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

• 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

• CHECK MIGRATION BEFORE CODE — Verify the constraint definition in priv/repo/migrations/ matches what the schema expects

• 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

• 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:

• Constraint name (e.g., users_email_index )

• Table name (e.g., users )

• Operation (insert, update, or delete)

• Conflicting values (if available in logs)

Step 2: Find the Migration

grep -r "constraint_name" priv/repo/migrations/

Also check: create unique_index, create index, add constraint

Verify: Does the migration constraint match the schema's unique_constraint/3 or foreign_key_constraint/3 call?

Step 3: Find the Schema

Look for the constraint handling in changeset

grep -r "unique_constraint|foreign_key_constraint|check_constraint" lib/

Step 4: Trace Insert Paths

Find ALL callers that insert/update this schema:

grep -r "Repo.insert|Repo.update|Repo.insert_all|cast_assoc.*:schema_name" lib/

Step 5: Identify the Cause

Symptom Likely Cause Fix Pattern

Same user triggers twice Race condition (double-click, retry) Upsert with on_conflict

Multiple parents share child cast_assoc doesn't dedup across changesets Dedup before building changesets

Concurrent API requests Missing transaction isolation Wrap in Repo.transaction or use upsert

Migration added constraint to existing data Data violates new constraint Backfill or clean data first

Step 6: Apply Fix

See references/constraint-patterns.md for detailed fix patterns.

Quick Fixes by Constraint Type

Unique violation → Upsert: Repo.insert(changeset, on_conflict: :replace_all, conflict_target: [:field])

Foreign key violation → Check: Does the referenced record exist? Was it deleted concurrently?

Check constraint → Validate: Does the value satisfy the constraint condition?

References

• references/constraint-patterns.md
• Detailed patterns for each constraint type