I'm pretty sure what sort of things can be done with NHibernate but I wanted to check with the community on this one.
If I have an entity Foo:
public class Foo
{
public int Id { get; set; }
public string Name { get; set; }
public bool HasRegistered { get; set; }
}
I would like the property HasRegistered to be true if there is a corresponding record in the Actions table (i.e. has the Foo's Id as a foreign key and a particular code 'BLAH' in another field), and to be false if there is not.
So, for example, HasRegistered would be true if there is an Action record with the following fields:
FooId (equal to the Foo's Id)
Code (equal to the value 'BLAH')
Is such mapping possible?
From my knowledge of NHibernate, this is not possible with a "transparent" mapping feature, like property or as you can do with bags os sets and so on..
The simplest way to that is using a formula over that property and make that property "un-mutable" so the property does'm involve in insert/update scenarios.
hope this helps.
so this could be an example:
<property
name="HasRegistered"
formula="sql formula to evaluate"
insert="false" update="false"/>
as source of more info about nhibernate mapping features about (property in this case) is the following:
NHibernate mapping: property
Related
Is there something analogous on NHibernate regarding Entity Framework's navigation property? For example, instead of:
s.Save(new Product { Category = s.Get<Category>("FD"), Name = "Pizza" });
I wish I could write:
s.Save(new Product { CategoryId = "FD", Name = "Pizza" });
Can I inform NHibernate not to use the Product's Category property as a mechanism to save the Product's category? I want to use CategoryId instead(Read: I don't want to use DTO). Entity Framework seems able to facilitate avoiding DTO patterns altogether, while at the same time offering the full benefit of ORM(can avoid joins using navigation properties). I want the EF's offering the best of both worlds(lean mechanism for saving objects, i.e. no need to retrieve the property's object) and navigation mechanism for querying stuff
Sample from EF: http://blogs.msdn.com/b/adonet/archive/2011/03/15/ef-4-1-code-first-walkthrough.aspx
public class Category
{
public virtual string CategoryId { get; set; }
public virtual string Name { get; set; }
public virtual IList<Product> Products { get; set; }
}
public class Product
{
public virtual int ProductId { get; set; }
public virtual string Name { get; set; }
public virtual string CategoryId { get; set; }
public virtual Category Category { get; set; }
}
[UPDATE]
Regarding James answer, I tried seeing the NHibernate's actions in SQL Server Profiler.
// this act didn't hit the Category table from the database
var c = s.Load<Category>("FD");
// neither this hit the Category table from the database
var px = new Product { Category = c, Name = "Pizza" };
// this too, neither hit the Category table from the database
s.Save(px);
Only when you actually access the Category object that NHibernate will hit the database
Console.WriteLine("{0} {1}", c.CategoryId, c.Name);
If I understand your question, you want to save a Product with a Category without hitting the database to load the Category object. NHibernate absolutely supports this and you almost have the right code. Here is how you do it in NHibernate:
s.Save(new Product { Category = s.Load<Category>("FD"), Name = "Pizza" });
This will not hit the database to fetch the actual Category, but it will simply save a Product with the correct Category.Id. Note that you don't need (and I would recommend getting rid of Product.CategoryId).
Now why does this work with session.Load(), but not session.Get()... With session.Get(), NHibernate has to return the object or null. In .NET, there is no way for an object to replace itself with null after the fact. So NHibernate is forced to go to the database (or L1 cache) to verify that the "FD" Category actually exists. If it exists, it returns an object. If not, it must return null.
Let's look at session.Load(). If the object is not present in the database, it throws an exception. So NHibernate can return a proxy object from session.Load() and delay actually hitting the database. When you actually access the object, NHibernate will check the database and can throw an exception at that point if the object doesn't exist. In this case, we're saving a Product to the database. All NHibernate needs is the Category's PK, which it has in the proxy. So it doesn't have to query the database for the Category object. NHibernate never actually needs to hydrate an actual Category object to satisfy the save request.
I had a similar question to Fluent NHibernate: How to create one-to-many bidirectional mapping? but I was interested in the situation when I have a one-to-one mapping. For instance
Umbrealla
ID
Owner
UmbreallaOwner
ID
Umbrella
As we know each umbrella can only be owned by one person and nobody owns more than one umbrella. In a fluent map I would have something like
UmbrellaMap()
{
Id(x=>x.ID);
References<UmbrellaOwner>(x=>x.Owner);
}
UmbrellaOwnerMap()
{
Id(x=>x.ID);
References<Umbrella>(x=>x.Umbrella);
}
When creating the tables fluent will create a field in umbrella referncing the ID of umbrellaOwner and a field in umbrellaOwner referencing umbrella. Is there any way to change the mapping such that only one foreign key will be created but the Umbrella property and the Owner property will both exist? The examples I have seen involve setting the relations up in both directions so adding a new Umbrella looks like
AddUmbrealla(UmbrellaOwner owner)
{
var brolly = new Umbrella();
brolly.Owner = owner;
owner.Umbrella = brolly;
session.Save(owner); //assume cascade
}
which seems logical but a bit cumbersome.
