app.rb

def merge(src, part)
  src.map do |src_item|
    new_item = src_item.dup
    part.each do |part_item|
      new_item.merge! part_item if part_item[:key] == src_item[:key]
    end
    new_item
  end
end

# или если ключи повторяються 

def merge(src = [], part = [])
  rest = src.size == 0 ? part.map(&:dup) : []
  
  result = src.map do |src_item|
    new_item = src_item.dup
    part.each do |part_item|
      if part_item[:key] == src_item[:key]
        new_item.merge! part_item.dup
      else
        rest << part_item.dup
      end
    end
    new_item
  end

  result.concat rest
end