# Source code for aiida.backends.sqlalchemy.migrations.versions.91b573400be5_prepare_schema_reset

# -*- coding: utf-8 -*-
###########################################################################
# This file is part of the AiiDA code.                                    #
#                                                                         #
# The code is hosted on GitHub at https://github.com/aiidateam/aiida-core #
# For further information on the license, see the LICENSE.txt file        #
# For further information please visit http://www.aiida.net               #
###########################################################################
# pylint: disable=invalid-name,no-member
"""Prepare schema reset.

Revision ID: 91b573400be5
Revises: 7b38a9e783e7
Create Date: 2019-07-25 14:58:39.866822

"""

# Remove when https://github.com/PyCQA/pylint/issues/1931 is fixed
# pylint: disable=no-name-in-module,import-error
from alembic import op
from sqlalchemy.sql import text

# revision identifiers, used by Alembic.
revision = '91b573400be5'
down_revision = '7b38a9e783e7'
branch_labels = None
depends_on = None

conn = op.get_bind()

# The following statement is trying to perform an UPSERT, i.e. an UPDATE of a given key or if it doesn't exist fall
# back to an INSERT. This problem is notoriously difficult to solve as explained in great detail in this article:
# https://www.depesz.com/2012/06/10/why-is-upsert-so-complicated/ Postgres 9.5 provides an offical UPSERT method
# through the ON CONFLICT keyword, but since we also support 9.4 we cannot use it here. The snippet used below
# taken from the provided link, is not safe for concurrent operations, but since our migrations always run in an
# isolated way, we do not suffer from those problems and can safely use it.
statement = text(
"""
INSERT INTO db_dbsetting (key, val, description)
SELECT 'schema_generation', '"1"', 'Database schema generation'
WHERE NOT EXISTS (SELECT * FROM db_dbsetting WHERE key = 'schema_generation');
"""
)
conn.execute(statement)