code: function(modelAlias, fields) {
        var model = this.getTrackedModel(modelAlias);
        if (model) {
          return {
            fields: fields,
            model: model
          };
        }
      }
docs: Returns a useful data structure that binds a tracked model to the fields being tracked on a mapping.
  @method __createModelConfig
  @param modelAlias
  @param fields {Array of Strings or undefined} the fields that the model is tracking. Can be undefined if tracking all fields.
  When creating a model config for a computed mapping, the fields refers to the fields being tracked only for that computed value.
  @return {Object} a binding between a tracked model and the fields its tracking for a mapping. If no tracked model is bound to the modelAlias,
  it will return undefined.
  @private