Mongoose Model如果不存在则不存储默认值

Question

team_id没有设置值，但是当我查看数据时，没有team_id === null

模型

var LeaguePosition = new Schema({
    team_id: { type: Schema.ObjectId, ref: 'Clubs', default: null },
    leagueID: String,
    position: Number,
    previousPosition: Number,
    gf: Number,
    pld: Number,
    ga: Number,
    gd: Number,
    pts: Number,
    w: Number,
    d: Number,
    l: Number,
    data: Date,
    week: {type: Number, default: 0}
})

数据

{
    "_id" : ObjectId("584c82b6d4a2102fd0fd1eb1"),
    "pts" : 16,
    "pld" : 10,
    "l" : 4,
    "d" : 1,
    "w" : 5,
    "gd" : 2,
    "ga" : 26,
    "gf" : 28,
    "leagueID" : "969975213",
    "previousPosition" : -1,
    "position" : 1,
    "week" : 1,
    "__v" : 0
}

保存

var pos = new LeaguePosition();
pos.position = index;
pos.previousPosition = previousPosition;
pos.leagueID = passData.leagueInfo.currentLeagueID;
pos.gf = item.gf;
pos.ga = item.ga;
pos.gd = item.gd;
pos.w = item.w;
pos.d = item.d;
pos.l = item.l;
pos.pld = item.pld;
pos.pts = item.pts;
pos.week = passData.leagueWeek+1;
return pos.save()