Ecto Constraint Debugging

Systematic approach to diagnosing constraint violations. Load when you see Ecto.ConstraintError, unique_constraint, foreign_key_constraint, or constraint-related changeset errors.

Iron Laws

1. READ THE CONSTRAINT NAME — The constraint name (e.g., links_url_index) tells you exactly which index/constraint failed. Parse it from the error message first
2. CHECK MIGRATION BEFORE CODE — Verify the constraint definition in priv/repo/migrations/ matches what the schema expects
3. TRACE ALL INSERT PATHS — Find every code path that inserts into the constrained table. The bug is often in a path you didn't consider
4. RACE CONDITION UNTIL PROVEN OTHERWISE — If validation passes but constraint fails, assume concurrent inserts until you prove a single-request cause

Step-by-Step Debugging

Step 1: Parse the Error

Extract from the error message: