Migration scripts referenced but not specified in detail

Validation checklist

• ER diagram shows all entities, relationships (||--o{, etc.), and field annotations (PK, FK, UK)
• Tables include TenantId FK, integer PK, GUID UK, audit timestamps (CreatedAt, UpdatedAt), and defaults
• Composite unique constraints: (TenantId, Name) or (TenantId, BusinessGuid)
• CHECK constraints validate business rules (capacity ranges, dates, prices, non-empty strings)
• FK constraints specify ON DELETE behavior (RESTRICT for parents, CASCADE for children)
• Indexes: TenantId first column, INCLUDE for projections, filtered WHERE for active records
• Design principles documented: tenant isolation, integer PKs, GUID UKs, audit fields, soft deletes
• Migration strategy specified: additive (backward-compatible) vs breaking (multi-step)