Difference between expect and expect_any_instance_of - ruby-on-rails

Question

I have method in my controller that looks like this:
def resend_confirmation
#current_user.send_confirmation_instructions
render json: nil, status: 200
end
I've written following spec for that method:
require 'rails_helper'
describe 'POST /api/v1/users/:id/resend_confirmation' do
let!(:current_user) { create(:user) }
before do
expect(current_user).to receive(:send_confirmation_instructions)
post resend_confirmation_api_v1_user_path(current_user),
headers: http_authorization_header(current_user)
end
describe 'response' do
it 'is empty' do
expect(response.body).to eq 'null'
end
end
it 'returns 200 http status code' do
expect(response.status).to eq 200
end
end
But the problem is that this spec is not passing. This line is failing:
expect(current_user).to receive(:send_confirmation_instructions)
when I change that to
expect_any_instance_of(User).to receive(:send_confirmation_instructions)
everything works pretty well. Could someone explain me why spec with expect syntax is not passing?
EDIT:
This how error looks:
Failures:
1) POST /api/v1/users/:id/resend_confirmation returns 200 http status code
Failure/Error: expect(current_user).to receive(:send_confirmation_instructions)
(#<User id: 4175, email: "test1#user.com", date_of_birth: "1990-01-01", created_at: "2016-07-18 06:56:52", updated_at: "2016-07-18 06:56:52", sex: "male", touch_id_enabled: false, first_name: "Test", last_name: "User", athena_health_patient_id: nil, photo_url: nil, admin: false, one_signal_player_id: "1", phone_number: nil, state: nil, address: nil, city: nil, zip_code: nil, phone_number_confirmed: false>).send_confirmation_instructions(*(any args))
expected: 1 time with any arguments
received: 0 times with any arguments
# ./spec/requests/api/v1/user/resend_confirmation_spec.rb:7:in `block (2 levels) in <top (required)>'
2) POST /api/v1/users/:id/resend_confirmation response is empty
Failure/Error: expect(current_user).to receive(:send_confirmation_instructions)
(#<User id: 4176, email: "test2#user.com", date_of_birth: "1990-01-01", created_at: "2016-07-18 06:56:53", updated_at: "2016-07-18 06:56:53", sex: "male", touch_id_enabled: false, first_name: "Test", last_name: "User", athena_health_patient_id: nil, photo_url: nil, admin: false, one_signal_player_id: "2", phone_number: nil, state: nil, address: nil, city: nil, zip_code: nil, phone_number_confirmed: false>).send_confirmation_instructions(*(any args))
expected: 1 time with any arguments
received: 0 times with any arguments
# ./spec/requests/api/v1/user/resend_confirmation_spec.rb:7:in `block (2 levels) in <top (required)>'

Answer 1

expects(...) sets the expectation on a particular instance. When the POST request is executed, your app will try to identify the user referenced in your request information, and will create an instance that represents it.
However, that instance is not the same you prepared in your test. It indeed references the same user object, but it's not the same Ruby object.
Therefore, inside your tests, the current_user that is used is not the one you set the expectations on.
Using expect_any_instance_of instead affects every instance of user created, hence also the one that will be created to satisfy the request.