Rails undefined method 'full_name'

I have this error, when I'am testing my controller

ActionView::Template::Error: undefined method `full_name' for nil:NilClass

My tests:

class PostsControllerTest < ActionController::TestCase
include Devise::TestHelpers

setup do
    @post = Post.create!(title: "my title", content: "bla bla bla")
    user = User.create!(email: "example@mail.com", first_name: "name", last_name: "surename",
                    password: "password", password_confirmation: "password")
    sign_in user
end

test "should show post" do
    get :show, id: @post
    assert_response :success
end

full_name method in model:

def full_name
  "#{first_name} #{last_name}"
end

and my view:

Wrote by
  = @post.user.full_name

Thanks for help