Inheritance / Polymorphism

[hussam789]

Given than i have these models:

public class Zoo {
  private int id;
  private Date openning_hour;
  private Date closing_hour;
  private String manager;
  private ArrayList<Animal> animals;
}

public abstract class Animal  {
  private String name;
  private int zoo_id;
}

public class Dog extends Animal {
  private boolean canSmellFromFar;
}

public class Spider extends Animal {
  private boolean bites;
}

and if i have two tables (Zoo, Animal), and we have a Query getAllAnimalIntTheZoo(int zooId).
can it return sub-types of Animal?

[AlecKazakova]

SQLDelight currently wont play well with joined table inheritance (the sql side solution to inheritance). The query will have to return a Mapper<GetAllAnimals> so it's really the return type that needs to inherit from Animal as well as GetAllAnimals. Heres what I would try:

-- AnimalParent.sq
CREATE TABLE animal_parent (
  name STRING NOT NULL PRIMARY KEY,
  zoo_id INTEGER REFERENCES zoo
);

-- Dog.sq
CREATE TABLE dog (
  name STRING PRIMARY KEY REFERENCES animal_parent,
  can_small_from_far INTEGER AS Boolean
);

-- Spider.sq
CREATE TABLE spider (
  name STRING PRIMARY KEY REFERENCES animal_parent,
  bites INTEGER AS Boolean
);

-- Animal.sq
animals_view:
CREATE VIEW animal AS
SELECT animal_parent.name, zoo_id, can_smell_from_far, bites
FROM animal_parent
LEFT JOIN dog USING (name)
LEFT JOIN spider USER (spider);

// Dog.java
@AutoValue public abstract class Dog implements Animal {
  @Override public Boolean bites() {
    throw new UnsupportedOperationException("Only available to spiders");
  }
}

// Spider.java
@AutoValue public abstract class Spider implements Animal {
  @Override public Boolean can_smell_from_far() {
    throw new UnsupportedOperationException("Only available to dogs");
  }
}

// Animal.java
public interface Animal extends AnimalModel {
  Factory FACTORY = new Factory();

  RowMapper<Animal> MAPPER = FACTORY.get_all_animals_in_the_zooMapper(new Get_all_animals_in_the_zooCreator<Animal>() {
    @Override public Animal create(String name, int zoo_id, Boolean can_smell_from_far, Boolean bites) {
      if (can_smell_from_far != null) {
        // This is a dog
        return new AutoValue_Dog(name, zoo_id, can_smell_from_far);
      }
      if (bites != null) {
        // This is a spider
        return new AutoValue_Spider(name, zoo_id, bites);
      }
      throw new AssertionError();
    }
  });
}

and then your queries are on the animal table and use the Animal.MAPPER mapper. So your getAllAnimalsInZoo would look like

get_all_animals_in_zoo:
SELECT *
FROM animal
WHERE zoo_id=?;

it's pretty verbose. There might be a slightly more elegant solution or something that could be built into sqldelight but I haven't encountered this problem myself so I'm not sure. Let me know if you have more questions/concerns.

[bangarharshit]

I also had the same use case in past. In my chat messenger app, when I am storing message it can be of different type Image, Text, Video (around 10). What I did is to write my custom mapper and marshal.

public final class MessageContainer {
  // Common meta-data like send time
   public final MessageTypeEnum messageType;
   public final BaseMessage baseMessage;  // BaseMessage is serialisable. 
}

My mapper and marshal are taking care of deserialising and serialising base message into and from string. It's not a scalable approach though and I am thinking of generating this code myself using some kind of converter like Converter<BaseMessage,String>. I need to check code - http://stackoverflow.com/questions/14308467/sqlite-convert-text-to-numbers (sql server has a really useful method which I couldn't find in sqlite - http://www.w3schools.com/sql/func_convert.asp)

@AlecStrong, Do you recommend committing model files along with .sq files into the version control. https://github.com/alexsimo/delightful-persistence/tree/4d17062afae866750133545368695a0de5205ad0/app/src/main/java/com/alexsimo/delightfulpersistence/database

[AlecKazakova]

If you mean the generated *Model.java files then no, those shouldn't be checked in (as they exist under the generated folder). If you mean the files you write which implement *Model then yes, those should be checked in.

[AlecKazakova]

working-kotlin branch has diverged significantly from this style so closing for now

[gabrielpaim]

Hi there,

I'd like to know if is there any update on this. Is there any plans for SqlDelight to support polymorphism/inheritance?

@AlecStrong Would you still recommend the same approach to deal with this still? That is, joining all inherited tables into a single view and treating it on mapper as a STI. I find it somehow uncomfortable, but maybe it still remains as good strategy to deal with it.

Thank you!