code: @SuppressWarnings("rawtypes")
    public MongoRx observableAdapter(final Function<Observable, Observable> adapter) {
      this.adapter = toAdapter(requireNonNull(adapter, "Adapter is required."));
      return this;
    }
docs: Set a {@link ObservableAdapter} to the {@link MongoDatabase} created by this module.
  
  @param adapter An {@link ObservableAdapter}.
  @return This module.