Well, a reference is a reference; one object has a reference to the other. The reverse is not necessarily true.
In your case, you MIGHT get away with a HasOne relationship. However, HasOne is normally for denormalized data. Say you wanted more info about the owner, but you could not change Owner's schema because other code depended on it. You'd create an AdditionalOwnerInfo object, and create a table in the schema in which the OwnerID field of the table was a foreign key to Owner, and also the primary key of the table.
Ayende recommends a two-sided References() relationship in 99.9% of one-to-one cases, where the second object is conceptually separate from the first, but there is an implicit "I alone own exactly one thing" type of relationship. You can enforce the "one and one only" nature of the reference using a Unique().Not.Nullable() modifier set on the References mapping.
To streamline the referential setup, consider defining one object (UmbrellaOwner) as the "parent" and the other (Umbrella) as the "child", and in the parent's property setter, set the child's parent to the current reference:
public class Umbrella
{
public virtual string ID { get; set; }
public virtual Owner Owner { get; set; }
}
public class UmbrellaOwner
{
public virtual string ID { get; set; }
private Umbrella umbrella;
public virtual Umbrella Umbrella
{
get{
return umbrella;
}
set{
umbrella = value;
if(umbrella != null) umbrella.Owner = this;
}
}
}
Now, when you assign the child to the parent, the backreference is automagically set up:
var owner = new UmbrellaOwner{Umbrella = new Umbrella()};
Assert.AreEqual(owner, owner.Umbrella.Owner); //true;
I have two different data models that map to the same Car entity. I needed to create a second entity called ParkedCar, which is identical to Car (and therefore inherits from it) in order to stop nhibernate complaining that two mappings exists for the same entity.
public class Car
{
protected Car()
{
IsParked = false;
}
public virtual int Id { get; set; }
public bool IsParked { get; internal set; }
}
public class ParkedCar : Car
{
public ParkedCar()
{
IsParked = true;
}
//no additional properties to car, merely exists to support mapping and signify the car is parked
}
The only issue is that when I come to retrieve a Car from the database using the Criteria API like so:
SessionProvider.OpenSession.Session.CreateCriteria<Car>()
.Add(Restrictions.Eq("Id", 123))
.List<Car>();
The query brings back Car Entities that are from the ParkedCar data model. Its as if nhibernate defaults to the specialised entity. And the mappings are defiantly looking in the right place:
<class name="Car" xmlns="urn:nhibernate-mapping-2.2" table="tblCar">
<class name="ParkedCar" xmlns="urn:nhibernate-mapping-2.2" table="tblParkedCar" >
How do I stop this?
I think you need to set the polymorphism property on the class mapping
<class "Car" polymorphism="explicit" ...
Since ParkedCar extends Car, a query for Car will return both Car and ParkedCar objects. You can restrict the type using HQL using the special class property, i.e. from Car c where c.class = Car. I don't think you can do this with the criteria API.
Alternatively you could filter the list after retrieving it if it's a reasonable size.
I have a scenario in NHibernate where I have a one-to-many relationship between entities Employee and EmployeeStatus.
Employee has properties eg: ID, Name and an IList of EmployeeStatus, whilst EmployeeStatus, for the purposes of this question, just has it's own ID and some free text.
I don't need to hold a reference to Employee from EmployeeStatus, the management of status' will be done purely through the Employee entity - adding to the IList property. IE: I want to quite simply be able to do the following;
Employee e = new Employee();
e.Name = "Tony";
e.StatusList.Add( new EmployeeStatus("Status A") );
e.StatusList.Add( new EmployeeStatus("Status B") );
session.Save(e);
I've tried various methods, including creating a one way one-to-many mapping where inverse is false, cascade set to all-delete-orphan, which all looks like it should work, but it generates an exception about being unable to set the EmployeeId in EmployeeStatus. I'm led to believe that this is because NHibernate wants to do an insert with EmployeeId as NULL and then update it to the ID of the parent.
I guess I'm missing something here, so quite simply - can anyone tell me what my mapping file should look like to achieve the above?
