Nullable fields not serialized

[Knotworking]

We require sending null values for fields within our request body json to our API. However, the nullable fields never get serialized. From our investigations this seems to be an auto-value-moshi issue.

I think our use case and issue is similar to issue-126.

We have tried the proposed solution from the issue above, but the problem is that within the generated toJson method, the writer (which has serializeNulls = true) is only accessed within a field != null if statement, so the nullable field is never added to the json object to be written.

Here is a sample project we've written to demonstrate the issue. Please see the SerializeTest test class.

[Knotworking]

So that you don't need to checkout our sample project and build it, here is the relevant code I think.

Our model class (Issue), we're intending to write null for the field title

@JsonClass(generateAdapter = true, generator = "avm")
@AutoValue
abstract class Issue {

    abstract fun getId(): String

    @Nullable
    abstract fun getTitle(): String?

    abstract fun toBuilder(): Builder

    @AutoValue.Builder
    abstract class Builder {
        abstract fun id(value: String): Builder
        abstract fun title(value: String?): Builder
        abstract fun build(): Issue
    }

    companion object {
        fun builder(): Builder = AutoValue_Issue.Builder()

        @JvmStatic
        fun jsonAdapter(moshi: Moshi): JsonAdapter<Issue> {
            return IssueJsonAdapter(moshi).serializeNulls()
        }
    }
}

The generated toJson method in the adapter. As you can see here the title field is only written when title != null

@Override
  public void toJson(JsonWriter writer, Issue value) throws IOException {
    writer.beginObject();
    writer.name("id");
    this.idAdapter.toJson(writer, value.getId());
    String title = value.getTitle();
    if (title != null) {
      writer.name("title");
      this.titleAdapter.toJson(writer, title);
    }
    writer.endObject();
  }

So, are we missing something, is it possible to serialize null values?