Question

I would like to know the best way to populate an object that has a collection of child objects and each child object may inturn have a collection of objects, from database without making multiple calls to the database to get child objects for each object. basically in hierarchical format something like for example a customer has orders and each order has order items. is it best to retrieve the data in xml format (SQL server 2005), or retrieve a dataset by joining the related tables together and then map that the data to the object? thanks in advance for your help.

Answer 1

There are a lot of variables still there:

1. Are the child objects of the same type? If so you can select them all at the same time and then set up the parent/child relationships in your object mapping layer.
2. Can the child objects have children of their own? If the nesting is unlimited, then you can’t get all the data at the same time unless you get all the data.

You could certainly do a join on all of the customers->orders->order items and break everything up in code, but that seems like that would be a lot of overhead in duplicated parent rows and a lot of work in processing that big mess.

Trying to avoid doing multiple calls might be a pre-mature optimization. Are you having performance problems with doing too many calls to the database?

Edit: Based on your comments, you should be able to do one query per hierarchy level:

Select * from orders 
where orders.customerid = my_customer_id

–Do some orm mappings and make a list of child object ids–

Select * from child_order_object 
where child_order_object_id in (list of child object ids)

–Do some more ORM mapping and link child objects to previous parent objects–

…

–Repeat for more levels–

You should be able to have just one query per relationship level rather than the exploding amount of queries to get just one object by id.