# Phase 4: Workout Modification

**Started:** 2026-03-01
**Goal:** Let users customize program workouts by swapping or adding exercises, creating personal forks

## Problem Statement

Users want flexibility within their program structure. Currently:
- Can't swap an exercise (e.g., replace bench press with dumbbell press due to equipment availability)
- Can't add exercises to a program workout (e.g., add face pulls to Push day)
- Any modification would require building a completely custom workout

## Solution: Workout Forking

When user modifies a program workout, we:
1. Copy the program workout to `custom_workouts` table
2. Store modifications in `custom_workout_exercises` table
3. Save workout logs with `source_type = 'custom'` to track lineage
4. Original program remains unchanged for future sessions

## Phase Plans

### 04-01: Database Schema Migration
- Create `custom_workouts` table
- Create `custom_workout_exercises` table
- Add `source_type` enum column to `workout_logs`
- Migration script with rollback

### 04-02: Backend API for Custom Workouts
- POST /api/custom-workouts (create from program workout)
- PUT /api/custom-workouts/:id (update exercises)
- GET /api/custom-workouts/:id (fetch with exercises)
- GET /api/custom-workouts (list user's custom workouts)
- Update workout log save to handle source_type

### 04-03: Frontend - Workout Edit Mode
- "Edit Workout" button on WorkoutSelectPage
- Exercise picker modal/component
- Swap exercise UI flow
- Add exercise UI flow
- Fork confirmation dialog

## Success Criteria

- [ ] User can replace any exercise in a program workout
- [ ] User can add exercises to a program workout
- [ ] Modified workout saves as custom_workout (original program unchanged)
- [ ] Subsequent workout sessions use the forked custom workout
- [ ] User can see which workouts are custom vs program originals

## Database Schema

```sql
-- custom_workouts: Stores the forked workout header
custom_workouts (
  id SERIAL PRIMARY KEY,
  user_id INTEGER REFERENCES users(id) ON DELETE CASCADE,
  name VARCHAR(100) NOT NULL,
  description TEXT,
  original_program_day_id INTEGER REFERENCES program_days(id),
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

-- custom_workout_exercises: Stores exercises in custom workout
custom_workout_exercises (
  id SERIAL PRIMARY KEY,
  custom_workout_id INTEGER REFERENCES custom_workouts(id) ON DELETE CASCADE,
  exercise_id INTEGER REFERENCES exercises(id) ON DELETE CASCADE,
  set_order INTEGER NOT NULL,
  sets INTEGER DEFAULT 3,
  reps INTEGER DEFAULT 10,
  weight_preset DECIMAL(5,2),
  UNIQUE(custom_workout_id, set_order)
);

-- workout_logs.source_type: Tracks where log came from
ALTER TABLE workout_logs ADD COLUMN source_type VARCHAR(20) DEFAULT 'program';
-- Values: 'program', 'custom'
```

## Out of Scope

- Building completely custom workouts from scratch (v2/CUS-01)
- Reusable custom workout templates (v2/CUS-02)
- Complex program modifications (reordering days, changing structure)