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

Answer 1

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.

Answer 2

say you want to find params[:user][:email] but it's not sure whether user is there in params or not. Then-

you can try:

params[:user].try(:[], :email)

It will return either nil(if user is not there or email is not there in user) or otherwise the value of email in user.

Answer 3

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].

Answer 4

Andrew's answer didn't work for me when I tried this again recently. Maybe something has changed?

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

The '[]' is in quotes instead of a symbol :[]

Answer 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'

Answer 6

@myvar = session.fetch(:comments, {}).fetch(@comment.id, {})["temp_value"]

From Ruby 2.0, you can do:

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

From Ruby 2.3, you can do:

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