[{"_id":ObjectId("6237e2103cd4b19dce804712"),"userId":"2701","shapes":[{"shape":"square","color":"blue","completed":true},{"shape":"circle","color":"red","name":"thisisred","completed":false},{"shape":"rectangle","color":"red","name":"thisisred","completed":false}]}]

db.collection.aggregate([{$match:{"shapes.color":"red"}},{$set:{shapes:{$map:{input:{$range:[0,{$size:"$shapes"}]},as:"s",in:{$mergeObjects:[{$arrayElemAt:["$shapes","$$s"]},{index:"$$s"}]}}}}},{$set:{shapes:{$filter:{input:"$shapes",as:"s",cond:{$eq:["$$s.color","red"]}}}}},{$set:{shapes:{$map:{input:"$shapes",as:"s",in:"$$s.index"}}}}])