Skip to main content

Saving NHibernate objects with assigned id’s

One of the great features of Nhibernate is that it manages persistance for us. You just attach an object to the session and NHibernate will figure out if this object is added or changed. But how does NHibernate knows the difference between a new and existing object?

By default it uses the value we assigned to the unsaved-value attribute on the id mapping. This means that if the id of our object is equal to our unsaved-value that NHibernate will detect this object as new and do an INSERT statement. If the id value is different from our unsaved-value NHibernate will generate an UPDATE statement instead.

 

   1:  <hibernate-mapping default-cascade="none" xmlns="urn:nhibernate-mapping-2.2">
   2:    <class name="Test.Data.Domain.Category, Test.Data" table="Categories" lazy="true">
   3:      <id name="CategoryID" type="System.Int32" column="CategoryID" unsaved-value="0">
   4:        <generator class="native" />
   5:      </id>
   6:    </class>
   7:  </hibernate-mapping>


Sounds easy but what if you are using a composite key? In that case using the unsaved-value makes no sense. If we have a look at the documentation NHibernate gives us a second option:

A version or timestamp property should never be null for a detached instance, so Hibernate will detact any instance with a null version or timestamp as transient, no matter what other unsaved-value strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid any problems with transitive reattachment in Hibernate, especially useful for people using assigned identifiers or composite keys!

So if you leave your version column empty, NHibernate will always detect the object as new.

Labels:

Popular posts from this blog

Podman– Command execution failed with exit code 125

After updating WSL on one of the developer machines, Podman failed to work. When we took a look through Podman Desktop, we noticed that Podman had stopped running and returned the following error message: Error: Command execution failed with exit code 125 Here are the steps we tried to fix the issue: We started by running podman info to get some extra details on what could be wrong: >podman info OS: windows/amd64 provider: wsl version: 5.3.1 Cannot connect to Podman. Please verify your connection to the Linux system using `podman system connection list`, or try `podman machine init` and `podman machine start` to manage a new Linux VM Error: unable to connect to Podman socket: failed to connect: dial tcp 127.0.0.1:2655: connectex: No connection could be made because the target machine actively refused it. That makes sense as the podman VM was not running. Let’s check the VM: >podman machine list NAME         ...
Read more

Azure DevOps/ GitHub emoji

I’m really bad at remembering emoji’s. So here is cheat sheet with all emoji’s that can be used in tools that support the github emoji markdown markup: All credits go to rcaviers who created this list.
Read more

Cache stampede: when our cache turned against us

While investigating some performance issues, we ran into an ASP.NET Core API that cached a fairly expensive aggregation query for 60 seconds. Under normal load, that was fine: one request rebuilds the cache, everyone else reads from it. Under peak load, dozens of requests would arrive in that same expiry window, all see a cache miss, and all fire the same expensive query in parallel. The database didn't like that. That was the moment when our caching layer stopped helping and started hurting. A burst of requests comes in at the same time, all miss the cache, and all go hammer the database or the downstream API at once. That's a cache stampede . The cache was supposed to protect our backend, and for a few hundred milliseconds it did the opposite. Why this happens IMemoryCache.GetOrCreate (and its async sibling) looks like it protects you, but it doesn't add any locking on its own. Look at the naive version: public async Task<Report> GetReportAsync(string key) ...
Read more