Polymorphic Interfaces with default implementation

[jenschude]

The code generator supports polymorphic interfaces like the one below:

scalar Currency
scalar Long

interface BaseMoney {
  type: String!
  currencyCode: Currency!
  centAmount: Long!
}

type HighPrecisionMoney implements BaseMoney {
  type: String!
  currencyCode: Currency!
  preciseAmount: Long!
  centAmount: Long!
  fractionDigits: Int!
}

type Money implements BaseMoney {
  type: String!
  currencyCode: Currency!
  centAmount: Long!
  fractionDigits: Int!
}

type ProductPrice {
  id: String
  value: BaseMoney!
  key: String
}

Using the generator is creates all the necessary types and query/projection classes. But there is one drawback, when only using fields from the interface you still have to do a projection against one of the types, cause the interface doesn't define a default implementation: .projection(p -> p.value().onMoney().centAmount()). The onMoney method in this case creates a projection query like this:

{
   price {
     value {
       ...on Money {
          __typeName
          centAmount
       }
     }
}

The __typeName field here is important to be able to correctly deserialize the polymorphic type:

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY,
    property = "__typename"
)
@JsonSubTypes({
    @JsonSubTypes.Type(value = HighPrecisionMoney.class, name = "HighPrecisionMoney"),
    @JsonSubTypes.Type(value = Money.class, name = "Money")
})
public interface BaseMoney {

Without the call to the on-method the __typeName field will not be queried and the deserializer fails with

Caused by: com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Could not resolve subtype of [simple type, class com.commercetools.graphql.api.types.BaseMoney]: missing type id property '__typename' (for POJO property 'value')

Adding the attribute defaultImpl to the BaseMoneys JsonTypeInfo and generating a defaultImpl class sufficient for the interface this would save the necessary call to one of the on methods in the projection .projection(p -> p.value().centAmount()) cause now the deserializer knows which class to instantiate in case there is no discriminator property available in the response.

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY,
    property = "__typename",
        defaultImpl = BaseMoneyImpl.class // <--- add this line
)
@JsonSubTypes({
    @JsonSubTypes.Type(value = HighPrecisionMoney.class, name = "HighPrecisionMoney"),
    @JsonSubTypes.Type(value = Money.class, name = "Money")
})
public interface BaseMoney {

Generate additionally this class as default implementation of the interface

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NONE
)
public class BaseMoneyImpl implements BaseMoney {