Databases · Middle · Question 57 of 75

What is polymorphic association, and what's its problem?

1A view over several tables
2A table with columns of different types
✓Correct answer: A table like comments(commentable_type, commentable_id) pointing to different tables; the database can't enforce a foreign key on it

Explanation

Alternatives: separate nullable foreign keys with a CHECK that exactly one is set, or a separate junction table per parent type.

Further reading

PostgreSQL: Constraints ↗

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