{"id":1069,"date":"2010-03-15T22:37:43","date_gmt":"2010-03-15T21:37:43","guid":{"rendered":"http:\/\/www.gamlor.info\/wordpress\/?p=1069"},"modified":"2021-03-11T09:45:22","modified_gmt":"2021-03-11T08:45:22","slug":"managing-1n-and-nn-object-relations","status":"publish","type":"post","link":"https:\/\/www.gamlor.info\/wordpress\/2010\/03\/managing-1n-and-nn-object-relations\/","title":{"rendered":"Managing 1:N and N:N Object Relations"},"content":{"rendered":"

\"object<\/a>When you design a domain model you normally have lots of 1:n and n:n relations. Many developers are quite familiar how to translate such relations into a relational model<\/a>. But how do you translate such relations into an object-model? There\u2019s no hard guideline for that. In this post I explain what I usually do.<\/p>\n

The Domain Model<\/h3>\n

For the examples I use a this small domain-model. Our application has authors, which write posts. An author has multiple posts and a post has only one author. Each post can have multiple tags and a tag is used for multiple posts.<\/p>\n

The Possibilities<\/h3>\n

The majority of object oriented language give two tools for this job, references and collections. And therefore a lot of possibilities.<\/p>\n

    \n
  1. Referencing from the \u2018child\u2019. In our example the post would have a reference to the author.<\/li>\n
  2. Referencing from the \u2018parent\u2019. In our example the author would have a collection of all its posts.<\/li>\n
  3. Bidirectional reference, in our example the post has a reference to the author and the author a collection of his posts.<\/li>\n<\/ol>\n

    And in the n:n scenario all this possibilities above also apply. Just that each participant uses collections.<\/p>\n

    \"\"<\/a>\"\"<\/a>\"\"<\/a><\/p>\n

    Navigation-Paths Dictate<\/h3>\n

    Which one of the 3 possibilities do you pick? The main criteria is how you\u2019re application navigates through the data. For example when you\u2019re accessing the blog posts and only want to know who has written this post, you add a reference to the author. When your always getting a post via its author, you add a collection of posts to the author. And when your application uses both navigation paths, you use a bidirectional relation.<\/p>\n

    The same applies for the post-tag relationship. When you only want to know which tag a post has, you add a collection to the post with the tags. When you want to know which post have a certain tag, you add a collection of all posts to the tag.<\/p>\n

    Bidirectional References: A “chicken or the egg” dilemma<\/h3>\n

    As soon as you have bidirectional references, you cannot avoid a \u2018chicken or the egg\u2019-dilemma. You have to create one instance first and then later assign it. Sometimes it\u2019s easy solvable by adding a factory-method to the parent-class. Like this: