How do I fetch multiple hash values at once?

Question

What is a shorter version of this?:

from = hash.fetch(:from)
to = hash.fetch(:to)
name = hash.fetch(:name)
# etc

Note the fetch, I want to raise an error if the key doesn't exist.

There must be shorter version of it, like:

from, to, name = hash.fetch(:from, :to, :name) # <-- imaginary won't work

It is OK to use ActiveSupport if required.

Answer 1

Use Hash's values_at method:

from, to, name = hash.values_at(:from, :to, :name)

This will return nil for any keys that don't exist in the hash.

Answer 2

Ruby 2.3 finally introduces the fetch_values method for hashes that straightforwardly achieves this:

{a: 1, b: 2}.fetch_values(:a, :b)
# => [1, 2]
{a: 1, b: 2}.fetch_values(:a, :c)
# => KeyError: key not found: :c

Answer 3

hash = {from: :foo, to: :bar, name: :buz}

[:from, :to, :name].map{|sym| hash.fetch(sym)}
# => [:foo, :bar, :buz]
[:frog, :to, :name].map{|sym| hash.fetch(sym)}
# => KeyError

Answer 4

my_array = {from: 'Jamaica', to: 'St. Martin'}.values_at(:from, :to, :name)
my_array.keys.any? {|key| element.nil?} && raise || my_array

This will raise an error like you requested

 my_array = {from: 'Jamaica', to: 'St. Martin', name: 'George'}.values_at(:from, :to, :name)
 my_array.keys.any? {|key| element.nil?} && raise || my_array

This will return the array.

But OP asked for failing on a missing key...

class MissingKeyError < StandardError
end
my_hash = {from: 'Jamaica', to: 'St. Martin', name: 'George'}
my_array = my_hash.values_at(:from, :to, :name)
my_hash.keys.to_a == [:from, :to, :name] or raise MissingKeyError
my_hash = {from: 'Jamaica', to: 'St. Martin'}
my_array = my_hash.values_at(:from, :to, :name)
my_hash.keys.to_a == [:from, :to, :name] or raise MissingKeyError

Answer 5

The simplest thing I would go for would be

from, to, name = [:from, :to, :name].map {|key| hash.fetch(key)}

Alternatively, if you want to use values_at, you can use a Hash with a default value block:

hash=Hash.new {|h, k| raise KeyError.new("key not found: #{k.inspect}") }
# ... populate hash
from, to, name = hash.values_at(:from, :to, :name) # raises KeyError on missing key

Or, if you're so inclined, monkey-patch Hash

class ::Hash
  def fetch_all(*args)
    args.map {|key| fetch(key)}
  end
end
from, to, name = hash.fetch_all :from, :to, :name

Answer 6

You could initialise your hash with a default value of KeyError object. This will return an instance of KeyError if the key you are trying to fetch is not present. All you need to do then is check its (value's) class and raise it if its a KeyError.

hash = Hash.new(KeyError.new("key not found"))

Let's add some data to this hash

hash[:a], hash[:b], hash[:c] = "Foo", "Bar", nil

Finally look at the values and raise an error if key not found

hash.values_at(:a,:b,:c,:d).each {|v| raise v if v.class == KeyError}

This will raise an exception if and only if key is not present. It'll not complain in case you have a key with nil value.

Answer 7

Pattern matching is an experimental feature in Ruby 2.7.

hash = { from: 'me', to: 'you', name: 'experimental ruby' }

hash in { from:, to:, name: }

See https://rubyreferences.github.io/rubyref/language/pattern-matching.html

Answer 8

I needed to have a hash being returned so I did a patch:

class Hash
  def multi_fetch(*keys)
    keys.map { |key| [key, fetch(key)] }.to_h
  end
end