在Rails中，将哈希压缩为嵌套哈希的最佳方法是什么

Question

Say I have this:

[
  { :user_id => 1, :search_id => a},
  { :user_id => 1, :search_id => b},
  { :user_id => 2, :search_id => c},
  { :user_id => 2, :search_id => d}
]

and I want to end up with:

[
  { :user_id => 1, :search_id => [a,b]},
  { :user_id => 2, :search_id => [c,d]}
]

What is the best way to do that?

Answer 1

Very strange requirement indeed. Anyway

[ { :user_id => 1, :search_id => "a"},
  { :user_id => 1, :search_id => "b"},
  { :user_id => 2, :search_id => "c"},
  { :user_id => 2, :search_id => "d"} ] \
    .map{ |h| h.values_at(:user_id, :search_id) } \
    .group_by(&:first) \
    .map{ |k, v| { :user_id => k, :search_id => v.map(&:last) } }

Answer 2

array.group_by{|x| x[:user_id] }.values.map do |val|
  { user_id:   val.first[:user_id], 
    search_id: val.inject([]){|me, el| me << el[:search_id]} }
end

Answer 3

First off, I think the cleaner output structure here is to allow the user IDs to be the hash keys and the list of search IDs to be the values:

{
  1 => [a, b],
  2 => [c, d]
}

There might be a clever way using Rails helpers to get this structure, but it's not too bad to do it manually, either:

output = {}
input.each do |row|
  key = row[:user_id]
  value = row[:search_id]

  output[key] ||= []
  output[key] << value
end

Answer 4

I agree with Matchus representation of the data and just want to suggest a shorter version for it (input being the initial array).

input.each.with_object(Hash.new {|h,k| h[k] = []}) do |k,o|
  o[k[:user_id]] << k[:search_id]
end

EDIT: this is Ruby > 1.9

Answer 5

input.inject({}) do
  |m, h| (m[h[:user_id]] ||= []) << h[:search_id]; m
end.inject([]) { |m, (k, v)| m << { :user_id => k, :search_id => v }; m }