I have a mant-to-many relationship modeled in the database (with a bridge table) between Student and Professor (_students_selected) , in my entites i have modeled it as a one-to-many relationship i.e. a Professor has one Student.
HasManyToMany<Student>(Reveal.Member<Professor>("_students"))
.Table("_students_selected").ChildKeyColumn("student_key").ParentKeyColumn("professor_key");
public class Professor
{
private IList<Students> _students;
public virtual Student Student
{
get { return _students.FirstOrDefault(); }
}
}
The above works when getting the data however when querying over the Professors i am unable to add a where condition on the students because the actual data is mapped to the private backing field _students. How do i query this? code below does not work.
_unitOfWork.Session.QueryOver<Professor>().Where(i => i.Student.Id == 24).List();
NHibernate can't translate your C# code inside the property to SQL, it can only work with mapped properties. Either use the collection in the statement (which needs to be public/internal then of course) or filter the results in memory (but be careful with select n + 1 problems then).
Related
I'm working in a project which involves a one to many relation in the database.
A simple example of this would be a teacher that teaches many courses but a course can be taught by just one teacher.
My question is what would be the best way to model this?
The first object is Teacher:
class Teacher{
public int id;
public String name;
public String lastName;
...
}
The thing is how would the course class look like?
Option 1:
class Course{
public int idCourse;
public String courseDescription;
**public int teacherId;**
...
}
Option 2:
class Course{
public int idCourse;
public String courseDescription;
**public Teacher teacher;**
...
}
Option 2 (having a reference to the teacher object) seems to be more functional from an OOP perspective.
In a usual flow, you would create a course object like :
Teacher t = new Teacher('first_name', 'last_name');
Course c = new Course('Math course');
c.setTeacher(t);
Your option 1 highlights what the relational database schema behind this might look like. Here's a good read on that :
https://www.lifewire.com/one-to-many-relationships-1019756#:~:text=A%20teacher%20can%20teach%20multiple,one%20teacher%20to%20multiple%20courses.
Once the code above is committed, it could fire two insert queries underlying to add the new teacher in teachers table and add the new course (along with the teacher ID), in the course table.
I have a project using NH 3.1 and have been using the QueryOver syntax for everything thus far.
One aspect of this project lives in a organization-wide database that I have read-only access to and is using a completely differently DBMS (Oracle vs MSSQL). So I store references from my objects (Foos) to their objects (Bars) using a standard many-to-many table
FooBars
FooID int not null PK
BarID int not null PK
And my domain object, instead of having a Iset<Bar> instead has an ISet<int> BarIDs which is manually mapped to the FooBars table. This prevents NH from trying to do the impossible and join all the way over to the Bars table (I can use a BarRepository.Get() to retrieve the details of the Bars later, if I need them, and in this case, I wouldn't, because I just need the IDs to filter the list of objects returned).
Given IList<int> SelectedBars how can I write a QueryOver<Foo> where BarIDs contains any element in SelectedBars?
SQL something like
...FROM foos INNER JOIN foobars on foo.fooID = foobars.fooID WHERE barID IN ( ... )
It is not possible with QueryOver. Two years ago, I had a similar question about filtering value collections. (Note: QueryOver is based on Criteria API).
I'm not 100% sure, but it probably works with HQL. It is much more powerful.
You may include an SQL statement into the QueryOver criteria.
I don't really understand why you don't map it as a list of entities. There is lazy loading to avoid unnecessary loading - although there are some trade offs sometimes. You can access the ID of NH proxies without hitting the database. Mapping ids makes usually life much harder.
Try:
session.QueryOver<Foo>()
.JoinQueryOver(x => x.FooBars)
.WhereRestrictionOn(x => x.BarId).IsIn( ... )
So 3 years later, I'm back to report how I did solve this.
public class Foo :Entity {
public virtual ISet<FooBar> BarIDs { get; protected internal set; }
} ...
public class FooBar :Entity {
protected internal FooBar() { }
protected internal FooBar(Foo f, int BarID) { ... }
public virtual Foo Foo { get; protected internal set; }
public virtual int BarID { get; protected internal set; }
}
This is basically what Stefan suggested, and what's hinted at in the related post. You just have to eat the overhead of writing an extra entity and referencing it. Remember that I'm storing BarIDs instead of full Bar objects, because I'm dealing with a hard boundary between two databases: the Bars are stored in an entirely different database on a different platform than the Foos. Otherwise of course, you're far better off just telling Foo that is has an ISet<Bar>.
Finding Foos by SelectedBarIDs is then easy, much like Thilak suggested:
session.QueryOver<Foo>().JoinQueryOver<FooBar>(f => f.BarIDs).
WhereRestrictionOn(b => b.BarID).IsIn(...)...
It's an interesting problem, working across the database boundary like this. I can't say I like having to do it, but if someone else is going to take the time to maintain a list of Bars and make it available for my use, it would be a giant waste of resources for me to do the same. So a small inefficiency in having the wrapper class is a very easy cost to justify.
I have a SQL 2008 R2 database with full text indexing set up and would like to use NHibernate to get back search results with ranking. I've figured out the SQL queries usng FULLTEXTTABLE to get result rankings, but I'm struggling with how to use NHibernate to get results with the ranking value since it doesn't map to an actual column in any table.
Any pointers?
(First of all the following syntax will be a bit hazy because this is from memory, please check the api)
well you can either construct some DTO class and map that on-the-fly
for example:
public class Person
{
public virtual String Name {get;set;}
public virtual String Surname {get;set;}
}
which is properly mapped to nhibernate
and the
PersonDTO : Person
{
public int FTSRanking {get;set;}
}
which is not mapped. Note that i'm inheriting from class Person although that is not necessary and i'm only doing it for ease.
