Question

I’ve looked at How to store data with dynamic number of attributes in a database, but it didn’t quite answer my question. The system I am implementing is rather large, so I’ll stick with the objects I am concerned about: Item and Category. An Item has a variable number of attributes depending on the Category that it belongs to. For instance, Items belonging to the “T-shirt” Category would have an attribute for size, while Items belonging to the “Car” Category would have an attribute for model. Someone logged into the system as an Admin can create new Categories with completely new attributes. What’s the best way to structure this both in my Java code and in my database to make this work?

If the categories weren’t dynamically created, I’d use inheritance so that I would have a TShirtItem object with its specific attributes filled in. But since it is dynamic, I’m puzzled. I saw a similar question that suggested using the Map data structure, but I am not sure how that would work…

The question I referred to at the top would suggest on the database end that I would have an Category table, and Attributes table, and relationship table linking what attributes go with what Category. This makes sense. But I’m tripped up again when thinking about the Item object. I could store the unique Category_ID in the Item table, but where would I store the attributes for each Item?

Answer 1

Here is a simple java approach to do it. While designing large systems I always suggest to look at the bigger picture. The issue here is dynamically changing attributes. It would not be easy but yeah it is interesting.

The structure of your item class has to be like the following:

class Item{
   private String itemName; // I assume all items will have a name.
   private Map<ItemAttibuteName , Object> attributeMap ; // this will be a dynamic map.

   public Map<ItemAttibuteName, Object> getAttributeMap(){//getter for attribute map
       if( null == attributeMap)
          return new HashMap<String, Object>(); 
       return attributeMap ;

   }
   // you can synchronize this if needed
   public void setAttribute(ItemAttibuteName name, Object value){ 
       attributeMap.put(name, value);
   }

   public Object getAttribute(ItemAttibuteName name){ 
       return attributeMap.get(name);
   }
}

public enum ItemAttibuteName{
    SIZE,
    COLOUR        
}

This approach suites your needs, further you can use a factory pattern to instantiate the Items depending on the category and make it an elegant code.

Please get back if you still have some doubts.

EDIT:
Now there will be a complicated way to get all the attributes of an element in the code while programming, because you don’t have getters or setters for the attributes. Well, the approach will help if you can maintain a Set of attributes added to the Item, in the Item class or you can query for the keySet of the item class’s attributeMap property.

private Set<ItemAttibuteName> attributes;

or

attributeMap.keySet();

Happy to Help

Dharam