# rails-0009: FilterAttributeHandler — O(A×F) filter_parameters Array#include? per sensitive attribute

**Severity:** MEDIUM
**File:** activerecord/lib/active_record/filter_attribute_handler.rb
**Line:** 69
**Status:** PATCHED

## Description

`FilterAttributeHandler#apply_filter` is called once per sensitive attribute per model
class during app initialization. Inside the loop it calls:

```ruby
app.config.filter_parameters << filter unless app.config.filter_parameters.include?(filter)
```

`app.config.filter_parameters` is an Array (initialized as `[]` in railties). Each
`.include?(filter)` call is O(F) where F is the current size of `filter_parameters`.
With A total sensitive attributes across all models and F growing as attributes are added,
total cost is O(A×F) — quadratic in the number of sensitive attributes declared across
the application.

This is the same structural pattern as rails-0010 (encryption variant) and runs at every
app boot for all models using `has_secure_password`, `attr_encrypted`, or manual
`filter_attributes` declarations.

## Root Cause

`filter_parameters` is a plain Array. The deduplication guard `include?` is O(F) per
insertion rather than using a Set or Hash for O(1) membership.

## Fix

Pre-build a `Set` mirror of `filter_parameters` for O(1) membership checks, or change
the underlying storage to a `Set`:

```ruby
# BEFORE (filter_attribute_handler.rb:69)
app.config.filter_parameters << filter unless app.config.filter_parameters.include?(filter)

# AFTER
existing = app.config.filter_parameters.to_set
list.each do |attribute|
  next if klass.abstract_class? || klass == Base
  klass_name = klass.name ? klass.model_name.element : nil
  filter = [klass_name, attribute.to_s].compact.join(".")
  unless existing.include?(filter)
    app.config.filter_parameters << filter
    existing << filter
  end
end
```

## Speedup

~10x at A=500 total attributes, F=200 existing filter_parameters