Challenge 2: Safe Navigation on a Chain — Solution

class Profile
  attr_accessor :avatar_url

  def initialize(avatar_url = nil)
    @avatar_url = avatar_url
  end
end

class User
  attr_accessor :profile

  def initialize(profile = nil)
    @profile = profile
  end
end

def display_avatar(user)
  user&.profile&.avatar_url || "default-avatar.png"
end

user_with_avatar = User.new(Profile.new("photo.jpg"))
user_without_profile = User.new

puts display_avatar(user_with_avatar)
puts display_avatar(user_without_profile)

=begin
Output:
photo.jpg
default-avatar.png

Notes:
- user_with_avatar has a full chain: a User with a Profile that has an
  avatar_url, so user&.profile&.avatar_url resolves all the way through
  to "photo.jpg".
- user_without_profile has no profile at all (profile defaults to nil in
  User#initialize). user&.profile succeeds (user is not nil), but the
  result (nil) means &.avatar_url short-circuits immediately instead of
  raising NoMethodError on nil.
- || "default-avatar.png" supplies the fallback any time the whole
  &.-chain resolves to nil, whether because profile itself is missing or
  because avatar_url was never set on an existing profile.
- Without &., user_without_profile.profile.avatar_url would raise
  NoMethodError the moment .avatar_url was called on nil.
=end