On Tue, Dec 16, 2008 at 5:29 AM, Symon Rottem <[email protected]> wrote:
> There are a couple of problems with this approach - it's pretty good, but I > it's still got a couple of holes. > > There are a couple of issues: > > > 1. The cast in the equals method will not necessarily result in the type > you're expecting: > > > > T other = obj as T; > > > > If the current instance is a DomesticCat and the passed instance is a Cat > proxy that, in fact, represents a DomesticCat instance then the cast would > fail and return null because the Cat proxy cannot be cast to DomesticCat. > This could be worked around using the NHibernateUtils.GetClass(entity) > method, but that might cause performance issues since the DB would need to > be hit for proxies... > > This doesn't happen in practice. Because it is Cat that inherit from EAHCP. > > > 2. This approach will still break if you have a transient entity that you > persist then evict from the Session thereby making it disconnected then > compare it with another loaded copy of same entity; the loaded entity and > the disconnected entity will be seen as equal but will have different > hashcodes breaking the contract which indicates that if equals() returns > true then hashcode comparison should also return true. > > This approach assume that you are using an entity in the context of a session. If you try to mix things, it is on your head to make sure everything works. > Certainly the approach will work for the majority of circumstances, but > it's probably worth being aware of the pitfalls just in case you fall into > them. :) > > Personally I've worked around the problem by making a base class for my > entities that has a read only "lifetime id" property that's allocated a GUID > value at instantiation and is used for equality and hashcode comparisons. > Note that this property is *not* used as the identity map - my entities > still have an Id property for that. The "lifetime id" property is persisted > and mapped using field access so the read only property can be set when a > persisted entity is loaded. > > > In effect, the GUID is generated when the transient instance is > instantiated and is then persisted with the object; at any point that the > persistent entity is reloaded the value is reloaded with it. If the entity > is evicted from the session or the session is closed making it a > disconnected entity the lifetime id doesn't change. If the entity is > deleted and made transient it still remains the same. You could even > re-persist it. > > Of course, the drawback is that every entity row must now store an > additional GUID, however it's not necessary to have an index on this column > as it will never be searched, so it's not *too* expensive. You might want > to make it unique, however, but I don't this it's essential as the > likelyhood of having two conflicting GUIDs in memory at the same time seems > rather low. > > There may be a better way of handling this, but I haven't found it. :) > > Cheers, > > Symon. > > > > > On Tue, Dec 16, 2008 at 5:33 AM, Ayende Rahien <[email protected]> wrote: > >> http://ayende.com/Blog/archive/2007/06/05/Generic-Entity-Equality.aspx >> >> >> On Mon, Dec 15, 2008 at 11:32 PM, Tim Barcz <[email protected]> wrote: >> >>> I am reading through a book on NHibernate (NHIbernate in Action, Manning) >>> and when talking about comparing entity values based on database identifier >>> (which is what EntityBase does) it strongly discourages equality based on >>> database Id's: >>> >>> Unfortunately, this solution has one huge problem: NHibernate doesn't >>>> assign identifier values until an >>>> entity is saved. So, if the object is added to an ISet before being >>>> saved, its hash code changes while it's >>>> contained by the ISet, contrary to the contract defined by this >>>> collection. In particular, this problem makes >>>> cascade save (discussed later in this chapter) useless for sets. We >>>> strongly discourage this solution (database >>>> identifier equality). >>> >>> >>> Generally DDD looks at an Entity's unique ID for determining equality. >>> However I'm a bit concerned at the strong warning from the NHibernate camp >>> about this type of equality comparison. >>> >>> What's the thought on this? I'd be interested in hearing arguments on >>> either side. >>> >>> Tim >>> >>> >> >> > > > -- > Symon Rottem > http://blog.symbiotic-development.com > > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "nhusers" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/nhusers?hl=en -~----------~----~----~----~------~----~------~--~---
