fixing bugs in asynchronous tasks

rowers/dataprepnodjango.py

@@ -715,6 +715,8 @@ def getsmallrowdata_db(columns,ids=[],debug=False):
     data = []
     columns = [c for c in columns if c != 'None']
 
+    df = pd.DataFrame()
+
     if len(ids)>1:
         for id, f in zip(ids,csvfilenames):
             try:
@@ -728,11 +730,13 @@ def getsmallrowdata_db(columns,ids=[],debug=False):
             df =  pd.concat(data,axis=0)
         except ValueError:
             df = pd.DataFrame()
-    else:
+    elif len(ids)==1:
         try:
             df = pd.read_parquet(csvfilenames[0],columns=columns,engine='pyarrow')
         except (OSError,IndexError):
             df = pd.DataFrame()
+    else:
+        df = pd.DataFrame()
 
 
     return df