Learn From Fix

Name: phx:learn
Rating: 92
Author: oliver-kriska

After fixing a bug or receiving a correction, capture the lesson to prevent future mistakes.

Usage

code

/phx:learn Fixed N+1 query in user listing - was missing preload
/phx:learn String vs atom key mismatch in params handling
/phx:learn LiveView assign_async needs render_async in tests

Workflow

Step 1: Identify the Pattern

Ask yourself:

•What was the root cause? (not the symptom)
•Is this a common mistake others might make?
•Can it be prevented with a simple rule?

Step 2: Check Existing Knowledge

bash

# Check if already documented
grep -ri "PATTERN_KEYWORD" plugins/elixir-phoenix/skills/
grep -i "PATTERN_KEYWORD" CLAUDE.md

Step 3: Add Prevention Rule

If not already documented, add to references/common-mistakes.md:

markdown

### Category: [Ecto/LiveView/OTP/Testing/etc]

**Mistake**: [What went wrong]

**Pattern**: Do NOT [bad pattern] - instead [good pattern]

**Example**:

    ```elixir
    # Bad
    bad_code()

    # Good
    good_code()
    ```

Step 4: Consider Broader Updates

If the lesson is significant:

• Update relevant agent instructions?
• Add to skill quick reference?
• Create new test case?

Output

After capturing, confirm:

text

Lesson captured in references/common-mistakes.md

Pattern: Do NOT use map["key"] for internal data - instead use map.key or Map.get(map, :key)
Category: Ecto

References

See existing lessons: references/common-mistakes.md

Category	Common Mistakes
Ecto	N+1 queries, missing preloads, string vs atom keys
LiveView	Blocking mount, missing render_async, stale assigns
OTP	Unnecessary processes, missing supervision, bottlenecks
Testing	async: false with global mocks, Process.sleep, insert vs build
Phoenix	Business logic in controllers, missing CSRF