Fixed #18375 -- Removed dict-ordering dependency for F-expressions

F() expressions reuse joins like any lookup in a .filter() call -
reuse multijoins generated in the same .filter() call else generate
new joins. Also, lookups can now reuse joins generated by F().

This change is backwards incompatible, but it is required to prevent
dict randomization from generating different queries depending on
.filter() kwarg ordering. The new way is also more consistent in how
joins are reused.

django/db/models/sql/expressions.py

@@ -1,14 +1,16 @@
 from django.core.exceptions import FieldError
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.fields import FieldDoesNotExist
+from django.db.models.sql.constants import REUSE_ALL
 
 class SQLEvaluator(object):
-    def __init__(self, expression, query, allow_joins=True):
+    def __init__(self, expression, query, allow_joins=True, reuse=REUSE_ALL):
         self.expression = expression
         self.opts = query.get_meta()
         self.cols = []
 
         self.contains_aggregate = False
+        self.reuse = reuse
         self.expression.prepare(self, query, allow_joins)
 
     def prepare(self):
@@ -50,9 +52,10 @@ class SQLEvaluator(object):
             try:
                 field, source, opts, join_list, last, _ = query.setup_joins(
                     field_list, query.get_meta(),
-                    query.get_initial_alias(), False)
+                    query.get_initial_alias(), self.reuse)
                 col, _, join_list = query.trim_joins(source, join_list, last, False)
-
+                if self.reuse is not None and self.reuse != REUSE_ALL:
+                    self.reuse.update(join_list)
                 self.cols.append((node, (join_list[-1], col)))
             except FieldDoesNotExist:
                 raise FieldError("Cannot resolve keyword %r into field. "

django/db/models/sql/query.py

@@ -1096,7 +1096,7 @@ class Query(object):
             value = value()
         elif isinstance(value, ExpressionNode):
             # If value is a query expression, evaluate it
-            value = SQLEvaluator(value, self)
+            value = SQLEvaluator(value, self, reuse=can_reuse)
             having_clause = value.contains_aggregate
 
         for alias, aggregate in self.aggregates.items():