Comparing an active record result with an array of object in a rspec test
Provided you sort the active record results (as you have done), comparing to an array works fine.  I'm not sure what you mean by 'flexible', but the test you've written looks pretty good (without knowing the internals of your 'sort' function).

I'd only suggest adding some more tests to cover reverse sorting, sorting by author.nickname (if that's part of the default scope, or pulled in via delegation), and passing invalid values to the sort function.

Being a little pedantic, you don't need to repeat the word "it" in the spec declaration.
The word "it" will be prepended to the message in the case of failures.

  describe "self.sort" do
    before(:each) do
      @tee = FactoryGirl.create :author, nickname: "tee jia hen", user: FactoryGirl.create(:user)
      @jon = FactoryGirl.create :author, nickname: "jon", user: FactoryGirl.create(:user)
      @tee_article1 = FactoryGirl.create :article, author: @tee, title: "3diablo"
      @tee_article2 = FactoryGirl.create :article, author: @tee, title: "1people"
      @jon_article = FactoryGirl.create :article, author:@jon, title: "2game"
    end

    it "should sort articles base on title" do
      Article.sort("title", "asc").should == [@tee_article2, @jon_article, @tee_article1]
    end

    it "should reverse-sort articles base on title" do
      Article.sort("title", "desc").should == [@tee_article1, @jon_article, @tee_article2]
    end
  end