Feature: Query preprocessing hook

[mlins]

Problem

Apps often need to transform search queries before they hit FTS - normalizing terms, stripping noise words, or applying domain-specific transformations. Currently this requires wrapping the search() method or preprocessing externally.

Use Case

In a food database:

• Strip size descriptors: "medium apple" → "apple" (avoid branded "MEDIUM APPLE SAUCE")
• Normalize terms: "garlic cloves" → "garlic" (USDA doesn't use "cloves")
• Reorder for USDA conventions: "cream cheese" → "cheese cream"

Proposed API

class Food < ApplicationRecord
  include ActiveRecord::Searchable

  searchable do
    field :description, weight: 2.0
    field :category
    
    # Option 1: Block
    preprocess_query do |query|
      query.gsub(/\b(small|medium|large)\b/i, '').strip
    end
    
    # Option 2: Lambda
    preprocess_query ->(q) { MyNormalizer.normalize(q) }
    
    # Option 3: Method reference
    preprocess_query :normalize_search_query
  end
  
  def self.normalize_search_query(query)
    # custom logic
  end
end

Implementation Notes

This would hook into QueryBuilder#sanitize_query, running the custom preprocessor before the existing sanitization logic:

def sanitize_query(terms)
  terms = terms.to_s
  terms = config.preprocess_query.call(terms) if config.preprocess_query
  terms = remove_invalid_characters(terms)
  # ... rest of sanitization
end

Benefits

• Clean extension point for domain-specific logic
• Keeps preprocessing close to the searchable configuration
• Testable in isolation
• Doesn't require monkey-patching or wrapper methods