Question

Is it possible to order a collection of parent objects by both a parent’s property and a child’s property?

Let’s say I have an Entity class that has this bit of code:

@OrderBy("age")
public List<Father> getFathers() {
    return fathers;
}

The Father Entity has an age property which we want to order by ascending age (as shown above).

A Father can have a Child. A Child would also have an age property. I would like getFathers() to return a Fathers list such that they are ordered by the Fathers’ age and then subsequently the Child’s age ascending.

If I keep the code above as is, the Fathers would be ordered by their own age correctly but a Father (aged 30) with a 10 year old Child could come before a Father (also aged 30) with a 5 year old Child.

I tried doing something like this:

@OrderBy("age, child.age")
public List<Father> getFathers() {
    return fathers;
}

But I would get the following exception:

org.hibernate.AnnotationException: property from @OrderBy clause not found: Father.child.age

Edit: Sorry, let me clarify with a bit more code. The getFathers() is not in the Child entity but in another entity. Let’s call it Village:

@Entity
public class Village {
    ...
    @OneToMany(mappedBy = "village",
               targetEntity = Father.class)
    @OrderBy("age")
    public List<Father> getFathers() {
        return fathers;
    }
}

So let’s say there is a Village and you can obtain the Fathers that live in this Village. For this example’s sake, let’s say that a Father for some odd Village rule can only have one child each.

So Father would look something like this:

@Entity
public class Father {
    private Child child;
    ...

    @OneToOne(mappedBy = "father")
    public Child getChild() {
        return child;
    }

    public void setChild(Child child) {
        this.child = child;
    }
}

So now in the original Village class, I want getFathers() to return all the fathers in order of age, plus, if there are any fathers with the same age, I want them to be then ordered by the age of their child.

Edit 2: I’ve also found that I can get the effect of what I want however it can’t be the most elegant way of doing it:

session.createFilter(village.getFathers(), "order by child.age");

Answer 1

It seems like you are missing two-way mapping. You should also map Child in Father.

By the way, there should be one-to-many relationship between Father and Child, traditionally speaking. Which means in Father class there should be a List<Child> children; property, and not the other way around.

[EDIT]

No, you can’t do like that. Why? Because when you define your @OrderBy("age, child.age"), the whole of it, child.age, would be considered as a property. It will be treated as a String. Hence, indeed, you don’t have any property name child.age in Father class. I hope you are getting my point. So, the Filter solution, you came up with, is a good way to go.