When designing database schemas, ensure relations and constraints are explicitly and correctly defined: 1. Every model must have a properly defined primary key: ```prisma model EmailAccount { email String @id // Primary key, not just @unique // OR id String @id @default(cuid()) // UUID as primary key email String @unique // Unique but not primary key } ``` 2. Always use explicit @relation directives with field references: ```prisma model EmailAccount { accountId String @unique account Account @relation(fields: [accountId], references: [id]) } ``` 3. Avoid redundant foreign keys that Prisma implicitly manages: ```diff - digestSchedule Schedule? - digestScheduleId String? @unique + digestSchedule Schedule? // Let Prisma handle the relation ``` 4. Carefully evaluate unique constraints to ensure they won't restrict valid use cases: ```prisma // ❌ Too restrictive - prevents multiple actions of same type @@unique([executedRuleId, actionType]) // ✅ Better - includes distinguishing field @@unique([executedRuleId, actionType, targetIdentifier]) ``` Proper relation definitions improve schema clarity, prevent migration issues, and avoid runtime errors from phantom fields or missing constraints.