ecto-changesets

Ecto changesets validate and cast external parameters into schema types, applying field validations like required, format, and number constraints before database operations. Use `cast/3` to convert incoming data, then chain validators like `validate_required/2` and `validate_format/3` to enforce rules and return structured error messages if validation fails.

Changesets map database-level constraints (unique, foreign key, check) to user-friendly error messages. Define validations in your schema, apply custom validators for password strength, and let `Repo.insert/1` or `Repo.update/1` catch constraint violations, surfacing them as changeset errors for clear user feedback.

Yes. Ecto changesets support validating embedded schemas and managing associations like `many_to_many` tags within one changeset, ensuring consistency across related data. Use association helpers to cast and validate nested records alongside parent schema validations in a single operation.

Create custom validators by writing functions that accept a changeset and return a modified changeset with added errors. Chain custom validators with built-in ones like `validate_inclusion/3` and `validate_number/3` to enforce domain-specific business rules and data integrity requirements.

Changeset validation catches schema-level errors early, but database-level constraints (unique indexes, foreign keys, check constraints) trigger only during `Repo.insert/1` or `Repo.update/1`. Add changeset-level constraint validations or use `unique_constraint/2` to translate database errors into user-facing messages before insertion.