Ayende, can you expand on this?
On Dec 16, 8:45 am, "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
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