Strategies for Mapping Views in NHibernate

Question

It seems that NHibernate needs to have an id tag specified as part of the mapping. This presents a problem for views as most of the time (in my experience) a view will not have an Id. I have mapped views before in nhibernate, but they way I did it seemed to be be messy to me.

Here is a contrived example of how I am doing it currently.

Mapping

  <class name="ProductView" table="viewProduct" mutable="false" >
    <id name="Id" type="Guid" >
      <generator class="guid.comb" />
    </id>
    <property name="Name" />
<!-- more properties -->
  </class>

View SQL

Select NewID() as Id, ProductName as Name, --More columns
From Product

Class

public class ProductView
{
    public virtual Id {get; set;}
    public virtual Name {get; set;}
}

I don't need an Id for the product or in the case of some views I may not have an id for the view, depending on if I have control over the View

Is there a better way of mapping views to objects in nhibernate?

Edit
Answer So Far

Mapping

  <class name="ProductView" table="viewProduct" mutable="false" >
    <id name="Id" type="Guid" />
    <property name="Name" />
    <!-- more properties -->
  </class>

Class

 public class ProductView
    {
        public virtual Name {get; set;}
        //more properties
    }

View SQL
Do I still need NewID()?

Select NewID() as Id, ProductName as Name, --More columns
From Product

score 5 · Accepted Answer · answered Jun 03 '10 at 05:41

5

You can make it just a little bit cleaner by not mapping the Id to a property and omitting the generator:

<id column="Id" type="guid"/>

That way, you keep the problem in the data layer, without leaking the implementation detail to your domain.

answered Jun 03 '10 at 05:41

Diego Mijelshon

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does that mean that I can remove the NewID() from the TSQL Select as it is only there to satisfy nhibernate's need to an ID – Nathan Fisher Jun 03 '10 at 05:53
2

No, that means your class doesn't need an Id property. – Ben Hoffstein Jun 03 '10 at 12:21
Ben's right. NHibernate does not need the Id to be mapped to a property, but it always needs a way to identify persistent instances. – Diego Mijelshon Jun 03 '10 at 15:09
thanks Diego, Ben. Ok let see if I understand correctly. I need to include the id tag in the mapping, I dont need an Id property in my class. Do I then need the need the newid() as part of sql or can that be droped as well and nhibernate will assign a guid to it? – Nathan Fisher Jun 03 '10 at 23:47
You still need the newid(); NHibernate can only assign Ids on save, not make them up on load. – Diego Mijelshon Jun 04 '10 at 11:24
Thanks Diego, I don't want nhibernate to save if it is a view. I only want it load. and if I have to explicitly include a newID() in the TSQL to get around this then so be it. unless there is another way that I am missing? – Nathan Fisher Jun 08 '10 at 05:13

score 2 · Answer 2 · answered Jun 03 '10 at 02:58

2

As far as I know, NHibernate will require either an id or a composite-id definition since it's the mechanism by which it uniquely identifies a given record. If there is no combination of columns that provides a key for each row in the view, I think you are stuck with hacky workarounds.

answered Jun 03 '10 at 02:58

Ben Hoffstein

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I wish that when you mark a class as `mutable=false` that the explicit need for an Id goes away. not sure if that is practical but would make scenarios like this easier to handle, imo. – Nathan Fisher Jun 03 '10 at 03:58

Strategies for Mapping Views in NHibernate

2 Answers2

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