Fix GroupConcat to put ORDER BY before SEPARATOR (#596)

Otherwise you end up with a syntax error if you set both, `separator` and `ordering`!

https://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_group-concat

Author: tuky

HISTORY.rst

@@ -15,6 +15,8 @@ Pending
   ``importlib.metadata.version("django-mysql")``
   (`docs <https://docs.python.org/3.8/library/importlib.metadata.html#distribution-versions>`__ /
   `backport <https://pypi.org/project/importlib-metadata/>`__).
+* Fix ``GroupConcat`` to work with both ``separator`` and ``ordering`` set.
+  (`PR #596 <https://github.com/adamchainz/django-mysql/pull/596>`__).
 
 3.2.0 (2019-06-14)
 ------------------

src/django_mysql/models/aggregates.py

@@ -52,16 +52,16 @@ def as_sql(self, compiler, connection, function=None, template=None):
 
         sql.append(expr_sql)
 
-        if self.separator is not None:
-            sql.append(" SEPARATOR '{}'".format(self.separator))
-
         if self.ordering is not None:
             sql.append(" ORDER BY ")
             sql.append(expr_sql)
             params.extend(params[:])
             sql.append(" ")
             sql.append(self.ordering.upper())
 
+        if self.separator is not None:
+            sql.append(" SEPARATOR '{}'".format(self.separator))
+
         sql.append(")")
 
         return "".join(sql), tuple(params)

tests/testapp/test_aggregates.py

@@ -132,3 +132,9 @@ def test_ordering_asc(self):
     def test_ordering_desc(self):
         out = self.shakes.tutees.aggregate(tids=GroupConcat("id", ordering="desc"))
         assert out == {"tids": ",".join(reversed(self.str_tutee_ids))}
+
+    def test_separator_ordering(self):
+        concat = GroupConcat("id", separator=":", ordering="asc")
+        out = self.shakes.tutees.aggregate(tids=concat)
+        concatted_ids = ":".join(self.str_tutee_ids)
+        assert out == {"tids": concatted_ids}