I am new to using Nhibernate and try to understand how i work with relationships when saving an object.

I have read a couple of places i need to set both side of a relationship, but i am not sure i understand it correctly as i think it will cause a lot of unnecessary selects if i always have to set it on both sides.

Will I only have to set both sides of a relationship if i am setting it through the Inverse side ?
In other words should the Primary side (the side that contains the foreign key) always be set ? and the other side only matters if i want to set i through that side ?

Heres is a very simple example

I have the following database tables

Users
-----
- Id (Primary Key)
- CountryId (Foreign key)

Countries
---------
- Id (Primary Key)
- Name

I have the following entities / models which maps to the tables

User
----
- Id (int)
- Country (Country)

Country
-------
- Id (int)
- Name (String)
- Users (IList<User>)


A User can have 1 country, and a Country can have many Users.
User is the Primary side (containing the CountryId foreign key), and Country is the inverse side of the relationship.

1) If I set Country on the User object i dont need to set it on the other side (- do not have to add it to the "IList<User> Users" collection on Country) ?

2) If I want to add the User to the Country (through the Country object) I need to add User to the "IList<User> Users" collection on the Country and set Country on the User ?


What i am wondering is how i can add a Country to a User without having to select both entities first ?

I like it to just fire something like "UPDATE User SET CountryId = @CountryId WHERE Id = @UserId"

If i always need to set up the relationship on both sides it will require a select of both the User and the Country (which i am trying to prevent), or at least only select the User and set the Country by doing something like "user.Country = session.Load<Country>(123); session.Save(user);"


Hope someone out there can help me understand how it works.


Best regards

Martin

Yes, on bidirectional relations according documentation you should always set both sides of a relationship.
If you set only the 'primary' side, the update on the database might be made correctly, but you could run into other
problems which are not visible immediately. Always set both sides if you want walk on the save side of the way.



From my experience, setting the association on both sides don't causes further selects.



That's possible with bulk updates.

Thanks for your reply.

What i mean about selects if i have to set relationship on both sides is that I will have to Select both sides of the relationship.

It just feels wrong to do 2 selects in order to do a simple "UPDATE Users SET CountryId = @CountryId WHERE (Id = @UserId)"

Please explain what you understand under 'select'.
Do you mean 'select' the object through load (session.get(...) or session.load(...) , em.getReference(...) , em.find(...))
or do you mean 'select' the object through a query (session.createQuery() or em.createQuery()) ?

Thanks again for your reply.

What i mean is that i will some how have to query both sides of the relationship (which requires two SQL select statements).

Etc in my example if i want to add a Country to my User, and i have both the User Id, and the Country Id, i will have to first query the User (any way i do it will require a SQL Select statement to be fired ? easiest way is using Get<>(Id) i guess ?), and then query the Country (Get<>(Id)) to be able to add the Country object to the User object.

As far as i know both - .Get(), .Load(), .CreateQuery() will make a SQL Select from the Database to get the data needed for creating the Object ?

Load() might not do it in the first place and only return some kind of proxy ? but when the proxy is accessed to set the relationship (country.Users.Add(theUser);) it will query the object by doing a SQL select anyway ?


ps. i am new to Nhibernate soo i might be misunderstanding something :)

No.
I just tried following code



and as expected the association is set without doing a query on b.




N.B.: I use Hiberante but I assume it is same behavior in NHibernate.

Conclusion: When setting a relation between two persistent objects then at least the owner side of the relation must be loaded (= 'selected' using your term) in persistent context. The non-owner side of the relation can be a non-materialized proxy,
in this way the 'select' on the 'target' object can be avoided.

If you don't want to load any side of the relation for setting the relation,
then you can use bulk-updates.

By the way: why you feel so bothering about the fact that a 'select' is issued before the update?
After all usually a select with condition on primary key just takes some milliseconds to execute.

Thanks again for your reply.

In your code example you only set up one side of the relationship - which i asked about earlier and was told that I need to set up both sides.


This will make it fire a unnessecary SQL Select when the AddUser() method on country is called - setting up both sides in a relationship



If i only set the primary side like in your code example (the side which contains the foreign key - in my example the User object) then it works fine and no extra SQL Select is fired. But then i dont set both side of the realtionship.

This works like in your example - but only sets up one side of the relationship



Is it only necessary to set up both side of the relationship if both objects are queryed (materialized) ? or when is it needed (i am even more confused now as it was what i tried to ask in the first post) :) ?

In case it is only needed to set up both side of the relationship when the object being added is already queryed / materialized, then how can i check if the object is already queryed (without it being queryed if it isnt) and set up the relationship ?



The reason i am concerned about the "Selects" is because I am going to have some tables with many columns (some with large texts), and i dont want to transfer the data every time when it is not needed.
Also, i am new to NHibernate soo i am pretty sure i must be doing something wrong, when i cant find out to make something this simple work :) , and I will like to learn how to use Nhibernate correctly.

I have created this small helper that i can use to test if an Object is an Uninitialized Proxy (or more important not a proxy or a proxy that have been initialized)



Then i can do something like



Then it works perfectly it seems.

But is this the correct way of doing it ? (only setting up both sides of a relationship if the inverse side is not a proxy or initialized) ?

Sorry, yes, my example was build on a unidirectional relation.



According hibernate documentation you always should set both sides of relations if it is bidirectional,
In practice I see that it is enough to set the owner side (at least on OneToMany relations) in order that the information is properly saved on database when data gets flushed.
But there are some side-effects to be aware, when not setting the inverse-side (=the collection) too.
Following are the side-effects which I know:
- the inverse-side collection it not up-to-date anymore (as you don't add the new item) in transaction
- the inverse-side collection continues to remain not up-to-date over the original transaction
if you detach (or reattach) the owning entity.
- the inverse-side collection is again up-to-date when you re-read the entity from database (not using reattachement)
- if the inverse-side collection is configured to be cached in 2L-cache then it would be completely rebuilded and replaced in cache instead do be simply updated.

Thanks a lot for your time and help.

Being used to write the Sql myself I just feel like a bad developer doing unessecary selects just to be able to updates and inserts.
But basically it doesnt matter as they are only needed when updating / inserting something which happens rarely, and i can make some relationships uni-directional.
Ill better focus on something else which is more important. I just wanted to know I was doing everything the right way.

Thanks again :)