implement revision_environment for merge

Fixed issue where the ``revision_environment`` directive in ``alembic.ini``
was ignored by the ``alembic merge`` command, leading to issues when other
configurational elements depend upon ``env.py`` being invoked within the
command.

Change-Id: Ifc6e2ae2197f61b8415cb0460854cc74ff9dc4cd
Fixes: #1299

alembic/command.py

@@ -329,6 +329,23 @@ def merge(
         "config": config  # Let templates use config for
         # e.g. multiple databases
     }
+
+    environment = util.asbool(config.get_main_option("revision_environment"))
+
+    if environment:
+
+        def nothing(rev, context):
+            return []
+
+        with EnvironmentContext(
+            config,
+            script,
+            fn=nothing,
+            as_sql=False,
+            template_args=template_args,
+        ):
+            script.run_env()
+
     return script.generate_revision(
         rev_id or util.rev_id(),
         message,

docs/build/unreleased/1299.rst

@@ -0,0 +1,8 @@
+.. change::
+    :tags: bug, commmands
+    :tickets: 1299
+
+    Fixed issue where the ``revision_environment`` directive in ``alembic.ini``
+    was ignored by the ``alembic merge`` command, leading to issues when other
+    configurational elements depend upon ``env.py`` being invoked within the
+    command.

tests/test_command.py

@@ -16,6 +16,7 @@
 from alembic import __version__
 from alembic import command
 from alembic import config
+from alembic import testing
 from alembic import util
 from alembic.script import ScriptDirectory
 from alembic.testing import assert_raises
@@ -203,6 +204,74 @@ def test_history_indicate_current(self):
         )
 
 
+class RevisionEnvironmentTest(_BufMixin, TestBase):
+    @classmethod
+    def setup(cls):
+        cls.env = staging_env()
+        cls.cfg = _sqlite_testing_config()
+        cls._setup_env_file()
+
+    def teardown(self):
+        self.cfg.set_main_option("revision_environment", "false")
+        clear_staging_env()
+
+    @classmethod
+    def _setup_env_file(self):
+        env_file_fixture(
+            r"""
+
+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
+)
+
+try:
+    with context.begin_transaction():
+        config.stdout.write(u"environment included OK\n")
+        context.run_migrations()
+finally:
+    connection.close()
+    engine.dispose()
+"""
+        )
+
+    def _assert_env_token(self, buf, expected):
+        if expected:
+            assert "environment included OK" in buf.getvalue().decode(
+                "ascii", "replace"
+            )
+        else:
+            assert "environment included OK" not in buf.getvalue().decode(
+                "ascii", "replace"
+            )
+
+    @testing.combinations(True, False, argnames="rev_env")
+    def test_merge_cmd_revision_environment(self, rev_env):
+        if rev_env:
+            self.cfg.set_main_option("revision_environment", "true")
+        self.cfg.stdout = buf = self._buf_fixture()
+        cfg = self.cfg
+        self.a, self.b, self.c = three_rev_fixture(cfg)
+        self.d, self.e, self.f = multi_heads_fixture(
+            cfg, self.a, self.b, self.c
+        )
+        command.merge(self.cfg, "heads", rev_id="merge_revision")
+        self._assert_env_token(buf, rev_env)
+        rev = ScriptDirectory.from_config(self.cfg).get_revision(
+            "merge_revision"
+        )
+
+        assert os.path.exists(rev.path)
+
+
 class CurrentTest(_BufMixin, TestBase):
     @classmethod
     def setup_class(cls):