Removed server default quoting from compare

Don't modify the metadata server default when comparing it in the
autogenerate process.
This impacts the value passes to user provided functions passed in
:paramref:`.EnvironmentContext.configure.compare_server_default`
and third party dialect that implement a custom ``compare_server_default``.

Fixes: #1178
Change-Id: Ib429efcf9077337f768ad5aad91659867e89391a

6 files changed, 55 insertions, 14 deletions

diff --git a/alembic/autogenerate/compare.py b/alembic/autogenerate/compare.py
index 595631c..b489328 100644
--- a/alembic/autogenerate/compare.py
+++ b/alembic/autogenerate/compare.py
@@ -1023,9 +1023,7 @@ def _compare_type(
 
 
 def _render_server_default_for_compare(
-    metadata_default: Optional[Any],
-    metadata_col: Column,
-    autogen_context: AutogenContext,
+    metadata_default: Optional[Any], autogen_context: AutogenContext
 ) -> Optional[str]:
 
     if isinstance(metadata_default, sa_schema.DefaultClause):
@@ -1039,11 +1037,7 @@ def _render_server_default_for_compare(
                 )
             )
     if isinstance(metadata_default, str):
-        if metadata_col.type._type_affinity is sqltypes.String:
-            metadata_default = re.sub(r"^'|'$", "", metadata_default)
-            return f"'{metadata_default}'"
-        else:
-            return metadata_default
+        return metadata_default
     else:
         return None
 
@@ -1190,7 +1184,7 @@ def _compare_server_default(
                 )
     else:
         rendered_metadata_default = _render_server_default_for_compare(
-            metadata_default, metadata_col, autogen_context
+            metadata_default, autogen_context
         )
 
         rendered_conn_default = (
diff --git a/alembic/context.pyi b/alembic/context.pyi
index c81a14f..621599d 100644
--- a/alembic/context.pyi
+++ b/alembic/context.pyi
@@ -22,6 +22,8 @@ if TYPE_CHECKING:
     from sqlalchemy.engine.base import Connection
     from sqlalchemy.engine.url import URL
     from sqlalchemy.sql.elements import ClauseElement
+    from sqlalchemy.sql.schema import Column
+    from sqlalchemy.sql.schema import FetchedValue
     from sqlalchemy.sql.schema import MetaData
     from sqlalchemy.sql.schema import SchemaItem
 
@@ -144,7 +146,20 @@ def configure(
         ]
     ] = None,
     compare_type: bool = False,
-    compare_server_default: bool = False,
+    compare_server_default: Union[
+        bool,
+        Callable[
+            [
+                MigrationContext,
+                Column,
+                Column,
+                Optional[str],
+                Optional[FetchedValue],
+                Optional[str],
+            ],
+            Optional[bool],
+        ],
+    ] = False,
     render_item: Optional[
         Callable[[str, Any, AutogenContext], Union[str, Literal[False]]]
     ] = None,
diff --git a/alembic/ddl/mysql.py b/alembic/ddl/mysql.py
index a452760..5e66f53 100644
--- a/alembic/ddl/mysql.py
+++ b/alembic/ddl/mysql.py
@@ -185,13 +185,22 @@ class MySQLImpl(DefaultImpl):
             and rendered_inspector_default == "'0'"
         ):
             return False
-        elif inspector_column.type._type_affinity is sqltypes.Integer:
+        elif (
+            rendered_inspector_default
+            and inspector_column.type._type_affinity is sqltypes.Integer
+        ):
             rendered_inspector_default = (
                 re.sub(r"^'|'$", "", rendered_inspector_default)
                 if rendered_inspector_default is not None
                 else None
             )
             return rendered_inspector_default != rendered_metadata_default
+        elif (
+            rendered_metadata_default
+            and metadata_column.type._type_affinity is sqltypes.String
+        ):
+            metadata_default = re.sub(r"^'|'$", "", rendered_metadata_default)
+            return rendered_inspector_default != f"'{metadata_default}'"
         elif rendered_inspector_default and rendered_metadata_default:
             # adjust for "function()" vs. "FUNCTION" as can occur particularly
             # for the CURRENT_TIMESTAMP function on newer MariaDB versions
diff --git a/alembic/runtime/environment.py b/alembic/runtime/environment.py
index 71a5309..3087377 100644
--- a/alembic/runtime/environment.py
+++ b/alembic/runtime/environment.py
@@ -15,6 +15,8 @@ from typing import Tuple
 from typing import TYPE_CHECKING
 from typing import Union
 
+from sqlalchemy.sql.schema import Column
+from sqlalchemy.sql.schema import FetchedValue
 from typing_extensions import Literal
 
 from .migration import _ProxyTransaction
@@ -79,6 +81,18 @@ OnVersionApplyFn = Callable[
     None,
 ]
 
+CompareServerDefault = Callable[
+    [
+        MigrationContext,
+        Column,
+        Column,
+        Optional[str],
+        Optional[FetchedValue],
+        Optional[str],
+    ],
+    Optional[bool],
+]
+
 
 class EnvironmentContext(util.ModuleClsProxy):
 
@@ -398,7 +412,7 @@ class EnvironmentContext(util.ModuleClsProxy):
             ProcessRevisionDirectiveFn
         ] = None,
         compare_type: bool = False,
-        compare_server_default: bool = False,
+        compare_server_default: Union[bool, CompareServerDefault] = False,
         render_item: Optional[RenderItemFn] = None,
         literal_binds: bool = False,
         upgrade_token: str = "upgrades",
diff --git a/docs/build/unreleased/1178.rst b/docs/build/unreleased/1178.rst
new file mode 100644
index 0000000..25789d3
--- /dev/null
+++ b/docs/build/unreleased/1178.rst
@@ -0,0 +1,9 @@
+.. change::
+    :tags: changed, autogenerate
+    :tickets: 1178
+
+    Don't modify the metadata server default when comparing it in the
+    autogenerate process.
+    This impacts the value passes to user provided functions passed in
+    :paramref:`.EnvironmentContext.configure.compare_server_default`
+    and third party dialect that implement a custom ``compare_server_default``.
diff --git a/tests/test_postgresql.py b/tests/test_postgresql.py
index 77ed4da..818dae7 100644
--- a/tests/test_postgresql.py
+++ b/tests/test_postgresql.py
@@ -846,7 +846,7 @@ class PostgresqlDetectSerialTest(TestBase):
 
         eq_(
             _render_server_default_for_compare(
-                tab.c.x.server_default, tab.c.x, self.autogen_context
+                tab.c.x.server_default, self.autogen_context
             ),
             c_expected,
         )
@@ -867,7 +867,7 @@ class PostgresqlDetectSerialTest(TestBase):
         server_default = diffs[0][0][4]["existing_server_default"]
         eq_(
             _render_server_default_for_compare(
-                server_default, tab.c.x, self.autogen_context
+                server_default, self.autogen_context
             ),
             c_expected,
         )