I'm new to rails tests and I'm trying to understand the call to assert_difference. From the documentation, I deduce that the method requires a numerical value for the difference between the previous and the final value of the expression. What if I just want to assert that there is a difference, no matter how big it is? Maybe something like assert_not (assert_no_difference ... )?
If you look at the documentation you'll notice that the signature for the method is:
assert_difference(expression, difference = 1, message = nil, &block)
The difference = 1 indicates that the method sets 1 as the default value for the difference argument if no value is provided.
The general idea behind this method is that there will be a specific change in the value of something that you want to verify in your test. If you want to just test that a value has changed you can use your own variables and asserts.
Maybe something like the following will suffice:
value = 0
new_value = some_function
assert_not_equal value, new_value
Best of luck!