1使用$where(性能较差)

db.factor.find(
  {
    journal: {$exists: true},
    $where: "(this.journal.length>20)"
  }
)

OR

db.factor.find(
  {
    $where: "(this.journal.length>20)"
  }
)

OR

db.factor.find(
  {
      $where: function() {
          return this.journal.length > 20;
      }
  }
)

2 使用$regex(性能较$where好一些)

db.factor.find({ journal: {$regex: /^.{21,}$/} })

• https://blog.csdn.net/user_longling/article/details/80565994