Detect and render autoincrement for alter_column()

The ``autoincrement=True`` flag is now rendered within the
:meth:`.Operations.alter_column` operation if the source column indicates
that this flag should be set to True.  The behavior is sensitive to
the SQLAlchemy version in place, as the "auto" default option is new
in SQLAlchemy 1.1.  When the source column indicates autoincrement
as True or "auto", the flag will render as True if the original column
contextually indicates that it should have "autoincrement" keywords,
and when the source column explcitly sets it to False, this is also
rendered.  The behavior is intended to preserve the AUTO_INCREMENT flag
on MySQL as the column is fully recreated on this backend.  Note that this
flag does **not** support alteration of a column's "autoincrement" status,
as this is not portable across backends.

Change-Id: I746c4841752adf9342bfdca7c9255aae5110b2ef
Fixes: #413

8 files changed, 245 insertions, 5 deletions

diff --git a/alembic/autogenerate/compare.py b/alembic/autogenerate/compare.py
index 267b017..e23b466 100644
--- a/alembic/autogenerate/compare.py
+++ b/alembic/autogenerate/compare.py
@@ -670,6 +670,19 @@ def _compare_nullable(
 
 
 @comparators.dispatch_for("column")
+def _setup_autoincrement(
+    autogen_context, alter_column_op, schema, tname, cname, conn_col,
+        metadata_col):
+
+    if metadata_col.table._autoincrement_column is metadata_col:
+        alter_column_op.kw['autoincrement'] = True
+    elif util.sqla_110 and metadata_col.autoincrement is True:
+        alter_column_op.kw['autoincrement'] = True
+    elif metadata_col.autoincrement is False:
+        alter_column_op.kw['autoincrement'] = False
+
+
+@comparators.dispatch_for("column")
 def _compare_type(
     autogen_context, alter_column_op, schema, tname, cname, conn_col,
         metadata_col):
diff --git a/alembic/autogenerate/render.py b/alembic/autogenerate/render.py
index 6e10792..94a1467 100644
--- a/alembic/autogenerate/render.py
+++ b/alembic/autogenerate/render.py
@@ -331,6 +331,7 @@ def _alter_column(autogen_context, op):
     server_default = op.modify_server_default
     type_ = op.modify_type
     nullable = op.modify_nullable
+    autoincrement = op.kw.get('autoincrement', None)
     existing_type = op.existing_type
     existing_nullable = op.existing_nullable
     existing_server_default = op.existing_server_default
@@ -366,6 +367,9 @@ def _alter_column(autogen_context, op):
     if nullable is None and existing_nullable is not None:
         text += ",\n%sexisting_nullable=%r" % (
             indent, existing_nullable)
+    if autoincrement is not None:
+        text += ",\n%sautoincrement=%r" % (
+            indent, autoincrement)
     if server_default is False and existing_server_default:
         rendered = _render_server_default(
             existing_server_default,
diff --git a/alembic/testing/requirements.py b/alembic/testing/requirements.py
index ad4f27c..be46a33 100644
--- a/alembic/testing/requirements.py
+++ b/alembic/testing/requirements.py
@@ -104,6 +104,13 @@ class SuiteRequirements(Requirements):
         )
 
     @property
+    def fail_before_sqla_110(self):
+        return exclusions.fails_if(
+            lambda config: not util.sqla_110,
+            "SQLAlchemy 1.1.0 or greater required"
+        )
+
+    @property
     def sqlalchemy_08(self):
 
         return exclusions.skip_if(
diff --git a/docs/build/changelog.rst b/docs/build/changelog.rst
index caf3f90..12f2242 100644
--- a/docs/build/changelog.rst
+++ b/docs/build/changelog.rst
@@ -7,6 +7,23 @@ Changelog
     :version: 0.9.0
     :released:
 
