fix: Report column defaults in introspection (#744)

Retrieve the column default when introspecting columns. Currently, the
default is hard-coded to None.

Fixes: #730

Author: waltaskew

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -1122,7 +1122,8 @@ def get_multi_columns(
 
         sql = """
             SELECT col.table_schema, col.table_name, col.column_name,
-                col.spanner_type, col.is_nullable, col.generation_expression
+                col.spanner_type, col.is_nullable, col.generation_expression,
+                col.column_default
             FROM information_schema.columns as col
             JOIN information_schema.tables AS t
                 USING (TABLE_CATALOG, TABLE_SCHEMA, TABLE_NAME)
@@ -1150,7 +1151,7 @@ def get_multi_columns(
                     "name": col[2],
                     "type": self._designate_type(col[3]),
                     "nullable": col[4] == "YES",
-                    "default": None,
+                    "default": col[6] if col[6] is not None else None,
                 }
 
                 if col[5] is not None:

test/system/test_basics.py

@@ -34,7 +34,7 @@
 )
 from sqlalchemy.orm import Session, DeclarativeBase, Mapped, mapped_column
 from sqlalchemy.types import REAL
-from sqlalchemy.testing import eq_, is_true, is_not_none
+from sqlalchemy.testing import eq_, is_true, is_not_none, is_none
 from sqlalchemy.testing.plugin.plugin_base import fixtures
 
 
@@ -47,7 +47,7 @@ def define_tables(cls, metadata):
             Column("number", Integer),
             Column("name", String(20)),
             Column("alternative_name", String(20)),
-            Column("prime", Boolean),
+            Column("prime", Boolean, server_default=text("FALSE")),
             Column("ln", REAL),
             PrimaryKeyConstraint("number"),
         )
@@ -120,12 +120,15 @@ def test_reflect(self, connection):
         eq_(5, len(table.columns))
         eq_("number", table.columns[0].name)
         eq_(BIGINT, type(table.columns[0].type))
+        is_none(table.columns[0].server_default)
         eq_("name", table.columns[1].name)
         eq_(String, type(table.columns[1].type))
         eq_("alternative_name", table.columns[2].name)
         eq_(String, type(table.columns[2].type))
         eq_("prime", table.columns[3].name)
         eq_(Boolean, type(table.columns[3].type))
+        is_not_none(table.columns[3].server_default)
+        eq_("FALSE", table.columns[3].server_default.arg.text)
         eq_("ln", table.columns[4].name)
         eq_(REAL, type(table.columns[4].type))
         eq_(1, len(table.indexes))