Fix bug with unions.

Author: jverswijver

datajoint/expression.py

@@ -625,9 +625,15 @@ def make_sql(self):
         if not arg1.heading.secondary_attributes and not arg2.heading.secondary_attributes:
             # no secondary attributes: use UNION DISTINCT
             fields = arg1.primary_key
-            return "({sql1}) UNION ({sql2})".format(
-                sql1=arg1.make_sql(fields),
-                sql2=arg2.make_sql(fields))
+            if isinstance(arg1, Union):
+                sql1 = arg1.make_sql()
+            else:
+                sql1 = arg1.make_sql(fields)
+            if isinstance(arg2, Union):
+                sql2 = arg2.make_sql()
+            else:
+                sql2 = arg2.make_sql(fields)
+            return "({sql1}) UNION ({sql2})".format(sql1=sql1, sql2=sql2)
         # with secondary attributes, use union of left join with antijoin
         fields = self.heading.names
         sql1 = arg1.join(arg2, left=True).make_sql(fields)

tests/test_relational_operand.py

@@ -1,3 +1,4 @@
+from os import stat
 import random
 import string
 import pandas
@@ -505,3 +506,12 @@ def test_joins_with_aggregation():
         session_dates = ((SessionDateA * (subj_query & 'date_trained<"2020-12-21"')) &
                          'session_date<date_trained')
         assert len(session_dates) == 1
+
+    @staticmethod
+    def test_union_multiple():
+        # https://github.com/datajoint/datajoint-python/issues/926
+        q1 = IJ & dict(j=2)
+        q2 = (IJ & dict(j=2, i=0)) + (IJ & dict(j=2, i=1)) + (IJ & dict(j=2, i=2))
+        x = set(zip(*q1.fetch('i', 'j')))
+        y = set(zip(*q2.fetch('i', 'j')))
+        assert_set_equal(x, y)