Support custom row-mapping projections

[Osvaldas Grigas]

I do not use DTO generation. For projecting query results into domain objects, I normally use either QBean or ConstructorExpression. However, there are times when these two do not provide enough flexibility, for example, when I need to transform row data in some way before putting it into the domain object.

In these cases, I'd like to be able to implement something like Spring's RowMapper interface for manual projection logic, only instead of ResultSet it should be strongly typed just like everything else in Querydsl, so that I could write things like:

QUserGroup ug = QUserGroup.userGroup;
from(ug).list(new MappingProjection<UserGroup, QUserGroup>(UserGroup.class, ug) {
    protected UserGroup map(Tuple row, QUserGroup columns) {
        UserGroup group = new UserGroup();
        group.setId(row.get(columns.id));
        group.setName(row.get(columns.name));
        group.setRoles(row.get(columns.roles).split(","));
        return group;
    }
});

[Osvaldas Grigas (o-grigas)]

I've attached a rather compact working prototype of MappingProjection - which is a FactoryExpression based on QTuple. Maybe I'm missing something, but I couldn't find an easier way to do that in Querydsl 2.2.0-beta4.

If you think it would make sense to include it into Querydsl (in one form or another), please do so. Also I would be very much interested to hear your opinion: is it the best way to go or do you see alternative solutions.

[Timo Westkämper (timo-westkamper)]

This is a very good addition, butI will generalize it a bit.

This should also work when using columns from multiple tables.

Maybe MappingProjection would just be an adapter to QTuple for further projection.

final QUserGroup ug = QUserGroup.userGroup;
MappingProjection<UserGroup> projection = new MappingProjection<UserGroup>(UserGroup.class, ug.all()){
    protected UserGroup map(Tuple row) {
        UserGroup group = new UserGroup();
        group.setId(row.get(ug.id));
        group.setName(row.get(ug.name));
        group.setRoles(row.get(ug.roles).split(","));
        return group;
    }
};

[Luis Fernando Planella Gonzalez (luisfpg)]

Seems nice.
The only thing to note is that QTuple currently has multiple constructors, and all those should be added to MappingProjection as well...

[Osvaldas Grigas (o-grigas)]

Updated the attached prototype implementation:
- generalized for multiple tables as Timo suggested
- added multiple constructors as Luis suggested
- added some JavaDocs

[Osvaldas Grigas (o-grigas)]

Other projections currently have a factory method that infers the generic type
argument and allows for a less verbose instantiation,
e.g. instead of: new ConstructorExpression<UserGroup>(UserGroup.class, ...);
  one can write: Projections.constructor(UserGroup.class, ...);

For the sake of consistency, I thought it would be nice to also have something
like that for MappingProjection (Projections.mapping(...)), but then I'm not
sure if it's possible to do with an abstract class.

[Timo Westkämper (timo-westkamper)]

Fix commited.

[Timo Westkämper (timo-westkamper)]

Released in 2.2.0