Create a deep nested hash using loops in Ruby

Question

I want to create a nested hash using four values type, name, year, value. ie, key of the first hash will be type, value will be another hash with key name, then value of that one will be another hash with key year and value as value.

The array of objects I'm iterating looks like this:

elements = [
    {
        year: '2018',
        items: [
            {
                name: 'name1',
                value: 'value1',
                type: 'type1',
            },
            {
                name: 'name2',
                value: 'value2',
                type: 'type2',
            },
        ]
    },
    {
        year: '2019',
        items: [
            {
                name: 'name3',
                value: 'value3',
                type: 'type2',
            },
            {
                name: 'name4',
                value: 'value4',
                type: 'type1',
            },
        ]
    }
]

And I'm getting all values together using two loops like this:

elements.each do |element|
    year = element.year
    element.items.each |item|
        name = item.name
        value = item.value
        type = item.type

        # TODO: create nested hash
    end
end

Expected output is like this:

{
    "type1" => {
        "name1" => {
            "2018" => "value1"
        },
        "name4" => {
            "2019" => "value4"
        }
    },
    "type2" => {
        "name2" => {
            "2018" => "value2"
        },
        "name3" => {
            "2019" => "value3"
        }
    }
}

I tried out some methods but it doesn't seems to work out as expected. How can I do this?

@AlekseiMatiushkin actually it is an array of objects, just corrected it — Dani Vijay
– Dani Vijay, Commented May 22, 2019 at 6:38

Cary Swoveland · Accepted Answer · 2019-05-22 07:17:42Z

4

elements.each_with_object({}) { |g,h| g[:items].each { |f|
    h.update(f[:type]=>{ f[:name]=>{ g[:year]=>f[:value] } }) { |_,o,n| o.merge(n) } } }
  #=> {"type1"=>{"name1"=>{"2018"=>"value1"}, "name4"=>{"2019"=>"value4"}},
  #    "type2"=>{"name2"=>{"2018"=>"value2"}, "name3"=>{"2019"=>"value3"}}}

This uses the form of Hash#update (aka merge!) that employs a block (here { |_,o,n| o.merge(n) } to determine the values of keys that are present in both hashes being merged. See the doc for definitions of the three block variables (here _, o and n). Note that in performing o.merge(n) o and n will have no common keys, so a block is not needed for that operation.

edited May 22, 2019 at 7:17

answered May 22, 2019 at 7:04

Cary Swoveland

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2 Comments

Aleksei Matiushkin Over a year ago

Good one! I wonder when will we have Hash#bury or like to avoid this ugly fence {=>{=>{=>{=> :)

Amadan Over a year ago

@AlekseiMatiushkin:

class Hash; def bury(*keys, val); keys.size > 1 ? self.dig(*keys[0..-2])[keys[-1]] = val : self[keys.first] = val; end; end

(the hash better be autovivifying, though, or it won't be too useful),

Aleksei Matiushkin · Accepted Answer · 2019-05-22 07:19:35Z

Assuming you want to preserve the references (unlike in your desired output,) here you go:

elements = [
  {
    year: '2018',
    items: [
      {name: 'name1', value: 'value1', type: 'type1'},
      {name: 'name2', value: 'value2', type: 'type2'}
    ]
},
{
    year: '2019',
    items: [
      {name: 'name3', value: 'value3', type: 'type2'},
      {name: 'name4', value: 'value4', type: 'type1'}
    ]
  }
]

Just iterate over everything and reduce into the hash. On the structures of known shape is’s a trivial task:

elements.each_with_object(
    Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) } # for deep bury
) do |h, acc|
  h[:items].each do |item|
    acc[item[:type]][item[:name]][h[:year]] = item[:value]
  end
end

#⇒ {"type1"=>{"name1"=>{"2018"=>"value1"},
#             "name4"=>{"2019"=>"value4"}},
#   "type2"=>{"name2"=>{"2018"=>"value2"},
#             "name3"=>{"2019"=>"value3"}}}

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