Extracting specific keys from a hash in Ruby

2015-07-16 ·

Let's take the following widget:

widget = {
  id: 1,
  name: "Widget 1",
  description: "This is widget 1",
  created_at: "1992-10-01 12:21"
}

In order to create a hash with a subset of keys from widget you can use select method which comes from Enumerable mixin included (among others) in Array and Hash.

keys_to_extract = [:id, :name]
widget.select { |key,_| keys_to_extract.include? key }
{
  id: 1,
  name: "Widget 1"
}

You can combine this with map to iterate over an array of hashes.

widgets = [
  {
    id: 1,
    name: "Widget 1",
    description: "This is widget 1",
    created_at: "1992-10-01 12:21"
  },
  {
    id: 2,
    name: "Widget 2",
    description: "This is widget 2",
    created_at: "1993-10-01 12:21"
  }
]
keys_to_extract = [:id, :name]
widgets.map do |w|
  w.select { |k,_| keys_to_extract.include? k }
end
[
  {
    id: 1,
    name: "Widget 1"
  },
  {
    id: 2,
    name: "Widget 2"
  }
]

This method can be also directly attached to any Hash instance using the following snippet.

class Hash
  def select_keys(*args)
    select {|k,_| args.include? k }
  end
end
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