I'm wondering what the best design would be for persisteing a new child entity with NHibernate without accidentally overwriting the parent in the database.
The problem I have is that the child entity will look something like this:
class Child
{
Parent Parent;
// other fields
}
My problem is that the child has been supplied from the UI layer along with the ID of the parent, and that means that the Parent ref is basically uninitialized: It will have the ID populated but everything else null - because the only way to populate its fields would be an extra round trip to the database to read them.
Now if I call Session.SaveOrUpdate(child) on NHibernate, what's going to happen with the parent. I don't want NHibernate to cascade save the uninitialized parent since that would just destroy the data in the database. How would people approach this problem? Any best practices?
You must use the session.Load(parentid) to get the aggregate root. In contrast to the session.Get() method, this does not actually fetch any data from the database, it just instantiates a Parent proxy object used to add Child objects to the correct Parent in the DB (eg. get the foreign key correctly).
Your code would probably look something like:
// Set the Parent to a nhibernate proxy of the Parent using the ParentId supplied from the UI
childFromUI.Parent = Session.Load<Parent>(childFromUI.Parent.Id);
Session.Save(childFromUI);
This article explains Get/Load and the nhibernate caches really well
You should probably be working with the aggregate root (probably the Parent) when doing Saves (or SaveOrUpdates etc).
Why not just:
Fetch the parent object using the parent id you have in the child from the UI layer
Add the child to the parents 'children' collection
I think you have to overview your mapping configuration for nhibernate. If you have defined on the reference by the child to the parent that hi has to Cascade all, it will update it!
So if you say Cascade.None he will do nothing. All other are bad ideas. Because you allready has the information of this parent. So why read from db agane?!
If your models looks like this
class Parent
{
}
class Child
{
Parent myParent;
}
and you are trying to set the parent and save the child without having a full parent object, just the ID.
You could try this:
session.Lock(child.myParent, LockMode.None);
before saving, this should tell nhibernate that there are no changes to the parent object to persist and it should only look at the object for the Id to persist the association between Parent and Child
Related
Grails 3.2.5. Is see from my sql dump that the hasOne relationship does an eager fetch. This used to be the case back in prior versions of Grails, and the behavior could not be overridden. Is this still the case? What is the recommended model for a 1:1 relationship where we want a lazy fetch on the dependent object?
A little background. My "Comment" object has a one-to-one relationship with a "CommentText" object, where the text object holds Oracle clob text - some of it large. I only wanted to get the text when explicitly required to do so. The fk was in the dependent database text object, hence the "hasOne". Fortunately I was able to move the fk to the owner side of the association via an embedded domain object and update the db schema.
Throughout, I was unable to get lazy loading of the hasOne dependent object. Tried fetch: 'lazy'; fetchMode: 'lazy, and other variations of things. I needed a full domain class association because of "find" actions that needed to traverse the association.
I would still prefer the hasOne approach, if loading were indeed lazy.
Old question, but I just encountered the same problem so I'll answer for later reference.
Basically, it is impossible to lazy-fetch a hasOne property in Grails 3 (tested with 3.3.11, assuming Hibernate). But there are some workarounds.
The immediate lazy-fetch N+1 problem
As soon as you put hasOne: [child: Child] on the parent class, GORM will force you to make the relationship into a bidirectional one-to-one, and it will put the foreign key on the child table.
When you then fetch entities of the parent, it will immediately fetch all of the child entities as well, but it will do a query for every child (N+1 problem).
So a statement like this
Parent.list(max: 10)
will issue one query to get the 10 parents, and then do a query where parent_id = ? for each of the 10 children.
Even if you put fetch: 'lazy' and batchSize: 10 on the mapping of the child in Parent.groovy, the behavior is the same.
Workaround 1: One-directional with FK on the parent table
This is the solution you mention in your post. If you don't need to access the parent from the child side, you can make the relationship one-directional, which will put the FK on the parent table.
Now when fetching the Parent entity it will fetch the child_id from the parent table automatically, but keep the child property as a Hibernate proxy.
The child entity behind the proxy will correctly only be fetched once you access it. The batchSize mapping seems to be ignored here though, so when you actually start accessing the .child entities it will again issue one query per Parent.
Workaround 2: One-to-many and just access the first element
If you really want to keep the FK on the child table and also have lazy loading, you can use this hackaround.
On the Parent.groovy you could specify it like this
static hasMany = [children: Child]
static transients = ['child']
Child getChild() {
children ? children.first() : null
}
static mapping = {
children batchSize: 100
}
Now when you fetch the Parent entities it will correctly ignore the child property, and if you e.g. loop through a list of Parent and access the .child on each, it will only issue one single query for batchSize Parents.
So code like this:
def parents = Parent.list(max: 10)
parents.each {
log.info it.child.subProperty
}
will do the first query to get the 10 parents, and then one single query to lazily batch-fetch the children for up to batchSize parents. With Sql logging enabled it should look something like this:
select child0_.parent_id, child0_.id, ... from child child0_ where child0_.parent_id in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
Workaround 3: The eager-fetch non-workaround
If your application code almost always uses the child property, then one option is to give up on lazy fetching and just specify child fetch: 'join' in Parent.groovy.
