在rails上使用has_many和在ruby中使用数组有什么区别？ （MongoDB的）

I want to implement a few simple models to my ruby on rails project. I have the model hierarchy, Topic and Project. A Topic will contain many Projects. I wanted to know what the difference was between adding a "has_many projects" relation in the Topic model vs just adding an array field in the Topic model that stores all of the projects within it. I want to implement a few simple models to my r