mongoose schema默认创建空数组？

我是mongoose和mongoDB的新手，通过教程，我有一个用户架构如下所示：

var userSchema = mongoose.Schema({
    local: {
        email: {
            type: String,
            index: {
                unique: true,
                dropDups: true
            }
        },
        password: String,
        displayName: String,
        avatar: {
            type: String,
            default: "./img/user.png"
        },
        role: {
            type: String,
            default: "student"
        },
        ask_history: [
            {
                question_id: {
                    type: mongoose.Schema.Types.ObjectId,
                    ref: 'questionAnswer'
                },
                favorite: Boolean,
                ask_time: Date
            }
        ],
        interest: [String]
    },
    facebook: {
        id: String,
        token: String,
        email: String,
        displayName: String,
        avatar: String,
        familyName: String,
        givenName: String,
        gender: String,
        ageMin: Number,
        role: {
            type: String,
            default: "student"
        },
        ask_history: [
            {
                question_id: {
                    type: mongoose.Schema.Types.ObjectId,
                    ref: 'questionAnswer'
                },
                favorite: Boolean,
                ask_time: Date
            }
        ],
        interest: [String]
    },
    twitter: {
        id: String,
        token: String,
        email: String,
        name: String,
        avatar: String,
        role: {
            type: String,
            default: "student"
        },
        ask_history: [
            {
                question_id: {
                    type: mongoose.Schema.Types.ObjectId,
                    ref: 'questionAnswer'
                },
                favorite: Boolean,
                ask_time: Date
            }
        ],
        interest: [String]
    }
}, {strict: true});

它通过用户注册的方式创建用户记录，或者提供本地电子邮件或使用第三方的oauth注册 .

如果我创建本地用户，上面的模式将生成如下所示的文档：

{
    "_id": {
        "$oid": "58792ee6e8a08204a0d68f7a"
    },
    "twitter": {                  ////////////////////////////////////////
        "interest": [],           //
        "ask_history": [],        //
        "role": "student"         //
    },                            // <-- nothing should created for a local user
    "facebook": {                 //
        "interest": [],           //
        "ask_history": [],        //
        "role": "student"         //
    },                            ////////////////////////////////////////
    "local": {
        "password": "$2a$10$OtdbF7t52TyNAuwZXFT0u.Q/A.E5TeV4T.shHCxSSxDll2nX4bCbW",
        "displayName": "aaa",
        "email": "aaa@gmail.com",
        "interest": [],
        "ask_history": [],
        "role": "admin",
        "avatar": "./img/user.png"
    },
    "__v": 0
}

我遇到的问题是

ask_history: [
            {
                question_id: {
                    type: mongoose.Schema.Types.ObjectId,
                    ref: 'questionAnswer'
                },
                favorite: Boolean,
                ask_time: Date
            }
        ],
interest: [String]

在模式中的那些数组声明中，文档对于任何类型的用户看起来都很好，例如，本地用户，文档看起来像这样（没有facebook或twitter字段）：

var userSchema = mongoose.Schema({
    local: {
        email: {
            type: String,
            index: {
                unique: true,
                dropDups: true
            }
        },
        password: String,
        displayName: String,
        avatar: {
            type: String,
            default: "./img/user.png"
        },
        role: {
            type: String,
            default: "student"
        }
    },
    facebook: {
        id: String,
        token: String,
        email: String,
        displayName: String,
        avatar: String,
        familyName: String,
        givenName: String,
        gender: String,
        ageMin: Number,
        role: {
            type: String,
            default: "student"
        }
    },
    twitter: {
        id: String,
        token: String,
        email: String,
        name: String,
        avatar: String,
        role: {
            type: String,
            default: "student"
        }
    }
}, {strict: true});

{
    "_id": {
        "$oid": "5874d1d0cd045371b4e96eca"
    },
    "local": {
        "password": "$2a$10$KpnIE2Uc2tX.YfcQCGm6EeDMexFFZXbKJVcQvxltBxrTkxb8E7mH.",
        "displayName": "aaa",
        "avatar": "./img/user.png",
        "email": "aaa@gmail.com",
        "role": "admin"
    },
    "__v": 0
}

这是我创建本地用户的功能：

passport.use('local-signup', new LocalStrategy({
    usernameField: 'email',
    passwordField: 'password',
    passReqToCallback: true 
}, function(req, email, password, done) {
    if (!req.body.name || req.body.name.length == 0) {
        return done(null, false, req.flash('signupMessage', 'Name can\'t be empty'));
    }
    User.findOne({
        'local.email': email
    }, function(err, user) {
        if (err) {
            return done(null, false, req.flash('signupMessage', 'Error: ' + err));
        } else {
            if (validator.validate(email)) {
                if (user) {
                    return done(null, false, req.flash('signupMessage', 'Email already registered.')); 
                } else {
                    var newUser = new User();
                    newUser.local.email = email;
                    newUser.hashPassword(password);
                    newUser.local.displayName = req.body.name;
                    newUser.save(function(err) {
                        if (err) {
                            throw err;
                        }
                        return done(null, newUser);
                    });
                }
            } else {
                return done(null, false, req.flash('signupMessage', 'Email not vaild'));
            }
        }
    });
}));

我的问题是如何设置架构到创建文档时，不会创建数组 .