This PersonDTO class is only used on queries but there are limitations, as the following hql shows.
NHSes.CreateQuery('select p.Name, p.Surname, p.FTSAlias as FTSRanking from Person p')
.SetResultTransformer(Transformers.AliasToBean<PersonDTO>())
will return a PersonDTO which nhibernate manages to assemble because every item in the select list matches a property (in name, casing, type) in the PersonDTO class. Also you will have to manually type the select list and also, since it is not a mapped class, nhibernate cannot assemble collections.
An other option would be to use the Criteria API in which you set projections (aka extend the select list)
IList<object[]> results = NHSes.CreateCriteria(typeof(Person))
.Add(Expression.SQL(" your fts clause here "))
.SetProjection(Projections.SQL(" add your fts ranking column here",,), Projections.( here add as a projection the main entity ))
.List<object[]>();
where in the results variable each returned row is an object[] and the first element (ie results[0][0]) is the ranking and the second element (ie results[0][1]) is a properly managed Person object
Supposing the following entities :
public class AppUser
{
public virtual int Id { get; set; }
public virtual string Login { get; set; }
}
// Mapped as joined-subclass
public class Person : AppUser
{
public virtual int Age { get; set; }
}
If I create 1 AppUser, and save it like this
var user = new AppUser() { Login = "test" };
session.Save( user ); // let's say Id = 1
How can I cast/convert/"promote" it to a Person, keeping the same ID ?
Now, i'm stuck with a row in my AppUser table, with Id = N. How can I populate the Person table with the same Id ? I can't delete the AppUser and recreate it as a Person, as AppUser may be referenced by foreign keys.
I could issue a "manual" SQL INSERT, but it's kind of ugly...
This is definitively a NHibernate question. I understand that from an OOP point of view, this makes little sense, hence the absence of other tags than nhibernate.
I don't believe nHibernate is going to be able to solve this problem for you. nHibernate is dealing with your data as an object and, especially with joined-subclass I don't believe there is anything built in that allows you to change the subclass type on the fly, or at least change the type and retain the original ID.
I think your best bet is to write a stored procedure that, given an ID and a NEW type, removes all entries from subclass tables and adds a new entry to the correct subclass table.
Once that proc runs, then reload the object in nHibernate (and make sure you have thrown away any cached data relating to it), it should now be of the correct type you want to work with, set its NEW properties and save it.
That way you have a relatively generic stored proc that just changes your subclass types, but you dont need to add all the crazy logic to handle various properties on your subclasses.
This has been discussed on SO before and I am quoting Jon Skeet for posterity:
No. A reference to a derived class
must actually refer to an instance of
the derived class (or null). Otherwise
how would you expect it to behave?
For example:
object o = new object();
string s = (string) o;
int i = s.Length; // What can this sensibly do?
If you want to be able to convert an
instance of the base type to the
derived type, I suggest you write a
method to create an appropriate
derived type instance. Or look at your
inheritance tree again and try to
redesign so that you don't need to do
this in the first place.
In Skeet's example, string's are objects and objects are not strings. So the "upcasting" would not work.
I'd like one of my entities to have a one-to-one relationship with a class hierarchy. Think of it like a Strategy pattern, where each strategy needs different parameters to be persisted. I tried using a combination of OneToOne and JoinedBase/JoinedKey, but I've come across a problem.
With this combination, the primary key of the main entity also appears as the primary key of the table representing the root class in the hierarchy, and as the primary key of the subclass:
Order --------------- TaxCalculator
([PrimaryKey]Id = 1234) ([PrimaryKey(PrimaryKeyType.Foreign)]OrderId = 1234)
^
|
|
UkTaxCalculator
([JoinedKey]UkTaxCalculatorId = 1234)
I can persist this fine, but then I can't change which subclass of TaxCalculator I have. When I do something like:
order.TaxCalculator = new OverseasTaxCalculator(order);
then try to flush, then ActiveRecord/NHibernate (understandably) gets unhappy that there are now two TaxCalculators with Id = 1234.
I can get around this by replacing the OneToOne with a HasMany/BelongsTo, and hiding the multiplicity from users of the Order object, but I'm interested to know if it's possible to do this with OneToOne.
There's a full code example on github. This code throws an exception when the second SessionScope is disposed. If you clone the project, it should run out-of-the-box.
first of all i am sorry, but i did not tried my solution. It is to late and i really need my sleep ;-). I think the only way the one-to-one could work would be a 'table-per-hierarchy'-approach using a discriminator column instead of table-per-subclass. Maybe this will enable you to morph the existing object to another subclass. An other way, something like a polymorphic delete-orphan unfortunately is not supported as you stated. So i'll guess this would be your (very) last option.
But if this fails why don't you map it as a one-to-many instead of many-to-one with a foreign key in the order table, reusing the TaxCalculators? I would imagine them as quite static.
Interesting idea though: polymorphic delete-orphan.
We do something very similar to what you are trying to do. I think it's your combination of one-to-one and the joined key that's causing the problem. Try this:
[ActiveRecord, JoinedBase]
public class TaxCalculator
{
protected int TaxCalculatorId;
[PrimaryKey]
public virtual int Id
{
get { return TaxCalculatorId; }
set { TaxCalculatorId = value; }
}
// common tax calculation fields, methods etc...
}
[ActiveRecord]
public class OverseasTaxCalculator : TaxCalculator
{
[JoinedKey]
public override int Id
{
get { return TaxCalculatorId; }
set { TaxCalculatorId = value; }
}
// overseas tax calculation specific methods, properties etc...
}