Equivalent of .try() for a hash to avoid "undefined method" errors on nil?

Question

In Rails we can do the following in case a value doesn't exist to avoid an error:

@myvar = @comment.try(:body)

What is the equivalent when I'm digging deep into a hash and don't want to get an error?

@myvar = session[:comments][@comment.id]["temp_value"] 
# [:comments] may or may not exist here

In the above case, session[:comments]try[@comment.id] doesn't work. What would?

Solution 1

You forgot to put a . before the try:

@myvar = session[:comments].try(:[], @comment.id)

since [] is the name of the method when you do [@comment.id].

Solution 2

The announcement of Ruby 2.3.0-preview1 includes an introduction of Safe navigation operator.

A safe navigation operator, which already exists in C#, Groovy, and Swift, is introduced to ease nil handling as obj&.foo. Array#dig and Hash#dig are also added.

This means as of 2.3 below code

account.try(:owner).try(:address)

can be rewritten to

account&.owner&.address

However, one should be careful that & is not a drop in replacement of #try. Take a look at this example:

> params = nil
nil
> params&.country
nil
> params = OpenStruct.new(country: "Australia")
#<OpenStruct country="Australia">
> params&.country
"Australia"
> params&.country&.name
NoMethodError: undefined method `name' for "Australia":String
from (pry):38:in `<main>'
> params.try(:country).try(:name)
nil

It is also including a similar sort of way: Array#dig and Hash#dig. So now this

city = params.fetch(:[], :country).try(:[], :state).try(:[], :city)

can be rewritten to

city = params.dig(:country, :state, :city)

Again, #dig is not replicating #try's behaviour. So be careful with returning values. If params[:country] returns, for example, an Integer, TypeError: Integer does not have #dig method will be raised.

Solution 3

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)".

Solution 4

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

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

Solution 5

Update: As of Ruby 2.3 use #dig

Most objects that respond to [] expect an Integer argument, with Hash being an exception that will accept any object (such as strings or symbols).

The following is a slightly more robust version of Arsen7's answer that supports nested Array, Hash, as well as any other objects that expect an Integer passed to [].

It's not fool proof, as someone may have created an object that implements [] and does not accept an Integer argument. However, this solution works great in the common case e.g. pulling nested values from JSON (which has both Hash and Array):

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

It can be used the same as Arsen7's solution but also supports arrays e.g.

json = { 'users' => [ { 'name' => { 'first_name' => 'Frank'} }, { 'name' => { 'first_name' => 'Bob' } } ] }

json.get_deep 'users', 1, 'name', 'first_name' # Pulls out 'Bob'

View more solutions

Comments

[oligan]

Related question: stackoverflow.com/questions/4371716/…

[user513951]

Ruby 2.3 introduced Hash#dig that makes try unnecessary here. @baxang has the best answer now.

[Markus Andreas]

Dig does not make try unnexessary, because it sill fails on other objects than hash. For exaple nil. But using dig in combination with the save operator does => session&.dig(:comments, @comment.id, "temp_value")

[sscirrus]

how about if I don't know if either [:comments] or [@comment.id] exist?

[bor1s]

in this case I think it would be better to create nested IF statements to check every parameter in session

[oligan]

-1 Why can't it be nested? try applies to any Object, and nil is an Object, so I suspect the following would work: nil.try(:do).try(:do_not).try(:there_is_a_try).

[oligan]

@sscirrus: You could do session[:comments][@comment.id]["temp_value"] if (session[:comments] and session[:comments][@comment.id])

[Pablo Castellazzi]

The "cant be nested" is wrong. But for your particular case my appreciation was correct. what you need to do is use try with :[], for use it with the key directly you need to use fetch.

[sscirrus]

This is fascinating - thanks Max! Are there any disadvantages to this you know of? Does anyone else have a perspective on this?

[sscirrus]

@AndrewGrimm - yeah, I figured that would work but I was hoping for something more concise (I would have a few similar expressions in one place, and it looks very code-heavy). I like your actual answer. :)

[Arsen7]

It will hide your problems with unexpected nils in other parts of your code. I would consider this method dangerous.

[riffraff]

actually, since nil is not a Hash you can probably simplify to fields.inject(self) {|acc,e| acc[e] if acc.is_a?(Hash)} But I have a feeling #respond_towould be better.

[Arsen7]

@riffraff: You are perfectly right about that acc & acc.is_a?() - consider that a mistake ;-). But respond_to would not work, because String and a lot of other objects also respond to :[], but the result of this method is not what is wanted here.

[svoop]

Since :[] looks a little weird within try, you could also write this as session[:comments].try(:fetch, @comment.id).

[rigyt]

fetch throws an error if the key is not found, unless you pass a default. So you would need to write: session[:comments].try(:fetch, @comment.id, nil)

[Jeff Dickey]

because that's not what .try does.

[Augustin Riedinger]

The point of using object.try is that object can be nil. Whereas in your case nil.get_deep will raise an exception. Your solution doesn't answer the question then.

[Arsen7]

The question says "when I'm digging deep into a hash", and I did not assume that a session could be nil, but if it could, then it would be perfectly OK to call session.try(:get_deep, :comments, @comment.id, "temp_value")

[user513951]

try is no longer necessary as of Ruby 2.3. @baxang has the best answer, below.

[emptywalls]

Looks like session doesn't implement dig: undefined method `dig' for #<ActionDispatch::Request::Session:0x007ffc6cafa698>

[JBlake]

Can't use dig on the session hash

[kamal]

what if nil? I want to return [], instead of nil.

[JustGage]

breaks if the hash is nil for those who where wondering

[epylinkn]

Note that this doesn't actually work with a Rails session as ActionDispatch::Request::Session doesn't implement #dig

[Renan]

also breaks if params[:country] is not a hash or nil (e.g. a string)

[thisismydesign]

&. does not break on nil (unlike .dig by itself). Therefore a safe implementation is: params&.dig(:country, :state, :city)

[hongchangfirst]

Sorry, I'm new to ruby, what's :[] here?

[svelandiag]

Best answer, working okay with regular hashes