Mocking AR's connection's insert
This approach won't win you any code cleanliness awards, but when you absolutely must tell whether or not your code is trying to insert records, this will get the job done.
flexmock( ActiveRecord::Base.connection ).should_receive( :insert ).never
# code to test
This doesn't make an assertion, but flexmock is setting an expectation that the method insert will never be called on AR's connection object before flexmock's teardown gets called.
Mocking model's create
This is a little higher level, but even more deeply coupled with your code.
flexmock( MyModel ).should_receive( :create, :create! ).never
# code to test
Doing the test this way seems a little cleaner at first, but there's one major caveat: create and create! are only two avenues to doing an insert in the database; this test would pass if the code was refactored to make a raw sql insert, if it used ar-extension's import method, etc. You could remember to change the test if either of those things were to ever happen, but it's best to just avoid this unless you have few options.
How does ActiveRecord test this?
After being unsatisfied with the last two ways to test this, I got the bright idea to look at AR's tests and see how they test this. Surely they need to test whether objects get created, right? Right. This is taken from AR 2.1.1's finder_test.rb, lines 598-604:
number_of_companies = Company.count
sig38 = Company.find_or_create_by_name("38signals")
assert_equal number_of_companies + 1, Company.count
assert_equal sig38, Company.find_or_create_by_name("38signals")
As you can see, to test whether a database record is created, they take the count of the table before and after the code they're testing, and make sure the final count matches their expectation. Simple.
I was a little underwhelmed by their solution. It's clean, but it's lacking something that I can't put my finger on. Overall. I like their way of testing this over my two solutions, so I've started refactoring my tests to work this way.
Has anyone out there run into this problem, and come up with a better solution?