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From "Bob Hanson" <mnbo...@gmail.com>
Subject Re: Kind of architecture question
Date Tue, 08 May 2007 16:04:44 GMT
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 <peix-listas@praxia.com.ar> 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:Tom.Nguyen@rels.info]
> Sent: Martes, 08 de Mayo de 2007 11:28 a.m.
> To: user-cs@ibatis.apache.org
> 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 <peix-listas@praxia.com.ar> 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:shanecourtrille@gmail.com]
> Sent: Lunes, 07 de Mayo de 2007 07:03 p.m.
> To: user-cs@ibatis.apache.org; peix-listas@praxia.com.ar
> 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 <peix-listas@praxia.com.ar> 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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