mysql count(): optimize requested fields

src/db/ModelQuery.ts

@@ -220,6 +220,7 @@ export default class ModelQuery<M extends Model> {
         this._sortBy = undefined;
         this._sortDirection = undefined;
 
+        this.fields.splice(0, this.fields.length);
         this.fields.push('COUNT(*)');
         let queryResult = await this.execute(connection);
         return queryResult.results.length;