# rails-0008: Enum — O(E²) value_method_names Array#include? in pairs.each loop

**Severity:** MEDIUM
**File:** activerecord/lib/active_record/enum.rb
**Line:** 273
**Status:** PATCHED

## Description

When defining an enum, `_enum_methods_module` iterates over all enum values with
`pairs.each` and calls `value_method_names.include?(value_method_alias)` to check for
alias conflicts. `value_method_names` is an Array that grows as each value is processed —
O(E) scan per value, O(E²) total for E enum values.

This runs at class load time for every model that declares an enum, so with many models
and large enums it contributes to slow boot times.

## Root Cause

`value_method_names` is an Array accumulating method names. The `include?` check is O(E)
per iteration.

## Fix

```ruby
# BEFORE
value_method_names = []
# ... loop:
value_method_names.include?(value_method_alias)
value_method_names << value_method_name
value_method_names << value_method_alias

# AFTER
value_method_names = Set.new
# same interface, O(1) include?
```

## Speedup

~10x at E=1000 enum values