Improve documentation for traversing nested properties in projections [DATACMNS-1030]

Anthony Foulfoin opened DATACMNS-1030 and commented

Hi,

Given the following classes :

interface CarProjection {
    String getName();
    List<Color> getColors();
    }    
}

class Car{
    String name;

    @OneToMany(mappedBy = "car")
    List<Color> colors;
}

class Color{
     String label;

     @ManyToOne
     Car car;
}

I want my CarProjection to return only the color labels instead of the full objects. At first I used an @Value with a spel expression :

@Value("#{target.colors.![label]}")
List<String> getColors();

Which works, but the SQL queries are not fully optimized because of the open projection and become extremely slow. I figured out that it was possible to do this directly, i.e. traversing properties in the method name :

List<String> getColorsLabel();

And it works like a charm :) But I found this solution randomly in a jira bug report, and not in the projections documentation, which could help other users to do the same thing.

EDIT:

Finally it does not work for traversing collections, I get duplicated cars objects, each one with only one color. Same thing if I don't traverse the property :

List<Color> getColors

I get duplicated results.

Affects: 1.12.8 (Hopper SR8)