Yes or no: Should models in MVC contain application logic?

Question

Yesterday I had some discussion with one of our developers regarding MVC, more precisely about the role of the model component in MVC.

In my opinion, a model should just contain properties and almost no functionality so there are as few methods in model classes as possible.

My collegue though believes that models could and should have more than that and offer a lot more functionality.

Here is an example we argued about.

Example 1

Let's say we wanted to create a blog. A blog has articles and tags. Each article can have multiple tags and each tag can belong to multiple articles. So we have a m:n relation here.

In pseudocode it'd probably look something like this:

class Article{     public int id;     public String title;     public String content;     public Tag[] tags;      // Constructor     public void Article(id, title, content, tags){         this.id = id;         this.title = title;         this.content = content;         this.tags = tags;     } }  class Tag{     public int id;     public String name;      // Constructor     public Tag(id, name){         this.id = id;         this.name = name;     } } 

Now, assume that we're working loose coupled here which means that it could happen that we have an instance of Article which has no Tags yet so we'll use an Ajax call (to our backend which has a database containing all the information) to get the tags that belong to our article.

Here comes the tricky part. I believe that getting the backend data via Ajax+JSON should be the controller's job using a dedicated class which deals with the ajax request using a parser:

class MyController{     private void whatever(articleID){         Article article = (Article) ContentParser.get(articleID, ContentType.ARTICLE);         doSomethingWith(article);     } }  public abstract class ContentParser{     public static Object get(int id, ContentType type){         String json = AjaxUtil.getContent(id, type.toString()); // Asks the backend to get the article via JSON         Article article = json2Article(json);          // Just in case         Tag[] tags = article.tags;         if (tags == null || tags.length <= 0){             json = AjaxUtil.getContent(article.id, ContentType.TAGS); // Gets all tags for this article from backend via ajax             tags = json2Tags(json);             article.tags = tags;         }          return article;     }      // Does funky magic and parses the JSON string. Then creates a new instance of Article     public static Article json2Article(String json){         /*          ...         */         return new Article(id, title, content, tags);     }      // Does funky magic and parses the JSON string. Then creates a new instance of Tag     public static Tag[] json2Tags(String json){         /*          ...         */         return tags;     }  } 

Example 2

My collegue believes that this breaks with the idea of MVC, he suggests that the model should take care about this:

class Blog{     public int id;     public String title;     public Article[] articles;      // Constructor     public Blog(id, title, articles){         this.id = id;         this.title = title;         this.articles = articles;     }      public void getArticles(){         if (articles == null || articles.length <= 0){             String json = AjaxUtil.getContent(id, ContentType.ARTICLE); // Gets all articles for this blog from backend via ajax             articles = json2Articles(json);         }         return articles;     }      private Article[] json2Articles(String json){         /*          ...         */         return articles;     }  }  class Article{     public int id;     public String title;     public String content;     public Tag[] tags;      // Constructor     public Article(id, title, content, tags){         this.title = title;         this.content = content;         this.tags = tags;     }      public Tag[] getTags(){         if (tags == null || tags.length <= 0){             String json = AjaxUtil.getContent(id, ContentType.TAGS); // Gets all tags for this article from backend via ajax             tags = json2Tags;         }         return tags;     }      // Does funky magic and parses the JSON string. Then creates a new instance of Tag     private Tag[] json2Tags(String json){         /*          ...         */         return tags;     } } 

And outside of the model you'd do: blog.getArticles(); or article.getTags(); to get the tags without bothering with the ajax call.

However, as handy as this might be I believe that this approach breaks with MVC because at the end of the day all models will be full of methods that do various funky stuff and the controller and helper classes do almost nothing.

In my understanding of MVC, models should only contain properties and a minimum of "helper methods" inside. For example a model "Article" could offer a method getNumOfTags() but it shouldn't do any Ajax calls on its own.

So, which approach is correct?

Answer 1

Generally I try to keep controllers simple in terms of logic too. If business logic is required, it will go up to 'service layer' classes to handle it. This also saves repeating any code/logic too, which ultimately makes the whole project more maintainable if business logic was to change. I just keep models purely as entity objects.

I think the answer above sums it up nicely though, it is easy to over engineer a project based on design patterns: Go with whatever works for you and is most maintainable/efficient.