What is the principle of gem?

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This gem is used to implement a tree structure, such as a category, and there are subcategories below, so you can use it at this time, and you can also use it when you reply to a post and others will reply to you. There is a submenu under a menu, and there can be a submenu under the submenu, and there are menu items under the submenu, which can realize a kind of tree structure menu. Its principle is also very simple, in the model is to achieve a self-association, if in a comment (comment) model, define a children (child), and then define a parent (parent),: class_name is' Comment'. So how do they achieve the connection, that is to say, the child needs to know who the father is, and the father needs to know what children it has. It is related through a field in the data store. In this gem, this field is called parent_id. If we create a root, root = Comment.create, at this time, the parent_id of root* is null, it is a root, children = root.children.create, and this children is a child of root. So children's parent_id is root's id.

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