docs: function activateField(model, fields) {
    let activeFields = [];
    if (fields === true) { // activate all
      return Object.keys(model.def.fields);
    } else if (Array.isArray(fields)) {
      activeFields = fields;
    } else if (typeof fields === 'string') {
      if (fields.indexOf('*') >= 0) { // activate all
        activeFields = Object.keys(model.def.fields);
      } else {
        activeFields = fields.trim().split(/\s*,\s*/);
      }
    }
    if (activeFields.length <= 0) throw new QueryError(model.name, 'fields required in find');
  
    // activate the related fields
    return Object.keys(activateRelatedFields(model, activeFields));
  }