why assert_equal() in Ruby on Rails sometimes seem to compare by Identity and sometimes by value?

Question

it was very weird that yesterday, I was do an integration test in Rails and

assert_equal array_of_obj1, array_of_obj2     # obj1 from db, obj2 created in test

and it failed. The values shown inside the array and objects were identical. If I change the test to

assert array_of_obj1 == array_of_obj2

Then it will pass.

But today, the first test actually passed. What reason could it be? Is assert_equal always using == or .equal? in Rails 2.2 or 2.3.5?