This will eliminate the N+1 lazy fetching problem, but as a downside Hibernate will now LEFT JOIN the child table and select all it's properties every time you request the Parent entity, even if you never touch the child property.
Workaround 4: Replace hasOne with mapping column: 'id'
class Face {
Nose nose // due to mapping, column 'nose_id' is not required
static mapping = {
nose column: 'id', insertable: false, updateable: false
}
}
class Nose {
static belongsTo = [face: Face] // due to mapping, column 'face_id' is not required
static mapping = {
// use the parent object's ID as this ID
// optional, but clean
id generator: 'foreign', params: [property: 'face']
// reference to the parent object so we don't need an extra column
face column: 'id', insertable: false, updateable: false
}
}
I'm trying to nail down the specifics of removing a child in an one-to-many relationship in CF using ORM. I've posted a small test app here:
https://github.com/pnahtanoj/cfrelationship
Regarding the removeChildren() function on ln47 of create.cfm - if I dump the parent object before and after, I see that the children have been removed. However, they are still in the DB after the close of the transaction. Not sure what I'm missing.
Using CF10, MySql 5.something...
Because you set inverse to true on the many side, that means only the child side is tracked by Hibernate.
You can set all the child's parent to null.
public void function removeChildren() {
transaction {
var children = getChilds();
for (var c in children)
c.setParent( javacast('null','') );
}
arrayClear(variables.childs);
}
Considering the simplified class below, lets assume:
that the parent can have a relatively large amount of children (e.g. 1000)
the child collection is lazy loaded
we have loaded one parent via it's Id from a standard Get method of a ParentRepository, and are now going to read the OldestChild property
class Parent
{
public IList<Child> Children { get; set; }
public Child OldestChild
{
get { return Children.OrderByDescending(c => c.Age).FirstOrDefault();
}
}
When using NHibernate and Repositories, is there some best practice approach to satisfy both:
a) Should select oldest child through the aggregate root (Parent) - [ie. without querying the children table independently via e.g. a ChildRepository using the Parent Id]
b) Should avoid loading the whole child collection into memory (ideally the oldest child query should be processed by the DB)
This seems something that should be both possible and easy, but I don't see an obvious way of achieving it. Probably I am missing something?
I'm using NHibernate 2.1, so a solution for that would be great, although will be upgrading to 3 soon.
I would create a specialized method on your repository, which returns the oldest child of a given parent.
You could map OldestChild using a Formula. Take a look at this to map a class to a formula: http://blog.khedan.com/2009/01/eager-loading-from-formula-in.html
I ve got the following setup:
public class ParentEntity
{
public ICollection<ChildEntity> {get; set; }
}
public class ChildEntity
{
// do i need to the parent here?
}
I managed to save the ParentEntity and cascaded the save to the added child entities which were saved as well. But in the db table the ParentId reference of the child was set to allow NULL. When setting it to NOT NULL the save failes since the ParentId in the child table is NULL.
What's happening there? ;)
When
You should map both sides of the relationship normally, and when you add a child to the parent's collection, you should also set the parent property on the child. Normally you would achieve this by writing a method like this:
public void AddChild(ChildEntity child)
{
this.Children.Add(child);
child.Parent = this;
}
NHibernate persists the ParentId column in the Child table based on the mapped property in the ChildEntity class. The definition of the one-to-many relationship merely allows NHibernate to load the collection from the database based on values in this column
I am having the same issue and need this to either have nHibernate expose the foreign key column, or do it in class via collection.
Problem: nHibernate creates the collection object (IList, for example) and you can not override the or listen to the add events of basic collections.
This becomes an issue only because it is required by the WCF RIA Services framework.
I'm having some trouble getting NH to persist my object graph.
I have (something like) this:
/*Tables*/
TABLE Parent
ParentID PK
LastEventID NULL
TABLE Event
EventID PK
ParentID FK NOT NULL
//Model Classes
public class Parent
{
public List<Event> Events; //Inverse
//Denormalized bit
public Event LastEvent; //not inverse
}
public class Event
{
public Parent Parent; //Makes the association up there Inverse
}
I'm creating a new Parent, creating a new Event, adding the new Event
to Parent.Events and setting Parent.LastEvent to the new Event.
When I tell NH to save the Parent I get an error about a transient
object needing to be saved first. I assume its because the association
between Parent and Event is not clear.
The way the SQL needs to go is to insert the Parent with a null
LastEvent, then insert the Event, then update Parent.LastEvent.
So how do I get NH to do this?
Without seeing your mapping schema, I'll have to guess.
Are you cascading your updates? From the reference:
To save or update all objects in a graph of associated objects, you must either
Save(), SaveOrUpdate() or Update() each individual object OR
map associated objects using cascade="all" or cascade="save-update".
Assuming you don't already have this, does adding cascade="all" or cascade="save-update" to the side marked inverse="true" fix the problem?