Right now the $rankFusion stage 'combination.weights' argument is either completely unspecified (implying each pipeline has the same weight of 1), or if any of the pipeline weights are specified, every pipeline must have a weight specified.

We would like to change the behavior so that a subset of the pipelines can have their weights specified. All unspecified pipelines will have an implied weight of 1, per the design doc.

So the following $rankFusion stage would be valid:

{
  $rankFusion: {
    input: {
      pipelines: {
        pipeline_1: {...},
        pipeline_2: {...}
      }
    }
    combination: {
      weights: {
        pipeline_1: 0.5
      }
    }
  }
}

With 'pipeline_1' having a weight of 0.5, and 'pipeline_2' having a weight of 1