code: function nestHandler (include, list) {
    let attributes = list
  
    if (_.isString(list)) {
      attributes = list.split('.')
    }
    let md = attributes[0]
  
    let model = {
      model: this.models[md],
      include: []
    }
    let existModel = _.find(include, m => m.model.name === md)
  
    if (!existModel) {
      include.push(model)
    } else {
      model = existModel
      if (!model.include) {
        model.include = []
      }
    }
  
    attributes.shift()
    attributes.length > 1 ? nestHandler.call(this, model.include, attributes) : _nestModelFactory.call(this, model, attributes[0])
  }
docs: 辅助处理函数
  可以处理的 where 条件与 whereFactory 类似
  
  @param {array} include, include集合
  @param {string} list, 每个 include 的详细属性
  @return {void}
  @this Model