django/DatabaseIntrospection: implement get_primary_key_column, get_relations

Implemented:
* DatabaseIntrospection.get_primary_key_column
* DatabaseIntrospection.get_relations

However, get_relations tests CANNOT yet be enabled because
Cloud Spanner doesn't yet have ON DELETE cascading for
FOREIGN KEYS which limits #313, which renders the method
useless as it will always return {} instead of being populated
with values that are derived from FOREIGN KEYs.

Fixes #312
Updates #311

Author: odeke-em

spanner/dbapi/autocommit_off_connection.py

@@ -170,17 +170,22 @@ def list_tables(self):
         # with a transaction otherwise we get back:
         #   400 Unsupported concurrency mode in query using INFORMATION_SCHEMA.
         # hence this specialized method.
-        self.run_prior_DDL_statements()
-
-        with self.__dbhandle.snapshot() as snapshot:
-            res = snapshot.execute_sql("""
+        return self.run_sql_in_snapshot("""
              SELECT
               t.table_name
             FROM
               information_schema.tables AS t
             WHERE
               t.table_catalog = '' and t.table_schema = ''
             """)
+
+    def run_sql_in_snapshot(self, sql):
+        # Some SQL e.g. for INFORMATION_SCHEMA cannot be run in read-write transactions
+        # hence this method exists to circumvent that limit.
+        self.run_prior_DDL_statements()
+
+        with self.__dbhandle.snapshot() as snapshot:
+            res = snapshot.execute_sql(sql)
             return list(res)
 
     def get_table_column_schema(self, table_name):

spanner/dbapi/autocommit_off_cursor.py

@@ -357,18 +357,11 @@ def __run_prior_DDL_statements(self):
         return self.__connection.run_prior_DDL_statements()
 
     def list_tables(self):
-        # We CANNOT list tables with
-        #   SELECT
-        #     t.table_name
-        #   FROM
-        #     information_schema.tables AS t
-        #   WHERE
-        #     t.table_catalog = '' and t.table_schema = ''
-        # with a transaction otherwise we get back:
-        #   400 Unsupported concurrency mode in query using INFORMATION_SCHEMA.
-        # hence this specialized method.
         return self.__connection.list_tables()
 
+    def run_sql_in_snapshot(self, sql):
+        return self.__connection.run_sql_in_snapshot(sql)
+
     def get_table_column_schema(self, table_name):
         return self.__connection.get_table_column_schema(table_name)
 

spanner/dbapi/autocommit_on_connection.py

@@ -89,27 +89,22 @@ def run_prior_DDL_statements(self):
         return self.__handle_update_ddl(ddl_statements)
 
     def list_tables(self):
-        # We CANNOT list tables with
-        #   SELECT
-        #     t.table_name
-        #   FROM
-        #     information_schema.tables AS t
-        #   WHERE
-        #     t.table_catalog = '' and t.table_schema = ''
-        # with a transaction otherwise we get back:
-        #   400 Unsupported concurrency mode in query using INFORMATION_SCHEMA.
-        # hence this specialized method.
-        self.run_prior_DDL_statements()
-
-        with self.__dbhandle.snapshot() as snapshot:
-            res = snapshot.execute_sql("""
-             SELECT
+        return self.run_sql_in_snapshot("""
+            SELECT
               t.table_name
             FROM
               information_schema.tables AS t
             WHERE
               t.table_catalog = '' and t.table_schema = ''
             """)
+
+    def run_sql_in_snapshot(self, sql):
+        # Some SQL e.g. for INFORMATION_SCHEMA cannot be run in read-write transactions
+        # hence this method exists to circumvent that limit.
+        self.run_prior_DDL_statements()
+
+        with self.__dbhandle.snapshot() as snapshot:
+            res = snapshot.execute_sql(sql)
             return list(res)
 
     def get_table_column_schema(self, table_name):

spanner/dbapi/autocommit_on_cursor.py

@@ -269,18 +269,11 @@ def __run_prior_DDL_statements(self):
         return self.__db_handle.run_prior_DDL_statements()
 
     def list_tables(self):
-        # We CANNOT list tables with
-        #   SELECT
-        #     t.table_name
-        #   FROM
-        #     information_schema.tables AS t
-        #   WHERE
-        #     t.table_catalog = '' and t.table_schema = ''
-        # with a transaction otherwise we get back:
-        #   400 Unsupported concurrency mode in query using INFORMATION_SCHEMA.
-        # hence this specialized method.
         return self.__db_handle.list_tables()
 
+    def run_sql_in_snapshot(self, sql):
+        return self.__db_handle.run_sql_in_snapshot(sql)
+
     def get_table_column_schema(self, table_name):
         return self.__db_handle.get_table_column_schema(table_name)
 

spanner/django/features.py

@@ -251,9 +251,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         # DatabaseIntrospection.get_table_description() isn't fully implemented:
         # https://github.com/orijtech/django-spanner/issues/248
         'introspection.tests.IntrospectionTests.test_get_table_description_col_lengths',
-        # DatabaseIntrospection.get_primary_key_column() isn't implemented:
-        # https://github.com/orijtech/django-spanner/issues/312
-        'introspection.tests.IntrospectionTests.test_get_primary_key_column',
         # DatabaseIntrospection.get_relations() isn't implemented:
         # https://github.com/orijtech/django-spanner/issues/311
         'introspection.tests.IntrospectionTests.test_get_relations',

spanner/django/introspection.py

@@ -52,3 +52,48 @@ def get_table_description(self, cursor, table_name):
             )
 
         return descriptions
+
+    def get_relations(self, cursor, table_name):
+        # TODO: PLEASE DO NOT USE THIS METHOD UNTIL
+        #   https://github.com/orijtech/django-spanner/issues/313
+        # is resolved so that foreign keys can be supported, as documented in:
+        #   https://github.com/orijtech/django-spanner/issues/311
+        """
+        Return a dictionary of {field_name: (field_name_other_table, other_table)}
+        representing all relationships in the table.
+        """
+        results = cursor.run_sql_in_snapshot(
+            '''
+            SELECT
+                tc.COLUMN_NAME as col, ccu.COLUMN_NAME as ref_col, ccu.TABLE_NAME as ref_table
+            FROM
+                INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS tc
+            JOIN
+                INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS as rc
+            ON
+                tc.CONSTRAINT_NAME = rc.CONSTRAINT_NAME
+            JOIN
+                INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE as ccu
+            ON
+                rc.UNIQUE_CONSTRAINT_NAME = ccu.CONSTRAINT_NAME
+            WHERE
+                tc.TABLE_NAME="%s"''' % self.connection.ops.quote_name(table_name),
+        )
+        return {column: (referred_column, referred_table) for (column, referred_column, referred_table) in results}
+
+    def get_primary_key_column(self, cursor, table_name):
+        results = cursor.run_sql_in_snapshot(
+            '''
+            SELECT
+                ccu.COLUMN_NAME
+            FROM
+                INFORMATION_SCHEMA.TABLE_CONSTRAINTS as tc
+            RIGHT JOIN
+                INFORMATION_SCHEMA.KEY_COLUMN_USAGE
+            AS
+                ccu ON tc.CONSTRAINT_NAME = ccu.CONSTRAINT_NAME
+            WHERE
+                tc.TABLE_NAME="%s" AND tc.CONSTRAINT_TYPE='PRIMARY KEY' AND tc.TABLE_SCHEMA=''
+            ''' % self.connection.ops.quote_name(table_name),
+        )
+        return results[0][0] if results else None