activerecord

# ActiveRecord

This skill provides comprehensive guidance for working with ActiveRecord in Rails applications. Use for writing migrations, defining associations, optimizing queries, preventing N+1 issues, implementing validations, and following database best practices.

## Quick Reference

### CRUD Operations

```ruby
# Create
user = User.create(name: "Alice", email: "alice@example.com")
user = User.create!(...)  # Raises on failure

# Read
User.find(1)              # Raises RecordNotFound
User.find_by(email: "x")  # Returns nil if not found
User.where(active: true)  # Returns Relation

# Update
user.update(name: "Bob")
user.update!(...)         # Raises on failure

# Delete (callbacks run)
user.destroy

# Delete (no callbacks)
user.delete
```

### Key Concepts

| Concept | Purpose |
|---------|---------|
| `belongs_to` | Child side of association (has foreign key) |
| `has_many` / `has_one` | Parent side of association |
| `has_many :through` | Many-to-many via join model |
| `includes` / `preload` | Eager loading (prevent N+1) |
| `scope` | Named query builder |
| `validates` | Model-level data validation |
| `before_save` / `after_commit` | Lifecycle callbacks |

## Eager Loading Decision Tree

```
Need to access associated data?
├── NO → Use `joins` (filtering only)
└── YES → Need to filter/sort by association?
          ├── NO → Use `preload` (separate queries)
          └── YES → Large dataset with many associations?
                    ├── YES → Use `includes` with `references`
                    └── NO → Use `eager_load` (single JOIN)
```

### Quick Comparison

| Method | Strategy | Best For |
|--------|----------|----------|
| `includes` | Auto-choose | Default choice |
| `preload` | Separate queries | Large datasets, no filtering |
| `eager_load` | LEFT OUTER JOIN | Filtering by association |
| `joins` | INNER JOIN | Filtering only, not accessing data |

```ruby
# N+1 problem
Post.all.each { |p| p.author.name }  # 1 + N queries

# Solution
Post.includes(