_getIndex (list, relation) {
    let index = -1
    list.forEach(function (_relation, i) {
      if (_relation === relation) {
        index = i
        return false
      } else if (utils.isObject(_relation)) {
        if (_relation.relation === relation) {
          index = i
          return false
        }
      }
    })
    return index
  }
Find the index of a relation in the given list

@method utils._getIndex
@param {string[]} list List to search.
@param {string} relation Relation to find.
@private
@returns {number}