I started using NHibernate today, but I cannot figure out how I setup a simple relation between two tables. I don't really know what it's called, it could be one-to-many or foreign key relation (I'm not that into database design and the terms used), but here's a very simple example.
I have a table Product with attributes Id (PK), ProductName and CategoryId. Then I have a table Categories with attributes Id (PK) and CategoryName.
I created these classes:
public class Product
{
public virtual int Id { get; set; }
public virtual string ProductName { get; set; }
public virtual int CategoryId { get; set; }
public virtual Category Category { get; set; }
public virtual string CategoryName
{
get { return this.Category == null ? String.Empty : this.Category.CategoryName; }
}
}
public class Category
{
public virtual int Id { get; set; }
public virtual string CategoryName { get; set; }
}
In other words, I simply want the Product to store to which category it belongs (via the CategoryId attribute which points to an Id in the Categories table). I don't need the Category class to hold a list of related Products, if that makes it any simpler.
To make it even more clear what I'm after, this is the SQL that I'm expecting:
SELECT Products.*, Categories.*
FROM Products INNER JOIN Categories ON Products.CategoryId = Categories.Id
at least that's what I think it should be (again, I'm not that good at database design or queries).
I can't figure out which kind of mapping I need for this. I suppose I need to map it in the Product.hbm.xml file. But do I map the CategoryId as well? And how do I map the Category property?
It seems like I would need a 'one-to-many' relation since I have ONE category per product (or is this reasoning backward?) but it seems like there is no one-to-many mapping...
Thanks for any help!
Addition:
I tried to add the many-to-one relation in the Person mapping, but I keep getting an exception saying "Creating proxy failed", and in the inner exception "Ambiguous match found".
I should maybe mention I am using an old version of NHibernate (1.2 I think) because that is the only one I got running with MS Access due to it not finding the JetDriver in newer versions.
I've put the mapping files, classes, and code where the error occurs in screenshots because I can't figure out how to post XML code here... It keeps reading it as html tags and skipping half of it. Anyway.
The mappings:
http://www.nickthissen.nl/Images/tmp7B5A.png
The classes:
http://www.nickthissen.nl/Images/tmpF809.png
The loading code where the error occurs:
http://www.nickthissen.nl/Images/tmp46B6.png
(As I said, the inner exception says "Ambiguous match found".
(Product in my example has been replaced by Person)
The Person and Category classes inherit Entity which is an abstract base class and defines the Id, Deleted, CreatedTime and UpdatedTime properties.
The code where the error occurs is in a generic 'manager' class (type parameter TEntity which must inherit Entity). It is simply supposed to load all entities with the Deleted attribute false. In this case, TEntity is 'Person'.
It works fine if I leave out the many-to-one Category mapping in the Person mapping, but then obviously the Category property is always null.
Oh yeah, sorry about the mix between C# and VB, the C# code is in a generic framework I use for multiple projects while the VB part is the actual implementation of that framework on my website and I just happened to use VB for that.
Help? Thanks!
In your Product class only needs to contain a Category object, you don't need a CategoryId property. Then in your Product mapping you need to have this entry
<many-to-one name="Category" column="CategoryId" />
UPDATE:
Your mappings appear to be missing the fully qualified name of the mapped class in the tag. See http://nhibernate.info/doc/nh/en/index.html#mapping-declaration-class
UPDATE 2:
See if this helps you NHibernate 1.2 in a .NET 4.0 solution
The 'Ambiguous match found' exception was caused by the project targeting .NET Framework 4, which does not seem to be compatible with NHibernate 1.2.1. I switched to 3.5 and that seems to solve that particular issue.
Now on to the next. As you can see, the Person class has a CategoryName property that should return the name of the current Category object, or an empty string if the category happens to be null. This is so I can databind a collection of Person objects to a grid, specifying 'CategoryName' as a property to bind a column to.
Apparently this does not work with NHibernate. Whenever I try to databind my collection of persons, I get this exception:
"Property accessor 'CategoryName' on object 'NHibernateWebTest.Database.Person' threw the following exception:'Could not initialize proxy - the owning Session was closed.'"
This occurs on the 'DataBind' method call in this code:
public virtual void LoadGrid()
{
if (this.Grid == null) return;
this.Grid.DataSource = this.Manager.Load();
this.Grid.DataBind();
}
(This is an ASP.NET project and 'Grid' is a GridView)
'this.Manager' returns an existing instance of NHibernateEntityManager, and I've already shown its Load method before, it contains this:
public virtual EntityCollection Load()
{
using (ISession session = this.GetSession())
{
var entities = session
.CreateCriteria(typeof (TEntity))
.Add(Expression.Eq("Deleted", false))
.List();
return new EntityCollection(entities);
}
}
(THere's some generic type parameters in there but this website seems to hide them (due to the html like tags I guess)... Sorry about that).
