code: function _nestModelFactory (model, v) {
    let attrs = v.split('?')[0]
  
    if (/\?/.test(v)) {
      let args = v.split('?')[1]
  
      if (!model.where) {
        model.where = {}
      }
      args.split('&').forEach(arg => {
        let field, value
  
        [field, value] = arg.split('=')
  
        if (field === 'as') {
          model[field] = value
          return
        }
        if (arg !== 'required') {
          _queryField2where.call(this, model.where, field, value, model.model.name)
        }
  
      })
  
      args.includes('required') ? model.required = true : model.required = false
    }
  
    if (attrs === '$null') {
      model.attributes = []
    } else if (attrs.startsWith('$exclude:')) {
      attrs = attrs.replace('$exclude:', '').split(',')
      model.attributes = {exclude: attrs}
    } else if (attrs !== '$all') {
      model.attributes = attrs.split(',')
    } else if (model.attributes && model.attributes.length !== 0 && attrs === '$all') {
      delete model.attributes
    }
  }
docs: 处理 include 的辅助函数, 帮助设置 include 中的 model 展示的 attributes
  @param {object} model, model 对象
  @param {string} v, 具体的 attributes 值
  @return {void}
  @this Model