Foreign Key Validation Without Relying on DataIntegrityViolationException

Problem

While building the Gift API at Kakao Tech Campus, I implemented a feature to add products to a wishlist. The question was: how should I handle invalid product IDs?

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Method 1: Relying on DB Constraints

Catch DataIntegrityViolationException when a foreign key constraint is violated.

@Transactional
public WishlistItemDto addItem(Member member, Long productId) {
    try {
        WishlistItem item = new WishlistItem(member, productId);
        wishlistRepository.save(item);
        return new WishlistItemDto(item);
    } catch (DataIntegrityViolationException e) {
        throw new EntityNotFoundException("Product does not exist.");
    }
}

Problems:

• DataIntegrityViolationException covers more than just FK violations
• The exception scope is too broad—hard to pinpoint the actual cause
• You’re hitting the DB just to find out something failed

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Method 2: Validation at Application Level

Check if the product exists before saving.

@Transactional
public WishlistItemDto addItem(Member member, Long productId) {
    Product product = productRepository.findById(productId)
        .orElseThrow(() -> new EntityNotFoundException("Product does not exist."));
    
    WishlistItem item = new WishlistItem(member, product);
    wishlistRepository.save(item);
    return new WishlistItemDto(item);
}

Advantages:

• Exceptions are precise
• Avoids unnecessary DB queries
• Business logic is explicit in the code

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Mentor Feedback

I asked my mentor which approach was better.

“From the perspective that we shouldn’t blindly trust external (DB-level) validation, application-level validation is the way to go.”

In short:

• DB constraints are the last line of defense
• Validate at the application level first
• Exception messages become more meaningful

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Lessons Learned

• Relying solely on DB constraints makes exception handling vague
• Application-level validation leads to clearer code and easier debugging
• I learned the defensive programming principle of “never trust external validation”

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From Kakao Tech Campus 3rd cohort Gift API clone coding, summarizing mentor feedback.