I figure Entity And Hash Code Provider in AA (Ayende's Acronyms) Gustavo.
On Tue, Dec 16, 2008 at 9:12 PM, Symon Rottem <[email protected]> wrote: > Excuse my ignorance, but what's EAHCP? > > Agreed, the second issue won't happen if you remain in the context of a > session, but this bit me when I was working with a non-web app and had > entities that had a lifetime in memory between sessions. I only made the > point because I was bitten and figure it's worth others knowing about. > > Cheers, > > Symon. > > > On Tue, Dec 16, 2008 at 3:45 PM, Ayende Rahien <[email protected]> wrote: > >> >> >> 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 >>> >>> >>> >>> >> >> >> > > > -- > 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 -~----------~----~----~----~------~----~------~--~---
