2 files changed, 53 insertions, 4 deletions
diff --git a/django/contrib/postgres/aggregates/general.py b/django/contrib/postgres/aggregates/general.py
index 806ecd1b78..4b2da0b101 100644
--- a/django/contrib/postgres/aggregates/general.py
+++ b/django/contrib/postgres/aggregates/general.py
@@ -1,14 +1,16 @@
 from django.contrib.postgres.fields import ArrayField, JSONField
 from django.db.models.aggregates import Aggregate
 
+from .mixins import OrderableAggMixin
+
 __all__ = [
     'ArrayAgg', 'BitAnd', 'BitOr', 'BoolAnd', 'BoolOr', 'JSONBAgg', 'StringAgg',
 ]
 
 
-class ArrayAgg(Aggregate):
+class ArrayAgg(OrderableAggMixin, Aggregate):
     function = 'ARRAY_AGG'
-    template = '%(function)s(%(distinct)s%(expressions)s)'
+    template = '%(function)s(%(distinct)s%(expressions)s %(ordering)s)'
 
     @property
     def output_field(self):
@@ -49,9 +51,9 @@ class JSONBAgg(Aggregate):
         return value
 
 
-class StringAgg(Aggregate):
+class StringAgg(OrderableAggMixin, Aggregate):
     function = 'STRING_AGG'
-    template = "%(function)s(%(distinct)s%(expressions)s, '%(delimiter)s')"
+    template = "%(function)s(%(distinct)s%(expressions)s, '%(delimiter)s'%(ordering)s)"
 
     def __init__(self, expression, delimiter, distinct=False, **extra):
         distinct = 'DISTINCT ' if distinct else ''
diff --git a/django/contrib/postgres/aggregates/mixins.py b/django/contrib/postgres/aggregates/mixins.py
new file mode 100644
index 0000000000..b270a5b653
--- /dev/null
+++ b/django/contrib/postgres/aggregates/mixins.py
@@ -0,0 +1,47 @@
+from django.db.models.expressions import F, OrderBy
+
+
+class OrderableAggMixin:
+
+    def __init__(self, expression, ordering=(), **extra):
+        if not isinstance(ordering, (list, tuple)):
+            ordering = [ordering]
+        ordering = ordering or []
+        # Transform minus sign prefixed strings into an OrderBy() expression.
+        ordering = (
+            (OrderBy(F(o[1:]), descending=True) if isinstance(o, str) and o[0] == '-' else o)
+            for o in ordering
+        )
+        super().__init__(expression, **extra)
+        self.ordering = self._parse_expressions(*ordering)
+
+    def resolve_expression(self, *args, **kwargs):
+        self.ordering = [expr.resolve_expression(*args, **kwargs) for expr in self.ordering]
+        return super().resolve_expression(*args, **kwargs)
+
+    def as_sql(self, compiler, connection):
+        if self.ordering:
+            self.extra['ordering'] = 'ORDER BY ' + ', '.join((
+                ordering_element.as_sql(compiler, connection)[0]
+                for ordering_element in self.ordering
+            ))
+        else:
+            self.extra['ordering'] = ''
+        return super().as_sql(compiler, connection)
+
+    def get_source_expressions(self):
+        return self.source_expressions + self.ordering
+
+    def get_source_fields(self):
+        # Filter out fields contributed by the ordering expressions as
+        # these should not be used to determine which the return type of the
+        # expression.
+        return [
+            e._output_field_or_none
+            for e in self.get_source_expressions()[:self._get_ordering_expressions_index()]
+        ]
+
+    def _get_ordering_expressions_index(self):
+        """Return the index at which the ordering expressions start."""
+        source_expressions = self.get_source_expressions()
+        return len(source_expressions) - len(self.ordering)