+    .. change:: 413
+      :tags: bug, autogenerate, mysql
+      :tickets: 413
+
+      The ``autoincrement=True`` flag is now rendered within the
+      :meth:`.Operations.alter_column` operation if the source column indicates
+      that this flag should be set to True.  The behavior is sensitive to
+      the SQLAlchemy version in place, as the "auto" default option is new
+      in SQLAlchemy 1.1.  When the source column indicates autoincrement
+      as True or "auto", the flag will render as True if the original column
+      contextually indicates that it should have "autoincrement" keywords,
+      and when the source column explcitly sets it to False, this is also
+      rendered.  The behavior is intended to preserve the AUTO_INCREMENT flag
+      on MySQL as the column is fully recreated on this backend.  Note that this
+      flag does **not** support alteration of a column's "autoincrement" status,
+      as this is not portable across backends.
+
     .. change:: 85
       :tags: bug, postgresql
       :tickets: 85
diff --git a/tests/_autogen_fixtures.py b/tests/_autogen_fixtures.py
index 93d7b5e..f731a7f 100644
--- a/tests/_autogen_fixtures.py
+++ b/tests/_autogen_fixtures.py
@@ -210,7 +210,8 @@ class AutogenFixtureTest(_ComparesFKs):
 
     def _fixture(
             self, m1, m2, include_schemas=False,
-            opts=None, object_filters=_default_object_filters):
+            opts=None, object_filters=_default_object_filters,
+            return_ops=False):
         self.metadata, model_metadata = m1, m2
         self.metadata.create_all(self.bind)
 
@@ -238,7 +239,11 @@ class AutogenFixtureTest(_ComparesFKs):
             autogenerate._produce_net_changes(
                 autogen_context, uo
             )
-            return uo.as_diffs()
+
+            if return_ops:
+                return uo
+            else:
+                return uo.as_diffs()
 
     reports_unnamed_constraints = False
 
diff --git a/tests/requirements.py b/tests/requirements.py
index 987c0c7..522ac5b 100644
--- a/tests/requirements.py
+++ b/tests/requirements.py
@@ -114,3 +114,12 @@ class DefaultRequirements(SuiteRequirements):
                 return False
 
         return exclusions.only_if(check_uuid_ossp)
+
+    @property
+    def autoincrement_on_composite_pk(self):
+        return exclusions.skip_if(["sqlite"], "not supported by database")
+
+    @property
+    def integer_subtype_comparisons(self):
+        """if a compare of Integer and BigInteger is supported yet."""
+        return exclusions.skip_if(["oracle"], "not supported by alembic impl")
diff --git a/tests/test_autogen_diffs.py b/tests/test_autogen_diffs.py
index 4816134..60dc6e2 100644
--- a/tests/test_autogen_diffs.py
+++ b/tests/test_autogen_diffs.py
@@ -754,9 +754,10 @@ class PKConstraintUpgradesIgnoresNullableTest(AutogenTest, TestBase):
         return cls._get_db_schema()
 
     def test_no_change(self):
-        diffs = []
+        uo = ops.UpgradeOps(ops=[])
         ctx = self.autogen_context
-        autogenerate._produce_net_changes(ctx, diffs)
+        autogenerate._produce_net_changes(ctx, uo)
+        diffs = uo.as_diffs()
         eq_(diffs, [])
 
 
@@ -1203,3 +1204,174 @@ class OrigObjectTest(TestBase):
         is_(op.to_index(), self.ix)
         is_(op.reverse().to_index(), self.ix)
 
