You may encounter a situation where you have an array of single-key hashes and need to restructure it into a grouped hash based on the keys.

Description

This Ruby method demonstrates an elegant and concise way to do that using each_with_object and Hash initialization. It transforms an array of hashes—each containing a single key-value pair—into a grouped hash where each key maps to an array of its corresponding values. It uses each_with_object({}) to accumulate the result in a clean and efficient way. The key part is item.shift, which destructures the first (and only) key-value pair from each hash. Then, (hsh[k] ||= []) << v appends the value to the correct key in the output hash. This is especially useful when dealing with normalized inputs or parameter arrays in APIs or forms.

Sample input:

  input = [
    { foo: 1 },
    { bar: 2 },
    { foo: 3 },
    { bar: 4 },
    { baz: 5 }
  ]

Sample Output:

  {
    foo: [1, 3],
    bar: [2, 4],
    baz: [5]
  }

Answer

  def convert_to_attr_hash(arr_with_hashes)
    arr_with_hashes.each_with_object({}) do |item, hsh|
      k, v = item.shift
      (hsh[k] ||= []) << v
    end
  end

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