This may have something to do with NHibernate itself, as I said I'm completely new to this. When I call my Load method I would expect it to return an EntityCollection(Of Person) with all its properties already set. It seems I have to keep the ISession open while I am databinding for some reason..? That seems a little strange...
Can I get around this? Can I make my Load method simply return a collection of persons already fully loaded, so that I can access CategoryName whenever I want?
Wait... Is this lazy loading perhaps?
Related
Consider the following simplified domain:
public class Movie
{
public virtual int Id { get; set; }
public virtual MovieDetail MovieDetail { get; set; }
}
public class MovieDetail
{
public virtual int Id { get; set; }
public virtual Movie Movie { get; set; }
}
A MovieDetail cannot exist without a Movie, but a Movie could exist without a MovieDetail (i.e. we have no details about it).
Our database has a separate table for Movie with columns Id, and a separate table for MovieDetail with columns Id and MovieId. There is also a foreign key from MovieDetail.MovieId to Movie.Id.
We've got this all mapped in NHibernate, but when getting a collection of Movie instances, we want a left outer join with MovieDetail. If not, we could have a N+1 problem when iterating over the Movie instances. That is the case now: there is a separate query for every call to the Movie.MovieDetail property.
I've tried one-to-one mapping, but that seems to be for the case when you have both instances. In our case, we don't always have a MovieDetail. Also, they don't share the same primary key.
I've researched formula's, but that would require me to make my MovieDetail implement IUserType, essentially putting NHibernate into my domain. I'd like to avoid that.
Maybe you could try adding a many-to-one relation in the Movie mapping to MovieDetail, it will act as a one to one mapping.
When you set the option 'not-null' to "false" it is also nullable I suppose.
I don't know if you are lazy loading or not, when this is so the MovieDetailis loaded when needed and not by a left join construction.
Shouldn't all the properties be virtual in both classes?
<many-to-one name="MovieDetail" column="Id" class="MovieDetail" not-null="false" lazy="false"/>
I'm in a bit of a hurry and I don't know if you can modify your domain / db schema but you might want to try and take a look at http://ayende.com/blog/3937/nhibernate-mapping-component.
It seems to me that a Movie can have at most one MovieDetail which might not be there. MovieDetail might have properties like Description, ReleaseDate, Actors, etc. I don't really understand why you separated these concepts. By bringing them together you would have 1 less table and 1 less FK to join on each time you want to list movies.
The component allows you to isolate your data into a separate entity while mapping to the same table as Movie.
I have a domain model that includes something like this:
public class Customer : EntityBase<Customer>, IAggregateRoot
{
public IList<Comment> Comments { get; set; }
}
public class Comment : EntityBase<Comment>
{
public User CreatedBy { get; set; }
public bool Private { get; set; }
}
I have a service layer through which I retrieve these entities, and among the arguments passed to that service layer is who the requesting user is.
What I'd like to do is be able to construct a DetachedCriteria in the service layer that would limit the Comment items returned for a given customer so the user isn't shown any comments that don't belong to them and are marked private.
I tried doing something like this:
criteria.CreateCriteria("Comments")
.Add(Restrictions.Or(Restrictions.Eq("Private", false),
Restrictions.And(Restrictions.Eq("Private", true),
Restrictions.Eq("CreatedBy.Id", requestingUser.Id))));
But this doesn't flow through to the lazy-loaded comments.
I'd prefer not to use a filter because that would require either interacting with the session (which isn't currently exposed to the service layer) or forcing my repository to know about user context (which seems like too much logic in what should be a dumb layer). The filter is a dirty solution for other reasons, too -- the logic that determines what is visible and what isn't is more detailed than just a private flag.
I don't want to use LINQ in the service layer to filter the collection because doing so would blow the whole lazy loading benefit in a really bad way. Lists of customers where the comments aren't relevant would cause a storm of database calls that would be very slow. I'd rather not use LINQ in my presentation layer (an MVC app) because it seems like the wrong place for it.
Any ideas whether this is possible using the DetachedCriteria? Any other ways to accomplish this?
Having the entity itself expose a different set of values for a collection property based on some external value does not seem correct to me.
This would be better handled, either as a call to your repository service directly, or via the entity itself, by creating a method to do this specifically.
To fit in best with your current model though, I would have the call that you currently make to get the the entities return a viewmodel rather than just the entities;
public class PostForUser
{
public Post Post {get; set;}
public User User {get; set;}
public IList<Comment> Comments}
}
And then in your service method (I am making some guesses here)
public PostForUser GetPost(int postId, User requestingUser){
...
}
You would then create and populate the PostForUser view model in the most efficient way, perhaps by the detached criteria, or by a single query and a DistinctRootEntity Transformer (you can leave the actual comments property to lazy load, as you probably won't use it)
I have an object Topic which is a self-related hierarchy where the child Topics are defined as
public class Topic : Entity {
public ISet<Topic> ChildTopics { get; internal set; }
public Topic ParentTopic { get; set; }
...
