Symon, how would this happen? Are you using session-per-request or session-per-conversation.
On Dec 16, 7:41 am, "Symon Rottem" <[email protected]> wrote: > Becuase the hashcode is generated from the lifetime id and would change if > you based it on the database allocated id at some point later. Since the > entity could still be in memory there's a chance that the hashcode could > change which would break the hashcode contract and possibly allow you to add > the same entity to a hash table more than once. > > Symon. > > > > On Tue, Dec 16, 2008 at 2:33 PM, epitka <[email protected]> wrote: > > > I interested in why you have to persist LifetimeId. Could you not use > > a lifetimeid only if instance does not have a "recordId"? > > > On Dec 16, 4:29 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... > > > > 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. > > > > 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 Rottemhttp://blog.symbiotic-development.com > > -- > Symon Rottemhttp://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 -~----------~----~----~----~------~----~------~--~---
