Can not insert fixture data to db

hi all,

I am doing some unit tests on my own ror environment(Ubuntu 11.10, ruby
1.9.3, rails 3.2.4), but I found that the fixture data did not insert
the database.

test ‘some test’ do
pp articles # I have an Article model
end #test

Then I type the command rake db:test:prepare and rake test:units, the
articles variable was en empty array and no exceptions raised.

Could you kindly tell me how can I fix this problem?