- Added/fixed support for MySQL "SET DEFAULT" / "DROP DEFAULT" phrases,

which will now be rendered if only the server default is changing
or being dropped (e.g. specify None to alter_column() to indicate
"DROP DEFAULT").  Also added support for rendering MODIFY rather than
CHANGE when the column name isn't changing.
fixes #103

1 files changed, 88 insertions, 25 deletions

diff --git a/alembic/ddl/mysql.py b/alembic/ddl/mysql.py
index 69b32b2..3d954f6 100644
--- a/alembic/ddl/mysql.py
+++ b/alembic/ddl/mysql.py
@@ -6,7 +6,8 @@ from ..compat import string_types
 from .. import util
 from .impl import DefaultImpl
 from .base import ColumnNullable, ColumnName, ColumnDefault, \
-            ColumnType, AlterColumn, format_column_name
+            ColumnType, AlterColumn, format_column_name, \
+            format_server_default
 from .base import alter_table
 
 class MySQLImpl(DefaultImpl):
@@ -26,22 +27,49 @@ class MySQLImpl(DefaultImpl):
                         existing_nullable=None,
                         existing_autoincrement=None
                     ):
-        self._exec(
-            MySQLAlterColumn(
-                table_name, column_name,
-                schema=schema,
-                newname=name if name is not None else column_name,
-                nullable=nullable if nullable is not None else
-                                existing_nullable
-                                if existing_nullable is not None
-                                else True,
-                type_=type_ if type_ is not None else existing_type,
-                default=server_default if server_default is not False
-                                            else existing_server_default,
-                autoincrement=autoincrement if autoincrement is not None
-                                            else existing_autoincrement
+        if name is not None:
+            self._exec(
+                MySQLChangeColumn(
+                    table_name, column_name,
+                    schema=schema,
+                    newname=name,
+                    nullable=nullable if nullable is not None else
+                                    existing_nullable
+                                    if existing_nullable is not None
+                                    else True,
+                    type_=type_ if type_ is not None else existing_type,
+                    default=server_default if server_default is not False
+                                                else existing_server_default,
+                    autoincrement=autoincrement if autoincrement is not None
+                                                else existing_autoincrement
+                )
+            )
+        elif nullable is not None or \
+            type_ is not None or \
+            autoincrement is not None:
+            self._exec(
+                MySQLModifyColumn(
+                    table_name, column_name,
+                    schema=schema,
+                    newname=name if name is not None else column_name,
+                    nullable=nullable if nullable is not None else
+                                    existing_nullable
+                                    if existing_nullable is not None
+                                    else True,
+                    type_=type_ if type_ is not None else existing_type,
+                    default=server_default if server_default is not False
+                                                else existing_server_default,
+                    autoincrement=autoincrement if autoincrement is not None
+                                                else existing_autoincrement
+                )
+            )
+        elif server_default is not False:
+            self._exec(
+                MySQLAlterDefault(
+                    table_name, column_name, server_default,
+                    schema=schema,
+                )
             )
-        )
 
     def correct_for_autogen_constraints(self, conn_unique_constraints, conn_indexes,
                                         metadata_unique_constraints,
@@ -53,7 +81,14 @@ class MySQLImpl(DefaultImpl):
                 conn_indexes.remove(idx)
 
 
-class MySQLAlterColumn(AlterColumn):
+class MySQLAlterDefault(AlterColumn):
+    def __init__(self, name, column_name, default, schema=None):
+        super(AlterColumn, self).__init__(name, schema=schema)
+        self.column_name = column_name
+        self.default = default
+
+
+class MySQLChangeColumn(AlterColumn):
     def __init__(self, name, column_name, schema=None,
                         newname=None,
                         type_=None,
@@ -68,12 +103,16 @@ class MySQLAlterColumn(AlterColumn):
         self.autoincrement = autoincrement
         if type_ is None:
             raise util.CommandError(
-                "All MySQL ALTER COLUMN operations "
+                "All MySQL CHANGE/MODIFY COLUMN operations "
                 "require the existing type."
             )
 
         self.type_ = sqltypes.to_instance(type_)
 
+class MySQLModifyColumn(MySQLChangeColumn):
+    pass
+
+
 @compiles(ColumnNullable, 'mysql')
 @compiles(ColumnName, 'mysql')
 @compiles(ColumnDefault, 'mysql')
@@ -84,14 +123,39 @@ def _mysql_doesnt_support_individual(element, compiler, **kw):
         )
 
 
-@compiles(MySQLAlterColumn, "mysql")
-def _mysql_alter_column(element, compiler, **kw):
-    return "%s CHANGE %s %s" % (
+@compiles(MySQLAlterDefault, "mysql")
+def _mysql_alter_default(element, compiler, **kw):
+    return "%s ALTER COLUMN %s %s" % (
+        alter_table(compiler, element.table_name, element.schema),
+        format_column_name(compiler, element.column_name),
+        "SET DEFAULT %s" % format_server_default(compiler, element.default)
+             if element.default is not None
+            else "DROP DEFAULT"
+    )
+
+@compiles(MySQLModifyColumn, "mysql")
+def _mysql_modify_column(element, compiler, **kw):
+    return "%s MODIFY %s %s" % (
+        alter_table(compiler, element.table_name, element.schema),
+        format_column_name(compiler, element.column_name),
+        _mysql_colspec(
+            compiler,
+            nullable=element.nullable,
+            server_default=element.default,
+            type_=element.type_,
+            autoincrement=element.autoincrement
+        ),
+    )
+
+
+@compiles(MySQLChangeColumn, "mysql")
+def _mysql_change_column(element, compiler, **kw):
+    return "%s CHANGE %s %s %s" % (
         alter_table(compiler, element.table_name, element.schema),
         format_column_name(compiler, element.column_name),
+        format_column_name(compiler, element.newname),
         _mysql_colspec(
             compiler,
-            name=element.newname,
             nullable=element.nullable,
             server_default=element.default,
             type_=element.type_,
@@ -105,10 +169,9 @@ def _render_value(compiler, expr):
     else:
         return compiler.sql_compiler.process(expr)
 
-def _mysql_colspec(compiler, name, nullable, server_default, type_,
+def _mysql_colspec(compiler, nullable, server_default, type_,
                                         autoincrement):
-    spec = "%s %s %s" % (
-        format_column_name(compiler, name),
+    spec = "%s %s" % (
         compiler.dialect.type_compiler.process(type_),
         "NULL" if nullable else "NOT NULL"
     )