add attr_accessible to comment

add attr_accessible to comment, unless my spec will fail as

Failure/Error: @comment = Comment.build_from(child_post, user.id, "This is a comment body")
     ActiveModel::MassAssignmentSecurity::Error:
       Can't mass-assign protected attributes: commentable, body, user_id
     # ./app/models/comment.rb:25:in `new'


my spec is as following

describe "When create a new comment" do
    it "should be able to create a comment" do
      @comment = Comment.build_from(child_post, user.id, "This is a comment body")
      @comment.save
      @comment.body.should == "This is a comment body"

      child_post.root_comments.size.should == 1
    end
  end

My rails version 3.2.13