问多$project mongodb

EN

db.questions.aggregate([
    { $project: { 
        total: { $add: [ "$answear_false", "$answear_true" ] }
    }},
    { $project: { 
        percent_true: { 
            $cond: [
                { $eq: [ "$total", null ] }, 
                0 ,
                { $divide: [ "$answear_true", "$total" ] } 
            ] 
        } 
    }}, 
    { $project: { _id: 1, total: 1, percent_true: 1 } }
])

{ "_id" : "1004121032231110394769", "percent_true" : 0 }
{ "_id" : "1004121035679127802289", "percent_true" : 0 }
{ "_id" : "1004121038562570811362", "percent_true" : 0 }