add check command for upgrade diffs

Added new Alembic command ``alembic check``. This performs the widely
requested feature of running an "autogenerate" comparison between the
current database and the :class:`.MetaData` that's currently set up for
autogenerate, returning an error code if the two do not match, based on
current autogenerate settings. Pull request courtesy Nathan Louie.

As this is a new feature we will call this 1.9.0

Fixes: #724
Closes: #1101
Pull-request: https://github.com/sqlalchemy/alembic/pull/1101
Pull-request-sha: 807ed545df70e7a10b913e2951a1b636f138a4ff

Change-Id: I03b146eaf762be464a0ff0858ff5730cc9366c84

1 files changed, 59 insertions, 0 deletions

diff --git a/tests/test_command.py b/tests/test_command.py
index e136c4e..5ec3567 100644
--- a/tests/test_command.py
+++ b/tests/test_command.py
@@ -9,7 +9,9 @@ from typing import cast
 
 from sqlalchemy import exc as sqla_exc
 from sqlalchemy import text
+from sqlalchemy import VARCHAR
 from sqlalchemy.engine import Engine
+from sqlalchemy.sql.schema import Column
 
 from alembic import __version__
 from alembic import command
@@ -537,6 +539,63 @@ finally:
         command.revision(self.cfg, sql=True)
 
 
+class CheckTest(TestBase):
+    def setUp(self):
+        self.env = staging_env()
+        self.cfg = _sqlite_testing_config()
+
+    def tearDown(self):
+        clear_staging_env()
+
+    def _env_fixture(self, version_table_pk=True):
+        env_file_fixture(
+            """
+
+from sqlalchemy import MetaData, engine_from_config
+target_metadata = MetaData()
+
+engine = engine_from_config(
+    config.get_section(config.config_ini_section),
+    prefix='sqlalchemy.')
+
+connection = engine.connect()
+
+context.configure(
+    connection=connection, target_metadata=target_metadata,
+    version_table_pk=%r
+)
+
+try:
+    with context.begin_transaction():
+        context.run_migrations()
+finally:
+    connection.close()
+    engine.dispose()
+
+"""
+            % (version_table_pk,)
+        )
+
+    def test_check_no_changes(self):
+        self._env_fixture()
+        command.check(self.cfg)  # no problem
+
+    def test_check_changes_detected(self):
+        self._env_fixture()
+        with mock.patch(
+            "alembic.operations.ops.UpgradeOps.as_diffs",
+            return_value=[
+                ("remove_column", None, "foo", Column("old_data", VARCHAR()))
+            ],
+        ):
+            assert_raises_message(
+                util.AutogenerateDiffsDetected,
+                r"New upgrade operations detected: \[\('remove_column'",
+                command.check,
+                self.cfg,
+            )
+
+
 class _StampTest:
     def _assert_sql(self, emitted_sql, origin, destinations):
         ins_expr = (