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custom_hash_eql.md

♾️ Custom Equality and Hashing for Complex Objects​

When you store custom objects in a Set, Ruby uses hash and eql? to decide uniqueness. By overriding these methods you can group objects by any combination of attributes, even nested structures. This lets you dedupe complex data without extra iteration.

require 'set'

class Event
  attr_reader :name, :timestamp, :metadata
  def initialize(name, timestamp, metadata = {})
    @name, @timestamp, @metadata = name, timestamp, metadata
  end

  # Only name and date portion of timestamp matter for uniqueness
  def hash
    [name, timestamp.to_date].hash
  end

  def eql?(other)
    other.is_a?(Event) && name == other.name && timestamp.to_date == other.timestamp.to_date
  end
end

events = Set.new
# Two events at different times on same day are treated as duplicates
events << Event.new('deploy', Time.now)
events << Event.new('deploy', Time.now - 3600)

puts events.size  # => 1