Our current input validation for $scoreFusion enforces that none of the input pipelines can modify the documents or their fields in any way (eg. $project, $group etc.). Because $scoreFusion gets desugared to a pipeline with stages like $group and $project that aren’t legal, nesting $scoreFusion doesn't work.
The same idea also applies to testing $rankFusion in $scoreFusion.
- is related to
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SERVER-93218 Allow nested $rankFusion
- Backlog