add null_ok to DatabaseIntrospection.get_table_description()

Introspect INFORMATION_SCHEMA.COLUMNS for COLUMN attributes.

INFORMATION_SCHEMA gives us information like:
* column_name
* is_nullable aka null_ok
* spanner_type

which can use to fill in the `null_ok` field for
spanner.django.introspection.DatabaseIntrospection.get_table_description

Updates #248
Updates #308

Author: odeke-em

spanner/dbapi/autocommit_off_connection.py

@@ -7,6 +7,7 @@
 from .autocommit_off_cursor import Cursor
 from .exceptions import Error
 from .periodic_auto_refresh import PeriodicAutoRefreshingTransaction
+from .utils import get_table_column_schema as get_table_column_schema_impl
 
 
 class Connection(object):
@@ -181,3 +182,6 @@ def list_tables(self):
               t.table_catalog = '' and t.table_schema = ''
             """)
             return list(res)
+
+    def get_table_column_schema(self, table_name):
+        return get_table_column_schema_impl(self.__connection, table_name)

spanner/dbapi/autocommit_off_cursor.py

@@ -369,6 +369,9 @@ def list_tables(self):
         # hence this specialized method.
         return self.__connection.list_tables()
 
+    def get_table_column_schema(self, table_name):
+        return self.__connection.get_table_column_schema(table_name)
+
 
 class Column:
     def __init__(self, name, type_code, display_size=None, internal_size=None,

spanner/dbapi/autocommit_on_connection.py

@@ -6,6 +6,7 @@
 
 from .autocommit_on_cursor import Cursor
 from .exceptions import Error
+from .utils import get_table_column_schema as get_table_column_schema_impl
 
 
 class Connection(object):
@@ -110,3 +111,6 @@ def list_tables(self):
               t.table_catalog = '' and t.table_schema = ''
             """)
             return list(res)
+
+    def get_table_column_schema(self, table_name):
+        return get_table_column_schema_impl(self.__dbhandle, table_name)

spanner/dbapi/autocommit_on_cursor.py

@@ -281,6 +281,9 @@ def list_tables(self):
         # hence this specialized method.
         return self.__db_handle.list_tables()
 
+    def get_table_column_schema(self, table_name):
+        return self.__db_handle.get_table_column_schema(table_name)
+
 
 class Column:
     def __init__(self, name, type_code, display_size=None, internal_size=None,

spanner/dbapi/utils.py

@@ -4,6 +4,12 @@
 # license that can be found in the LICENSE file or at
 # https://developers.google.com/open-source/licenses/bsd
 
+from collections import namedtuple
+
+from google.cloud.spanner_v1 import param_types
+
+ColumnDetails = namedtuple('column_details', ['null_ok', 'spanner_type'])
+
 
 class PeekIterator(object):
     """
@@ -40,3 +46,28 @@ def __next__(self):
 
     def __iter__(self):
         return self
+
+
+def get_table_column_schema(spanner_db, table_name):
+    with spanner_db.snapshot() as snapshot:
+        rows = snapshot.execute_sql(
+            '''SELECT
+                COLUMN_NAME, IS_NULLABLE, SPANNER_TYPE
+            FROM
+                INFORMATION_SCHEMA.COLUMNS
+            WHERE
+                TABLE_SCHEMA = ''
+            AND
+                TABLE_NAME = @table_name''',
+            params={'table_name': table_name},
+            param_types={'table_name': param_types.STRING},
+        )
+
+        column_details = {}
+        for column_name, is_nullable, spanner_type in rows:
+            column_details[column_name] = ColumnDetails(
+                null_ok=is_nullable == 'YES',
+                spanner_type=spanner_type,
+            )
+
+        return column_details

spanner/django/introspection.py

@@ -32,17 +32,23 @@ def get_table_description(self, cursor, table_name):
         interface.
         """
         cursor.execute("SELECT * FROM %s LIMIT 1" % self.connection.ops.quote_name(table_name))
-        return [
-            FieldInfo(
-                line.name,
-                line.type_code,
-                # TODO: Fill these in as they're implemented.
-                None,  # display_size
-                None,  # internal_size
-                None,  # precision
-                None,  # scale
-                None,  # null_ok
-                None,  # default
+        column_details = cursor.get_table_column_schema(table_name)
+        descriptions = []
+        for line in cursor.description:
+            column_name, type_code = line[0], line[1]
+            details = column_details[column_name]
+            descriptions.append(
+                FieldInfo(
+                    column_name,
+                    type_code,
+                    # TODO: Fill these in as they're implemented.
+                    None,  # display_size
+                    None,  # internal_size
+                    None,  # precision
+                    None,  # scale
+                    details.null_ok,
+                    None,  # default
+                )
             )
-            for line in cursor.description
-        ]
+
+        return descriptions