I think it really boils down to how you choose to implement your
domain. In your example below, why does Order reference customer? Is
Order an aggregate? If not, all access to Order should be through
Customer and then you don't need a reference.
foreach ( Order order in customer.Orders ) {
Console.WriteLine( "Order #{0} from {1}", order.Number, customer.Name );
}
Otherwise, here are some things I have done: (.NET 2.0)
My domain classes implement IEquatable<T> which does a deep
comparison. I might not have an equal reference so I check for data
equality instead. The simplest implementation could just compare Ids
but that presents a problem when checking for "dirty" data.
Reference example: I have an Application class which contains a list
of ApplicationUsers (sub-class of User). An ApplicationUser is a User
of an Application (has a reference to the Application) with a list of
Roles for that Application. My domain defines an interface
IApplicationRepository. I have a single repository implementation
(IBatisRepository.Repository) which implements all of my repository
interfaces. The repository method that returns an application (with
all of its application users via a sql join) assigns a reference AFTER
the query completes:
public Application GetApplication(string applicationName)
{
Application application =
SqlMapper.Instance().QueryForObject<Application>("SelectApplicationByName",
applicationName);
if (application != null)
{
foreach (ApplicationUser user in application.Users)
{
user.Application = application;
}
}
return application;
}
On 5/8/07, Carlos Peix <[EMAIL PROTECTED]> wrote:
Hi Tom,
Yes, I know that it's not IBatis concern, I'm not asking Gilles to add
support for this.
But this operation mode (to stop calling this an issue :-) not only gives me
different instances of the same object (I can fall in the trap modifying one
object and reading another unmodified instance in may code) but it have
performance implications.
Think for a minute in the example I posted (it's below you answer in this
post). Imagine that the customer holds a reference to the collection of
orders, something like:
Customer {
// Mapped collection, with getter and setter, of course.
private IList orders;
}
and the order has a reference to the Customer, just like the example below.
if I code this way:
Customer customer = CustomerRepository.GetById( 1 );
and then I do (in another method, down the code path):
foreach ( Order order in customer.Orders ) {
Console.WriteLine( "Order #{0} from {1}", order.Number,
order.Customer.Name );
}
I'm loading the customer again with all the orders (I know I can use lazy
load for this or that I can send the original customer instance).
I just want to know how other people is handling this.
Thanks
Carlos
________________________________
From: Nguyen, Tom [mailto:[EMAIL PROTECTED]
Sent: Martes, 08 de Mayo de 2007 11:28 a.m.
To: [email protected]
Subject: RE: Kind of architecture question
I agreed with Sam that this should not be a concern for iBatis. I've also
battled with the same issues in the pass, but I've found that it's much
easier to use SQL for controlling of identity and concurrency (through
various methods such as comparison of OriginalValue or LastModifiedDate).
Repository implementation is a can of worm in itself. I prefer to use light
weight object and don't have to worry about issues such as multi-threading,
caching and releasing references for Garbage Collector when it's not being
reference. Also, you can't really ensure identical object references in a
webfarm or multiple application server SOA implementation.
On the other hand, I had a very specific need of repository base on lookup
business objects such as OrderType, BusinessType, ContactType, etc… In this
case, I implement custom in-memory caching (repository). It's a very
specific implementation which allow me to lazy load objects by identity
(key) and type, cache, timely clearing of cache for GC, etc..
Regards,
Tom Nguyen
Sr. Developer
On 5/7/07, Carlos Peix <[EMAIL PROTECTED]> wrote:
Hi Shane, thanks for taking the time to answer.
Yes, you are talking about an Identity Map I guess, but this solution is
harder to implement and tune. It's my last resort though.
Of course, definitely the select="CustomerById" needs to be removed. In
fact, the command CustomerById uses a different cache that the command
CustomerAll so, if we use cache for both commands we can stop IBatis
building different instances.
One approach I used is for resolving this is implementing the
CustomerRepository.GetAll() with an IBatis query (cached) and the
CustomerRepository.GetById() resolved in memory. But I'm not very happy with
this solution either
Of course, the Order ResultMap just get the Customer Id and I resolve the
property getter or setter that way.
class Order {
// This field is mapped in IBatis
object customerId;
// this is a transient field
private Customer customer;
public Customer Customer
{
get {
if ( customer == null )
customer = CustomerRepository.GetById( customerId );
return customer;
}
set { customerId = value.Id; }
}
}
Carlos
________________________________
From: Shane Courtrille [mailto:[EMAIL PROTECTED]
Sent: Lunes, 07 de Mayo de 2007 07:03 p.m.
To: [email protected]; [EMAIL PROTECTED]
Subject: Re: Kind of architecture question
I'm just learning iBatis.Net now but I have used the repository <-> mapper
pattern in the past. Usually what I have done is have the repository
contain a reference to a cache. It checks the cache before using the
mapping layer to retrieve the item. The problem I see is your Order
select="CustomerById". Without knowing more about iBatis I would suggest
you may need to remove that and instead have your repository fill in the
reference after it gets the Order. Definitely not a solution I'm in love
with though.
Shane
On 5/7/07, Carlos Peix <[EMAIL PROTECTED]> wrote:
Hi all,
I used IBatis.NET with success in various projects now but I am still not
very happy with my implementations. I'll try to explain my concerns.
My environment is .NET 1.1 but I think .NET 2.0 is the same thing. I should
say that I try to work guided by the DDD principles, so I access the final
store through repositories.
I have, for example, an entity Customer (aggregate root or simply a
persistent identifiable object) and a CustomerRepository with the following
interface:
Customer CustomerRepository.GetById( object id );
IList CustomerRepository.GetAll();
...
and I also have a Order and OrderRepository with the following interface
OrderRepository.GetByNumber( int number );
...
The problem I face all the time with IBatis is the I get different instances
if I do:
// implemented with a Mapper.QueryForObject( "CustomerById", id );
Customer customer1 = CustomerRepository.GetById( object id );
// implemented with a Mapper.QueryForList( "CustomerAll" );
Customer customer2 = CustomerRepository.GetAll()[0];
// implemented with a Mapper.QueryForObject( "OrderByNumber", number );
// and Customer mapped this way in the OrderResultMap:
// <result property="customer" column="CustomerId" select="CustomerById" />
Customer customer3 = OrderRepository.GetByNumber( 100 ).Customer;
But customer1.Id, customer2.Id and customer3.Id are the same.
Ok, this is a situation that need to be controlled, otherwise I could modify
or check different instances of the same object. I was told previously that
this is not an IBatis problem and I see why (in fact IBatis doesn't know
anything about object identity, so it can't control this).
The question is: how are you structuring your code to control that
situation? there are some "recommended practices"? I started with some ideas
(one of them include IBatis cache configuration) but I'm not happy with any
of them.
Sorry about the long post and thanks in advance.
Carlos Peix
________________________________
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