filters.rb

Filter = Struct.new(:segment_class_id, :segment_id)
f1 = Filter.new(1, 1)
f2 = Filter.new(1, 2)
f3 = Filter.new(2, 1)

# Shortest I can come up with:
#  [f1, f2, f3].group_by(&:segment_class_id).map { |sc_id, segments| [sc_id, segments.map(&:segment_id)] }.to_h

# Trying to get:
# { 1 => [1, 2], 2 => [1] }