Define database models and schemas with proper naming, data types, constraints, relationships, and indexing strategies. Use this skill when creating or modifying database models, entity definitions, or schema files. When working on files like models.py, models/*.ts, entities/*.ts, schema.prisma, or ORM model definitions. When defining table structures, setting up foreign key relationships, adding database constraints (NOT NULL, UNIQUE), choosing appropriate data types, or implementing model-level validation.