+
+class AutoincrementTest(AutogenFixtureTest, TestBase):
+    __backend__ = True
+    __requires__ = 'integer_subtype_comparisons',
+
+    def test_alter_column_autoincrement_none(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table('a', m1, Column('x', Integer, nullable=False))
+        Table('a', m2, Column('x', Integer, nullable=True))
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        assert 'autoincrement' not in ops.ops[0].ops[0].kw
+
+    def test_alter_column_autoincrement_pk_false(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('x', Integer, primary_key=True, autoincrement=False))
+        Table(
+            'a', m2,
+            Column('x', BigInteger, primary_key=True, autoincrement=False))
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        is_(ops.ops[0].ops[0].kw['autoincrement'], False)
+
+    def test_alter_column_autoincrement_pk_implicit_true(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('x', Integer, primary_key=True))
+        Table(
+            'a', m2,
+            Column('x', BigInteger, primary_key=True))
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        is_(ops.ops[0].ops[0].kw['autoincrement'], True)
+
+    def test_alter_column_autoincrement_pk_explicit_true(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('x', Integer, primary_key=True, autoincrement=True))
+        Table(
+            'a', m2,
+            Column('x', BigInteger, primary_key=True, autoincrement=True))
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        is_(ops.ops[0].ops[0].kw['autoincrement'], True)
+
+    def test_alter_column_autoincrement_nonpk_false(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('id', Integer, primary_key=True),
+            Column('x', Integer, autoincrement=False)
+        )
+        Table(
+            'a', m2,
+            Column('id', Integer, primary_key=True),
+            Column('x', BigInteger, autoincrement=False)
+        )
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        is_(ops.ops[0].ops[0].kw['autoincrement'], False)
+
+    def test_alter_column_autoincrement_nonpk_implicit_false(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('id', Integer, primary_key=True),
+            Column('x', Integer)
+        )
+        Table(
+            'a', m2,
+            Column('id', Integer, primary_key=True),
+            Column('x', BigInteger)
+        )
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        assert 'autoincrement' not in ops.ops[0].ops[0].kw
+
+    @config.requirements.fail_before_sqla_110
+    def test_alter_column_autoincrement_nonpk_explicit_true(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('id', Integer, primary_key=True),
+            Column('x', Integer, autoincrement=True)
+        )
+        Table(
+            'a', m2,
+            Column('id', Integer, primary_key=True),
+            Column('x', BigInteger, autoincrement=True)
+        )
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        is_(ops.ops[0].ops[0].kw['autoincrement'], True)
+
+    def test_alter_column_autoincrement_compositepk_false(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('id', Integer, primary_key=True),
+            Column('x', Integer, primary_key=True, autoincrement=False)
+        )
+        Table(
+            'a', m2,
+            Column('id', Integer, primary_key=True),
+            Column('x', BigInteger, primary_key=True, autoincrement=False)
+        )
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        is_(ops.ops[0].ops[0].kw['autoincrement'], False)
+
+    def test_alter_column_autoincrement_compositepk_implicit_false(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('id', Integer, primary_key=True),
+            Column('x', Integer, primary_key=True)
+        )
+        Table(
+            'a', m2,
+            Column('id', Integer, primary_key=True),
+            Column('x', BigInteger, primary_key=True)
+        )
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        assert 'autoincrement' not in ops.ops[0].ops[0].kw
+
+    @config.requirements.autoincrement_on_composite_pk
+    def test_alter_column_autoincrement_compositepk_explicit_true(self):
+        m1 = MetaData()
+        m2 = MetaData()
+
+        Table(
+            'a', m1,
+            Column('id', Integer, primary_key=True, autoincrement=False),
+            Column('x', Integer, primary_key=True, autoincrement=True),
+            # on SQLA 1.0 and earlier, this being present
+            # trips the "add KEY for the primary key" so that the
+            # AUTO_INCREMENT keyword is accepted by MySQL.  SQLA 1.1 and
+            # greater the columns are just reorganized.
+            mysql_engine='InnoDB'
+        )
+        Table(
+            'a', m2,
+            Column('id', Integer, primary_key=True, autoincrement=False),
+            Column('x', BigInteger, primary_key=True, autoincrement=True)
+        )
+
+        ops = self._fixture(m1, m2, return_ops=True)
+        is_(ops.ops[0].ops[0].kw['autoincrement'], True)
diff --git a/tests/test_autogen_render.py b/tests/test_autogen_render.py
index 7689d36..03d8d04 100644
--- a/tests/test_autogen_render.py
+++ b/tests/test_autogen_render.py
@@ -4,7 +4,7 @@ from alembic.testing import TestBase, exclusions, assert_raises
 
 from alembic.operations import ops
 from sqlalchemy import MetaData, Column, Table, String, \
-    Numeric, CHAR, ForeignKey, DATETIME, Integer, \
+    Numeric, CHAR, ForeignKey, DATETIME, Integer, BigInteger, \
     CheckConstraint, Unicode, Enum, cast,\
     UniqueConstraint, Boolean, ForeignKeyConstraint,\
     PrimaryKeyConstraint, Index, func, text, DefaultClause
@@ -1074,6 +1074,19 @@ class AutogenRenderTest(TestBase):
             "existing_type=sa.Integer(), nullable=True, schema='foo')"
         )
 
+    def test_render_modify_type_w_autoincrement(self):
+        op_obj = ops.AlterColumnOp(
+            "sometable", "somecolumn",
+            modify_type=Integer(), existing_type=BigInteger(),
+            autoincrement=True
+        )
+        eq_ignore_whitespace(
+            autogenerate.render_op_text(self.autogen_context, op_obj),
+            "op.alter_column('sometable', 'somecolumn', "
+            "existing_type=sa.BigInteger(), type_=sa.Integer(), "
+            "autoincrement=True)"
+        )
+
     def test_render_fk_constraint_kwarg(self):
         m = MetaData()
         t1 = Table('t', m, Column('c', Integer))