{"slug":"ecto-constraint-debug","title":"ecto-constraint-debug","summary":"Debug Ecto constraint violations - trace triggers, check migrations, find duplicate data. Use when seeing unique_constraint, foreign_key_constraint, or check_constraint errors.","platform":"Claude","tags":[],"authorName":"LLM Mart","authorSlug":"llm-mart","score":0,"source":"github","price":null,"verified":false,"createdAt":"2026-10-04T15:14:12.27068Z","repo":{"url":"https://github.com/oliver-kriska/claude-elixir-phoenix","stars":560,"forks":44,"license":"MIT","updatedAt":"2026-10-02T04:11:38Z"},"bodyHtml":"
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name: ecto-constraint-debug\ndescription: Debug Ecto constraint violations - trace triggers, check migrations, find duplicate data. Use when seeing unique_constraint, foreign_key_constraint, or check_constraint errors.\neffort: medium

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Ecto Constraint Debugging

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Ash projects: Ash surfaces DB constraints through its own error DSL. Use the ash-framework skill — mix usage_rules.search_docs \"constraint\" -p ash_postgres.

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

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Iron Laws

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  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
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  2. CHECK MIGRATION BEFORE CODE — Verify the constraint definition in priv/repo/migrations/ matches what the schema expects
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  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
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  4. RACE CONDITION UNTIL PROVEN OTHERWISE — If validation passes but constraint fails, assume concurrent inserts until you prove a single-request cause
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Step-by-Step Debugging

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Step 1: Parse the Error

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Extract from the error message:

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Step 2: Find the Migration

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Use Grep to search for the constraint name in priv/repo/migrations/. Also check for create unique_index, create index, add constraint.

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

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Step 3: Find the Schema

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Use Grep to find constraint handling in changesets (unique_constraint, foreign_key_constraint, check_constraint) in lib/.

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Step 4: Trace Insert Paths

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Find ALL callers that insert/update this schema:

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Use Grep to find all insert/update paths (Repo.insert, Repo.update, Repo.insert_all, cast_assoc) in lib/.

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Step 5: Identify the Cause

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SymptomLikely CauseFix Pattern
Same user triggers twiceRace condition (double-click, retry)Upsert with on_conflict
Multiple parents share childcast_assoc doesn't dedup across changesetsDedup before building changesets
Concurrent API requestsMissing transaction isolationWrap in Repo.transaction or use upsert
Migration added constraint to existing dataData violates new constraintBackfill or clean data first
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Step 6: Apply Fix

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See ${CLAUDE_SKILL_DIR}/references/constraint-patterns.md for detailed fix patterns.

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Quick Fixes by Constraint Type

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

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

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

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References

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