[{"_id":"1","content":{"a":1,"b":2}}]

db.collection.aggregate([{$match:{_id:"1"}},{"$lookup":{"from":"collection","let":{docId:"2"},"pipeline":[{$match:{$expr:{$eq:["$_id","$$docId"]}}}],"as":"doc2"}},{$match:{doc2:[]}},{$project:{doc2:false}},{$addFields:{someNewFields:"..."}},{"$merge":{"into":"collection","on":"_id","whenMatched":"replace"}}])