Merge branch 'OskarPersson-exists' into azure-1.11

Author: michiya

README.rst

@@ -234,7 +234,6 @@ Limitations
 The following features are currently not supported:
 
 - Altering a model field from or to AutoField at migration
-- `Exists <https://docs.djangoproject.com/en/1.11/ref/models/expressions/#django.db.models.Exists>`__ subqueries
 
 Notice
 ------

sql_server/pyodbc/compiler.py

@@ -1,7 +1,7 @@
 from itertools import chain
 
 from django.db.models.aggregates import Avg, Count, StdDev, Variance
-from django.db.models.expressions import OrderBy, Ref, Value
+from django.db.models.expressions import Exists, OrderBy, Ref, Value
 from django.db.models.functions import ConcatPair, Greatest, Least, Length, Substr
 from django.db.models.sql import compiler
 from django.db.transaction import TransactionManagementError
@@ -39,6 +39,14 @@ def _as_sql_least(self, compiler, connection):
 def _as_sql_length(self, compiler, connection):
     return self.as_sql(compiler, connection, function='LEN')
 
+def _as_sql_exists(self, compiler, connection, template=None, **extra_context):
+    # MS SQL doesn't allow EXISTS() in the SELECT list, so wrap it with a
+    # CASE WHEN expression. Change the template since the When expression
+    # requires a left hand side (column) to compare against.
+    sql, params = self.as_sql(compiler, connection, template, **extra_context)
+    sql = 'CASE WHEN {} THEN 1 ELSE 0 END'.format(sql)
+    return sql, params
+
 def _as_sql_order_by(self, compiler, connection):
     template = None
     if self.nulls_last:
@@ -243,6 +251,8 @@ def _as_microsoft(self, node):
             as_microsoft = _as_sql_least
         elif isinstance(node, Length):
             as_microsoft = _as_sql_length
+        elif isinstance(node, Exists):
+            as_microsoft = _as_sql_exists
         elif isinstance(node, OrderBy):
             as_microsoft = _as_sql_order_by
         elif isinstance(node, StdDev):

sql_server/pyodbc/operations.py

@@ -5,7 +5,6 @@
 
 from django.conf import settings
 from django.db.backends.base.operations import BaseDatabaseOperations
-from django.db.models.expressions import Exists
 from django.db.models.functions import Greatest, Least
 from django.utils import timezone
 from django.utils.encoding import force_text
@@ -83,13 +82,6 @@ def check_expression_support(self, expression):
                     raise NotImplementedError(
                         'SQL Server has no support for %s function.' %
                         f.function)
-        # SQL Server doesn't allow to use EXISTS in a selection list
-        unsupported_expressions = (Exists, )
-        for e in unsupported_expressions:
-            if isinstance(expression, e):
-                raise NotImplementedError(
-                    "the backend doesn't support %s expression." %
-                    e.__name__)
 
     def combine_duration_expression(self, connector, sub_expressions):
         lhs, rhs = sub_expressions