Add function get columns for layer's query

lib/cartodb/models/aggregation/aggregation-mapconfig.js

@@ -38,7 +38,7 @@ module.exports = class AggregationMapConfig extends MapConfig {
         return AggregationMapConfig.SUPPORTED_GEOMETRY_TYPES.includes(geometryType);
     }
 
-    constructor (config, datasource) {
+    constructor (config, connection, datasource) {
         super(config, datasource);
 
         const validate = aggregationValidator(this);
@@ -50,6 +50,8 @@ module.exports = class AggregationMapConfig extends MapConfig {
         validate('placement', includesValidPlacementsValidator);
         validate('threshold', positiveNumberValidator);
         validate('columns', aggregationColumnsValidator);
+
+        this.connection = connection;
     }
 
     getAggregatedQuery (index) {
@@ -120,4 +122,26 @@ module.exports = class AggregationMapConfig extends MapConfig {
 
         return featureCount >= threshold;
     }
+
+    getLayerColumns (index, skipGeoms, callback) {
+        const geomColumns = ['the_geom', 'the_geom_webmercator'];
+        const limitedQuery = ctx => `SELECT * FROM (${ctx.query}) __cdb_schema LIMIT 0`;
+        const layer = this.getLayer(index);
+
+        this.connection.query(limitedQuery({ query: layer.options.sql }), (err, result) => {
+            if (err) {
+                return callback(err);
+            }
+
+            let columns = result.fields || [];
+
+            columns = columns.map(({ name, type }) => ({ name, type }));
+
+            if (skipGeoms) {
+                columns = columns.filter((column) => !geomColumns.includes(column));
+            }
+
+            return callback(err, columns);
+        });
+    }
 };