Thanks in advance
Tony
-- edit: Heres a rough idea of the classes as requested --
public class Employee
{
private IList<EmployeeStatus> _statusList;
public Employee()
{
_statusList = new List<EmployeeStatus>();
}
public virtual int Id{ get; set; }
public virtual string Name{ get; set; }
public virtual IList<EmployeeStatus> StatusList
{
get
{
return _statusList;
}
}
}
public class EmployeeStatus
{
public virtual int Id{ get; set; }
public virtual string StatusText{ get; set; }
public EmployeeStatus()
{
}
public EmployeeStatus(string statusText)
{
StatusText = statusText;
}
}
The scenario you've described is just a basic one-to-many mapping. Here is the Fluent NHibernate mapping for this:
public class EmployeeMap : ClassMap<Employee>
{
public EmployeeMap()
{
WithTable("Employee");
HasMany(employee => employee.StatusList)
.Cascade.All();
}
}
You do not need to maintain a reference from EmployeeStatus back to Employee to achieve this.
Turns out that what I want to do isn't possible - you have to have a bi-directional association, and must set the child's parent reference. Not a massive problem I suppose, but didn't want to hold references in the child that I don't need within my code directly.
I may not of explained clearly, but an employee status cannot be linked to more than one employee. It's definitely 1 (employee) to many (status')
In the physical database, the status entity has an employeeID field, which isn't in the domain - IE: I hold no reference back to employee from the status entity, but the physical field should be inferred from the owner of the collection - In fact, it does do this if I set the EmployeeID field in the status table to nullable - it actually executes 2 SQL statements - an insert and then an update, the EmployeeID being set in the update.
Thanks,
Tony
Can you post the code for the classes?
Are you trying to keep a history of statuses for an Employee?
-- Edit --
Looks like you are going to need many-to-many, since the child in the relationship (EmployeeStatus) has no reference back to the parent (Employee).
-- Edit 2 --
If you want the insert to be done as 1 call to the DB, you are going to need to add an Employee property to the EmployeeStatus class, and set the Inverse=true. And I'm pretty sure that you are going to need to add some logic which sets the bi-directional relationship in the objects. I.E.
public void AddStatus(EmployeeStatus status)
{
this.StatusList.Add(status);
status.Employee = this;
}
I have a legacy DB which uses a guid to map children to the parent entity.
In my domain layer, I'd prefer to obscure this quirk, so I'd like to have my parent entity look like this:
public class Parent
{
virtual public int Id {get; protected set; }
virtual public string ParentContent { get; set; }
virtual public Guid ReferenceId { get; set; }
virtual public IDictionary<int,Child> Children { get; set; }
}
public class Child
{
virtual public int Id { get; protected set; }
virtual public Parent { get; set; }
virtual public string ChildContent { get; set; }
}
The parent would then map each Child.Id to Child in the Children dictionary. The Child mapping works fine, but I can't seem to find a reasonable mapping for the parent.
A field named ParentReferenceID exists in both Parent and Child tables, so I've attempted to map this with something like this:
mapping.HasMany<Broker>(x => x.Children)
.Table("Child")
.KeyColumn("ParentReferenceID")
.Inverse()
.AsMap<long>(index=>index.Id,val=>val.Type<Broker>());
Unfortunately, this produces an error:
The type or method has 2 generic parameter(s), but 1 generic argument(s) were provided. A generic argument must be provided for each generic parameter.
To simplify my problem, I started by trying Bag semantics, replacing the Parent's IDictionary with an IList. This was mapped using something like:
mapping.HasMany<Broker>(x => x.Brokers)
.Table("Child")
.KeyColumn("ParentReferenceId")
.Inverse()
.AsBag();
That produces the more obvious exception,
System.Data.SqlClient.SqlException: Operand type clash: uniqueidentifier is incompatible with int
Unfortunately, I can't seem to figure out the right way to tell it to join on the ReferenceID field. What's the right way to do that? I'd prefer the dictionary, but I'd be reasonably happy if I could even get the bag to work.
For clarity, I'm using a build of Fluent that is bundled with a recent SharpArchitecture pulled from git. The Fluent dll is marked version 1.0.0.594, but if a more recent build would help, I'm flexible.
Further digging has led me to a solution for the Bag case, though the dictionary is still giving me a bit of trouble.
The solution requires a patch to Fluent NHibernate's OneToManyPart mapping class. (Hat tip to This bug report: Could not map a one-to-many relationship where the key is not the primary key.
mapping.HasMany(x => x.Children)
.Table("Child").KeyColumn("ParentReferenceId")
.PropertyRef("ReferenceId")
.Inverse()
.AsBag();
Theoretically, AsMap should work almost the same way, but for some reason that I'm not entirely clear on, it doesn't work for me. I'll explore that later, but I'm open to suggestions.