You got it exactly right. :-) Larry
On 8/24/05, Alan Chandler <[EMAIL PROTECTED]> wrote: > Larry Meadors writes: > > ... > > > > The groupBy attribute is used to deal with the "N+1" selects problem. > > The "N+1 Selects" problem is caused by trying to load child records > > that are related to a list of parent records. So, if you run one query > > to get the parent records, and there are some number "N" of them, then > > you have to run "N" more queries to get the child records for the > > parent records resulting in "N+1 selects". > > Yes I FULLY understand the above > > > > > > Here is another example that does an Account -> Order -> OrderItem > > mapping using this technique: > > > > <resultMap id="ResultAccountInfoNMap" class="AccountInfo" > > groupBy="account.accountId" > > > <result property="account.accountId" column="accountId" /> > > <result property="orderList" resultMap="Ch7.ResultOrderInfoNMap" /> > > </resultMap> > > > > <resultMap id="ResultOrderInfoNMap" class="OrderInfo" > > groupBy="order.orderId" > > > <result property="order.orderId" column="orderId" /> > > <result property="orderItemList" resultMap="Ch7.ResultOrderItemNMap" /> > > </resultMap> > > > > <resultMap id="ResultOrderItemNMap" class="OrderItem"> > > <result property="orderId" column="orderId" /> > > <result property="orderItemId" column="orderItemId" /> > > </resultMap> > > > > <select id="getAccountInfoListN" resultMap="ResultAccountInfoNMap"> > > select > > account.accountId as accountid, > > orders.orderid as orderid, > > orderitem.orderitemid as orderitemid > > from account > > join orders on account.accountId = orders.accountId > > join orderitem on orders.orderId = orderitem.orderId > > order by accountId, orderid, orderitemid > > </select> > > I may be a little dense here - can I just confirm. In class "AccountInfo", > you have a property called "account" which is itself an object of some > class(doesn't precisely matter, the class definition for AccountInfo will > have imported the definition). This sub object has a property called > "accountID" (and similarly for "order"). > > [only asking because this seems to make the example slightly more complex > than it needs for explanation, and I am just making sure I understand > correctly]. > > > > > Does that make it any clearer? > > > Well... I still don't understand exactly what the groupBy attribute is > actually saying here. Let me run one proposition by you and see if I have > it right. > > Is it saying account.accountID is the common field that the query is joined > on, and therefore there will be a lot of records where this will be a common > factor - and to take all of them and use them to only create a single > instance of class AccountInfo and to populate orderList with all the > variants of this particular common field (and of course a similar position > this with OrderInfo and orderItemList)? > > If that right, then I think I've got it. I know when I need to use it. > > If not ... > > ... then you will have to try again:-( > > > > > -- > Alan Chandler > [EMAIL PROTECTED] > > >
