Equivalent of .try() for a hash to avoid “undefined method” errors on nil? [duplicate]

Answer 1

Another approach:

@myvar = session[:comments][@comment.id]["temp_value"] rescue nil

This might also be consider a bit dangerous because it can hide too much, personally I like it.

If you want more control, you may consider something like:

def handle # just an example name, use what speaks to you
    raise $! unless $!.kind_of? NoMethodError # Do whatever checks or 
                                              # reporting you want
end
# then you may use
@myvar = session[:comments][@comment.id]["temp_value"] rescue handle

Answer 2

The proper use of try with a hash is @sesion.try(:[], :comments).

@session.try(:[], :comments).try(:[], commend.id).try(:[], 'temp_value')

Answer 3

When you do this:

myhash[:one][:two][:three]

You're just chaining a bunch of calls to a "[]" method, an the error occurs if myhash[:one] returns nil, because nil doesn't have a [] method. So, one simple and rather hacky way is to add a [] method to Niclass, which returns nil: i would set this up in a rails app as follows:

Add the method:

#in lib/ruby_extensions.rb
class NilClass
  def [](*args)
    nil
  end
end

Require the file:

#in config/initializers/app_environment.rb
require 'ruby_extensions'

Now you can call nested hashes without fear: i'm demonstrating in the console here:

>> hash = {:foo => "bar"}
=> {:foo=>"bar"}
>> hash[:foo]
=> "bar"
>> hash[:doo]
=> nil
>> hash[:doo][:too]
=> nil

Answer 4

The most beautiful solution is an old answer by Mladen Jablanović, as it lets you to dig in the hash deeper than you could with using direct .try() calls, if you want the code still look nice:

class Hash
  def get_deep(*fields)
    fields.inject(self) {|acc,e| acc[e] if acc}
  end
end

You should be careful with various objects (especially params), because Strings and Arrays also respond to :[], but the returned value may not be what you want, and Array raises exception for Strings or Symbols used as indexes.

That is the reason why in the suggested form of this method (below) the (usually ugly) test for .is_a?(Hash) is used instead of (usually better) .respond_to?(:[]):

class Hash
  def get_deep(*fields)
    fields.inject(self) {|acc,e| acc[e] if acc.is_a?(Hash)}
  end
end

a_hash = {:one => {:two => {:three => "asd"}, :arr => [1,2,3]}}

puts a_hash.get_deep(:one, :two               ).inspect # => {:three=>"asd"}
puts a_hash.get_deep(:one, :two, :three       ).inspect # => "asd"
puts a_hash.get_deep(:one, :two, :three, :four).inspect # => nil
puts a_hash.get_deep(:one, :arr            ).inspect    # => [1,2,3]
puts a_hash.get_deep(:one, :arr, :too_deep ).inspect    # => nil

The last example would raise an exception: "Symbol as array index (TypeError)" if it was not guarded by this ugly "is_a?(Hash)".

Answer 5

Try to use

@myvar = session[:comments][@comment.id]["temp_value"] if session[:comments]

Answer 6

As of Ruby 2.3 this gets a little easier. Instead of having to nest try statements or define your own method you can now use Hash#dig (documentation).

h = { foo: {bar: {baz: 1}}}

h.dig(:foo, :bar, :baz)           #=> 1
h.dig(:foo, :zot)                 #=> nil

Or in the example above:

session.dig(:comments, @comment.id, "temp_value")

This has the added benefit of being more like try than some of the examples above. If any of the arguments lead to the hash returning nil then it will respond nil.