How do you enforce data validation in a document database?

Enforcing data validation in a document database requires a combination of database-level constraints, application logic, and middleware tools. Document databases like MongoDB or Couchbase are schema-flexible by design, but this flexibility can lead to inconsistent data if validation isn’t implemented. The primary approach is to use built-in schema validation features provided by the database. For example, MongoDB allows defining JSON Schema validation rules at the collection level. These rules can enforce requirements like field presence, data types, format constraints (e.g., email patterns), or custom validation logic using expressions. For instance, you could ensure a user document always includes a name field as a string and an age field as a number within a specific range.

Another layer of validation occurs at the application level. Developers can implement checks in their code before inserting or updating documents. Libraries like Zod (TypeScript) or Pydantic (Python) help define data models and validate incoming data against those models. For example, a REST API might validate user input using a schema that rejects invalid email formats or missing required fields before the data reaches the database. This reduces invalid writes and keeps the application logic decoupled from database-specific validation rules. However, relying solely on application logic can leave gaps if other services or tools interact directly with the database, so combining this with database-level validation is ideal.

Finally, middleware or database triggers can enforce validation during data operations. Tools like Mongoose for MongoDB provide schema-based modeling and pre-save hooks to validate or transform data before persistence. Alternatively, cloud-based document databases like Firebase Firestore allow writing security rules to validate document structures and field values during read/write operations. For example, a Firestore security rule could enforce that a post document must have a createdAt timestamp and a userId matching the authenticated user. Combining these approaches ensures validation at multiple layers, reducing the risk of invalid data while maintaining the flexibility of a document database.