问无法访问Typescript中的mongoose架构上下文

EN

export interface IUser extends mongoose.Document {
    email: string;
    password: string;
    firstName: string;
    lastName: string;
    comparePassword(password: string, callback: Function): void;
}

class UserSchema {
    static get schema(): mongoose.Schema {
        let userSchema = new mongoose.Schema({
            email: {
                type: String,
                unique: true,
                lowercase: true
            },
            firstName: {
                type: String,
                required: true
            },
            lastName: {
                type: String,
                required: true
            },
            password: {
                type: String,
                select: false
            }
        });

        userSchema.pre('save', this.saveHandler);        
        userSchema.method('comparePassword', this.comparePassword);

        return userSchema;
    }

    static saveHandler(next) {
        let user = this.schema;
        if(!user.isModified('password')) {
              return next();
         }
          bcrypt.genSalt(10, (err, salt) => {
              bcrypt.hash(user.password, salt, (err, hash) => {
                  user.password = hash;
                  next();
              });
          });
    }

    static comparePassword(password: string, callback: Function) {
        let user = this.schema;
        bcrypt.compare(password, user.password, (err, isMatch) => {
            callback(err, isMatch);
        });
    }
}

export const UserModel = mongoose.model<IUser>('User', UserSchema.schema);

var schema = new Schema({
    email: { type: String, unique: true, lowercase: true },
    password: { type: String, select: false },
    firstName: String,
    lastName: String,
});

schema.pre('save', function(next) {
    var user = this;
    if(!user.isModified('password')) {
        return next();
    }
    bcrypt.genSalt(10, function(err, salt) {
        bcrypt.hash(user.password, salt, function(err, hash) {
            user.password = hash;
            next();
        });
    });
});