Fix dropping of indexed columns in sqlite/sa08

Version 0.8 of SQLAlchemy added support of indexes
on expressions in addition to plain table columns,
which changed the way indexes are created.

This broke support of dropping columns of composite
indexes for SQLite: due to limitations of ALTER in
SQLite every time a column is dropped, we recreate
the whole table without the given column; if a
column is a part of a composite index, we change the
index definition to omit that column and then indexes
are recreated too.

SQLAlchemy versions starting from 0.8 no more pay
attention to 'columns' attribute of Index instances
when generating DDL for indexes, so when one of columns
of a composite index is dropped, we try to create a
new index on the column that doesn't exist anymore,
which of course fails.

Closes-Bug: #1241038

Change-Id: I777b8ce36e36f49bfb0889908811a063cf1a527b

3 files changed, 39 insertions, 3 deletions

diff --git a/migrate/changeset/__init__.py b/migrate/changeset/__init__.py
index 80ea152..256d704 100644
--- a/migrate/changeset/__init__.py
+++ b/migrate/changeset/__init__.py
@@ -14,6 +14,7 @@ warnings.simplefilter('always', DeprecationWarning)
 
 _sa_version = tuple(int(re.match("\d+", x).group(0)) for x in _sa_version.split("."))
 SQLA_07 = _sa_version >= (0, 7)
+SQLA_08 = _sa_version >= (0, 8)
 
 del re
 del _sa_version
diff --git a/migrate/changeset/schema.py b/migrate/changeset/schema.py
index c467cc5..5a77208 100644
--- a/migrate/changeset/schema.py
+++ b/migrate/changeset/schema.py
@@ -11,7 +11,7 @@ from sqlalchemy.schema import ForeignKeyConstraint
 from sqlalchemy.schema import UniqueConstraint
 
 from migrate.exceptions import *
-from migrate.changeset import SQLA_07
+from migrate.changeset import SQLA_07, SQLA_08
 from migrate.changeset.databases.visitor import (get_engine_visitor,
                                                  run_single_visitor)
 
@@ -573,7 +573,9 @@ populated with defaults
                 if col.name!=self.name:
                     columns.append(col)
             if columns:
-                index.columns=columns
+                index.columns = columns
+                if SQLA_08:
+                    index.expressions = columns
             else:
                 to_drop.add(index)
         table.indexes = table.indexes - to_drop
diff --git a/migrate/tests/changeset/test_changeset.py b/migrate/tests/changeset/test_changeset.py
index 3989ea3..bd7f034 100644
--- a/migrate/tests/changeset/test_changeset.py
+++ b/migrate/tests/changeset/test_changeset.py
@@ -18,6 +18,7 @@ class TestAddDropColumn(fixture.DB):
     """
     level = fixture.DB.CONNECT
     table_name = 'tmp_adddropcol'
+    table_name_idx = 'tmp_adddropcol_idx'
     table_int = 0
 
     def _setup(self, url):
@@ -26,14 +27,27 @@ class TestAddDropColumn(fixture.DB):
         self.table = Table(self.table_name, self.meta,
             Column('id', Integer, unique=True),
         )
+        self.table_idx = Table(
+            self.table_name_idx,
+            self.meta,
+            Column('id', Integer, primary_key=True),
+            Column('a', Integer),
+            Column('b', Integer),
+            Index('test_idx', 'a', 'b')
+        )
         self.meta.bind = self.engine
         if self.engine.has_table(self.table.name):
             self.table.drop()
+        if self.engine.has_table(self.table_idx.name):
+            self.table_idx.drop()
         self.table.create()
+        self.table_idx.create()
 
     def _teardown(self):
         if self.engine.has_table(self.table.name):
             self.table.drop()
+        if self.engine.has_table(self.table_idx.name):
+            self.table_idx.drop()
         self.meta.clear()
         super(TestAddDropColumn,self)._teardown()
 
@@ -257,7 +271,26 @@ class TestAddDropColumn(fixture.DB):
 
         col.drop(self.table)
 
-# TODO: remove already attached columns with indexes, uniques, pks, fks ..
+# TODO: remove already attached columns with uniques, pks, fks ..
+    @fixture.usedb(not_supported='postgresql')
+    def test_drop_column_of_composite_index(self):
+        # NOTE(rpodolyaka): postgresql automatically drops a composite index
+        #                   if one of its columns is dropped
+        self.table_idx.c.b.drop()
+
+        reflected = Table(self.table_idx.name, MetaData(), autoload=True,
+                          autoload_with=self.engine)
+        index = next(iter(reflected.indexes))
+        self.assertEquals(['a'], [c.name for c in index.columns])
+
+    @fixture.usedb()
+    def test_drop_all_columns_of_composite_index(self):
+        self.table_idx.c.a.drop()
+        self.table_idx.c.b.drop()
+
+        reflected = Table(self.table_idx.name, MetaData(), autoload=True,
+                          autoload_with=self.engine)
+        self.assertEquals(0, len(reflected.indexes))
 
     def _check_index(self,expected):
         if 'mysql' in self.engine.name or 'postgres' in self.engine.name: