_setupAttr(attr, value) {
    const isAssociation =
      this.associationKeys.has(attr) || this.associationIdKeys.has(attr);

    if (!isAssociation) {
      this.attrs[attr] = value;
      // define plain getter/setters for non-association keys
      this._definePlainAttribute(attr);
    }
  }
model.attrs represents the persistable attributes, i.e. your db
    table fields.
    @method _setupAttr
    @param attr
    @param value
    @private
    @hide