Support for nested .select() calls (#2737)

* Reproduction for #1567

* Recursive handling of nested pointer keys in select

* Better support for multi-level nested keys

* Adds support for selecting columns natively (mongo)

* Support for postgres column selections

* Filter-out empty keys for pg

src/Adapters/Storage/Postgres/PostgresStorageAdapter.js

@@ -921,8 +921,8 @@ export class PostgresStorageAdapter {
     });
   }
 
-  find(className, schema, query, { skip, limit, sort }) {
-    debug('find', className, query, {skip, limit, sort});
+  find(className, schema, query, { skip, limit, sort, keys }) {
+    debug('find', className, query, {skip, limit, sort, keys });
     const hasLimit = limit !== undefined;
     const hasSkip = skip !== undefined;
     let values = [className];
@@ -954,7 +954,19 @@ export class PostgresStorageAdapter {
       sortPattern = `ORDER BY ${where.sorts.join(',')}`;
     }
 
-    const qs = `SELECT * FROM $1:name ${wherePattern} ${sortPattern} ${limitPattern} ${skipPattern}`;
+    let columns = '*';
+    if (keys) {
+      // Exclude empty keys
+      keys = keys.filter((key) => {
+        return key.length > 0;
+      });
+      columns = keys.map((key, index) => {
+        return `$${index+values.length+1}:name`;
+      }).join(',');
+      values = values.concat(keys);
+    }
+
+    const qs = `SELECT ${columns} FROM $1:name ${wherePattern} ${sortPattern} ${limitPattern} ${skipPattern}`;
     debug(qs, values);
     return this._client.any(qs, values)
     .catch((err) => {