What does &. (ampersand dot) mean in Ruby?

I came across this line of ruby code. What does &. mean in this?

@object&.method 

7 Answers

It is called the Safe Navigation Operator. Introduced in Ruby 2.3.0, it lets you call methods on objects without worrying that the object may be nil(Avoiding an undefined method for nil:NilClass error), similar to the try method in Rails.

So you can write

@person&.spouse&.name 

instead of

@person.spouse.name if @person && @person.spouse 

From the Docs:

my_object.my_method

This sends the my_method message to my_object. Any object can be a receiver but depending on the method's visibility sending a message may raise a NoMethodError.

You may use &. to designate a receiver, then my_method is not invoked and the result is nil when the receiver is nil. In that case, the arguments of my_method are not evaluated.

Note: Even though @Santosh gave a clear and full answer, I would like add some more background and add an important note regarding its use with non instance variables.

It is called "Safe Navigation Operator" (aka "Optional chaining operator", "Null-conditional operator", etc.). Matz seems to call it "lonely operator". It was introduced in Ruby 2.3. It sends a method to an object only if it is not nil.

Example:

# Call method `.profile` on `user` only if `user` is not `nil` @user&.profile # Equivalent to unless @user.nil? @user.profile end 

"Edge case" with local variables:

Please note, above code uses instance variables. If you want to use safe navigation operator with local variables, you will have to check that your local variables are defined first.

# `user` local variable is not defined previous user&.profile # This code would throw the following error: NameError: undefined local variable or method `user' for main:Object 

To fix this issue, check if your local variable is defined first or set it to nil:

# Option 1: Check the variable is defined if defined?(user) user&.profile end # Option 2: Define your local variable. Example, set it to nil user = nil user&.profile # Works and does not throw any errors 

Method background

Rails has try method that basically does the same. It uses send method internally to call a method. Matz suggested that it is slow and this should be a built-in language feature.

Many other programming languages have similar feature: Objective C, Swift, Python, Scala, CoffeeScript, etc. However, a common syntax is ?. (question dot). But, this syntax could not be adopted by Ruby. Because ? was allowed in method names and thus, ?. symbol sequence is already a valid Ruby code. For example:

2.even?.class # => TrueClass 

That's why Ruby community had to come up with different syntax. It was an active discussion and different options were considered (.?, ?, &&, etc.). Here is a list of some considerations:

u.?profile.?thumbnails u\profile\thumbnails u!profile!thumbnails u ? .profile ? .thumbnails u && .profile && .thumbnails # And finally u&.profile&.thumbnails 

While choosing the syntax, developers looked at different edge cases and the discussion is quite useful to go through. If you want to go through all variants and nuance of the operator, please see this feature introduction discussion on official Ruby issue tracker.

Be wary! Though the safe navigation operator is convenient it can also be easy to trick yourself into changing your logic with it. I recommend avoiding the use of it in flow control. Example:

str = nil puts "Hello" if str.nil? || str.empty? # The above line is different than the below line puts "Hello" if str&.empty? 

In the first example, str.nil? returns true and str.empty? is never called, causing the puts statement to be executed. In the second example however, str&.empty? returns nil which is falsey, and the puts statement is never executed.

it used for nil check, such as in kotlin and swift For example; with Object -> Swift and Kotlin

model = car?.model 

this model can be nil(Swift) or null(Kotlin) if we have not defined the model value in car class. we use that ampersand instead of question mark in ruby

model = car&.model 

if use car.model without ampersand and if model is nil the system cannot continue running.

safe navigation operator (&.): tells Ruby to only call the next method if the receiver isn’t nil. Otherwise, the expression returns nil.

Practical In Action

Let’s construct a Roster object for a Sports team. The Roster will contain multiple Player objects.

class Roster attr_accessor :players end class Player attr_accessor :name, :position def initialize(name, position) @name = name @position = position end end 

With these two objects, we can create a roster for a 2-on-2 women’s basketball tournament:

moore = Player.new("Maya Moore", "Forward") taurasi = Player.new("Diana Taurasi", "Guard") tourney_roster1 = Roster.new tourney_roster1.players = [moore, taurasi] 

If we want to know the forward for our 2-on-2 team, we might find the name this way:

if tourney_roster1.players.first.position == "Forward" puts "Forward: #{tourney_roster1.players.first.name}" end 

But what if our opposing roster isn’t set correctly?

tourney_roster2 = Roster.new if tourney_roster2.players.first.position == "Forward" puts "Forward: #{tourney_roster1.players.first.name}" end 

tourney_roster2 hasn’t yet been set with any players. The preceding code will raise a NoMethodError because tourney_roster2.players returns nil. We can add conditional statements to avoid this, but it makes our if statement verbose and unclear:

if tourney_roster2.players && tourney_roster2.players.first && tourney_roster2.players.first.position == "Forward" 

Instead, we can use the safe navigation operator to avoid the NoMethodError:

if tourney_roster2.players&.first&.position == "Forward" puts "Forward: #{tourney_roster1.players.first.name}" end 

Thus,

 >> tourney_roster2.players&.first == nil #=> true >> tourney_roster2.players&.first&.position == nil #=> true 

Some legitimate use cases: The safe navigation operator comes in handy when working with multiple objects, as shown here, and when chaining methods together.

what does mean object&.an_attribute in ruby on rails?

I am new in Ruby on rails and I saw this kind of code but I don't understand it:

In Ruby, like in most mainstream programming languages, user code cannot modify the fundamental workings of the programming languages, nor can it change the programming language's syntax.

Since Ruby on Rails is just Ruby code, it should be immediately obvious that this cannot possibly have anything to do with Ruby on Rails.

Therefore, we need to look at Ruby for an explanation, not Ruby on Rails.

The safe navigation operator or safe navigator is specified in language/safe_navigator_spec.rb of the ruby/spec, in particular here:

context "when context is nil" do it "always returns nil" do eval("nil&.unknown").should == nil eval("[][10]&.unknown").should == nil end it "can be chained" do eval("nil&.one&.two&.three").should == nil end it "doesn't evaluate arguments" do obj = Object.new obj.should_not_receive(:m) eval("nil&.unknown(obj.m) { obj.m }") end end 

It is documented in the Calling Methods section of the Ruby Syntax documentation:

&., called “safe navigation operator”, allows to skip method call when receiver is nil. It returns nil and doesn't evaluate method's arguments if the call is skipped.

Here's a short-read (3 mins) I found on this - it is pretty good.

To add to the above, it acts like the try! method in Rails, not the try method.

Because it will raise a NoMethodError exception if the receiver is not nil and does not implement the tried method.

Example taken from the above article:

account = Account.new(owner: Object.new) account&.owner&.address # => NoMethodError: undefined method `address' for #<Object:0x00559996b5bde8>` account.try(:owner).try(:address) # => nil account.try!(:owner).try!(:address) # => NoMethodError: undefined method `address' for #<Object:0x00559996b5bde8>`