PetaPOCO and more than 4 joins

问题

Is petapoco capable of achieving the following : 1.Unlimited joins in one query 2.Unlimited One to Many relations in one query

I have looked at PetaPOCO and it seems like it is not capable of doing more than 4 joins, the longest signature looks like : db.Query<T1, T2, T3 , T4>

Also seems like it supports a one to many relation , but only for one composite object such as below : db.FetchOneToMany<T1, T2> where T2 is a foreign key of T1

I'm testing some of the micro ORMs out there to stick to the best one. Do you know of any of them that can handle these situations and if none of the micro ORMs are supporting this feauture, how do you deal with an object that is like the following :

class A
{
    List<B> member1;
    List<C> member2; 
    Z  member3; //Composit object
    Z1 member4; //Composit object
    Z2 member5; //Composit object
    Z3 member6; //Composit object
    Z4 member7; //Composit object
}

And then even more complicated is , what if member one (type B) has some composite object within itself ? What if we have :

class B
{
    G member0;
}

Please don't propose a solution to hit database multiple times, coz it's going to be way too many calls when the objects become just a little bit complex.

Oh and i also know that one other way of tackling the case of unlimited joins is creating a very flat object that hols all fields combined. It's not an elegant solution at all.

回答1:

The T1..T$ Query() overloads all pass through to the main Query(..Type[]..) method. You can either add more Query() overloads yourself to handle more T parameters, or pass in all the types you need in a Type array (which is what the T1-T4 functions do) :

Query<TRet>( new Type[]{typeof(Poco1), typeof(Poco2), typeof(Poco3), typeof(Poco4), typeof(Poco5)}, null, sql, args);

You can have multiple one to many relationships but Schotime is right, you need to be very careful of swathes of duplicate data coming back in your result set. Write the sql query and look at the result set, is the amount of duplication acceptable to you? If so then in Petapoco there is a concept of relator callbacks where you write a small class that handles the different pocos in a single result row and add each poco to the list properties on the parent poco.

http://www.toptensoftware.com/Articles/115/PetaPoco-Mapping-One-to-Many-and-Many-to-One-Relationships

I've never had to do this with multiple one to many but quoted from the above

"If you're joining more than two tables you'll need something more complex but it's really just extensions of the above."

Another option is to have a stored procedure that does all the work in a single database request and have it return multiple result sets which I believe Schotime has achieved in his branch of petapoco but I've not used it myself yet so I can't really comment on if it will help here :

http://schotime.net/blog/index.php/2011/11/20/petapoco-multiple-result-sets/

If I absolutely had to wire up all the data in one go for objects as complex and nested as you are suggesting then I would use a stored procedure (a single db call) and stitch it all together with code. Only then would I figure out how to do this in Petapoco. However if your UI doesn't show all the nested data until the user clicks on an expander button (or similar) I'd use an AJAX call at that point rather than get all the data initially.

回答2:

The answer is correct, but I came to this page from another forum and no one there could make this work, so I thought I would chip in what I did to make things clearer. Basically, I had code like the following:

var sql = "select * from someTable where tableId = @0";
var listOfStuff = _petapoco.Fetch<FirstType, SecondType, ThirdType, FourthType, FirstType>(new RelatorClass().MapIt, sql, idVar);

Since I needed to add in a fifth poco, and all the Fetch methods eventually lead to the master Query method listed above in the accepted answer, I had to do this:

var sql = "select * from someTable where tableId = @0";
Func<FirstType, SecondType, ThirdType, FourthType, FifthType, FirstType> mapIt = new RelatorClass().MapIt;
var listOfStuff = _petapoco.Query<FirstType>(new[] { typeof (FirstType), typeof (SecondType), typeof (ThirdType), typeof (FourthType), typeof(FifthType)}, mapIt, sql, idVar).ToList();

Now I can query with 5 pocos and I didn't have to modify the PetaPoco code. The only other thing to do would be to add to your relator class so you can tell PetaPoco where to map the new data and you're good to go.

来源：https://stackoverflow.com/questions/11268519/petapoco-and-more-than-4-joins

标签

orm

petapoco