}
I'm writing a form (MVC3) to produce a drop-down list (Html.DropDownListFor) of the first-level topics (theoretically this will eventually AJAX into a second drop-down for second-level topics), but when it goes to save, it produces the ever-popular "Cannot cast..." exception (see question title).
The usual cause of this is that you used List or Set instead of IList or ISet, but I am using ISet, and it specifically says it can't cast to ISet.
The reason this is a set is because you wouldn't want a Topic being a child of another Topic more than once. The table mapping create by Fluent NH automapping is correct with this override:
mapping.HasMany<Topic>(t => t.ChildTopics).AsSet().Inverse().KeyColumn("TopicId");
In my project, as of NHibernate 3.2.0.400, this error still occurs if I use System.Collections.Generic.ISet<T> instead of Iesi.Collections.Generic.ISet<T>. Simply changing the references around is minimal work and solves the problem.
I'm just trying to get my head around nHibernate and have a query. When setting up the mappings file (with Fluent or regular .hbm.xml files) you specify relationships (bags; one-to-many, etc) and sub-types - the idea being (I believe) is that when you fetch an object it also fetches and matching data. My question is can I programmatically tell my query to ignore that relationship?
So, below, there is a Foo class with a list of Bar objects. Within the mappings file this would be a one-to-many relationship and sometimes I want to retrieve a Foo with all Bars BUT sometimes I want to just retrieve the Foo object without the Bar, for performance reasons. How can I do this?
public class Foo { public int Id { get; set; } public List<Bar> { get; set; } }
public class Bar { public int Id { get; set; }
Cheers
The relationship shouldn't be loaded automatically unless you turn off Lazy Loading or specify it to be eager loaded in the query.
Edit:
To answer your questions in the comment below.
1) It's done as part of the query. An basic example using QueryOver in NHibernate 3.0 would look something like:
var result = Session.QueryOver()
.Fetch(x => x.Category).Eager
.Where(x => x.Price > 10)
.List();
I think with ICriteria it's "SetFetchMode("Category", FetchMode.Eager)"
2) If you turn off lazy-loading on the mapping for an object, it will effectively always be eager loaded. Tho I suggest you eager load on a query-by-query basis to avoid the possibility of having a massive chain of data loaded, or loading data you don't actually need.
I would like to know if there is a way to disable automatic loading of child records in nHibernate ( for one:many relationships ).
We can easily switch off lazy loading on properties but what I want is to disable any kind of automatic loading ( lazy and non lazy both ). I only want to load data via query ( i.e. HQL or Criteria )
I would still like to define the relationship between parent child records in the mapping file to facilitate HQL and be able to join parent child entities, but I do not want the child records to be loaded as part of the parent record unless a query on the parent record
explicitly states that ( via eager fetch, etc ).
Example:
Fetching Department record from the database should not fetch all employee records from the database because it may never be needed.
One option here is to set the Employees collection on Department as lazy load. The problem with this approach is that once the object is given to the calling API it can 'touch' the lazy load property and that will fetch the entire list from the db.
I tried to use 'evict' - to disconnect the object but it does not seem to be working at all times and does not do a deep evict on the object.
Plus it abstracts the lazy loaded property type with a proxy class that plays havoc later in the code where we are trying to operate on the object via reflection and it encounters unexpended type on the object.
I am a beginner to nHibernate, any pointers or help would be of great help.
Given your request, you could simply not map from Department to Employees, nor have an Employees property on your department. This would mean you always have to make a database hit to find the employees of a database.
Aplogies if these code examples don't work out of the box, I'm not near a compiler at the moment
So, your department class might look like:
public class Department
{
public int Id { get; protected set; }
public string Name { get; set; }
/* Equality and GetHashCode here */
}
and your Employee would look like:
public class Employee
{
public int Id { get; protected set; }
public Name Name { get; set; }
public Department Department { get; set; }
/* Equality and GetHashCode here */
}
Any time you wanted to find Employees for a department, you've have to call:
/*...*/
session.CreateCriteria(typeof(Employee))
.Add(Restrictions.Eq("Department", department)
.List<Employee>();
Simply because your spec says "Departments have many Employees", doesn't mean you have to map it as a bi-directional association. If you can keep your associated uni-directional, you can really get your data-access to fly too.
Google "Domain Driven Design" Aggregate, or see Page 125 of Eric Evan's book on Domain Driven Design for more information
You can have the lazy attribute on the collection. In your example, Department has n employees, if lazy is enabled, the employees will not be loaded by default when you load a department : http://www.nhforge.org/doc/nh/en/#collections-lazy
You can have queries that explicitly load department AND employees together. It's the "fetch" option : http://www.nhforge.org/doc/nh/en/#performance-fetching-lazy