assigns is only defined for controller specs and that's done via the rspec-rails gem. There is no general mechanism to test instance variables in RSpec, but you can use Kernel's instance_variable_get to access any instance variable you want.
So in your case, if object were the object whose instance variable you were interested in checking, you could write:
expect(object.instance_variable_get(:@person)).to eql(user)
As for getting ahold of the UserMailer instance, I can't see any way to do that. Looking at the method_missing definition inside https://github.com/rails/rails/blob/master/actionmailer/lib/action_mailer/base.rb, a new mailer instance will be created whenever an undefined class method is called with the same name as an instance method. But that instance isn't saved anywhere that I can see and only the value of .message is returned. Here is the relevant code as currently defined on github:
Class methods:
def respond_to?(method, include_private = false) #:nodoc:
super || action_methods.include?(method.to_s)
end
def method_missing(method_name, *args) # :nodoc:
if respond_to?(method_name)
new(method_name, *args).message
else
super
end
end
Instance methods:
attr_internal :message
# Instantiate a new mailer object. If +method_name+ is not +nil+, the mailer
# will be initialized according to the named method. If not, the mailer will
# remain uninitialized (useful when you only need to invoke the "receive"
# method, for instance).
def initialize(method_name=nil, *args)
super()
@_mail_was_called = false
@_message = Mail.new
process(method_name, *args) if method_name
end
def process(method_name, *args) #:nodoc:
payload = {
mailer: self.class.name,
action: method_name
}
ActiveSupport::Notifications.instrument("process.action_mailer", payload) do
lookup_context.skip_default_locale!
super
@_message = NullMail.new unless @_mail_was_called
end
end
