Do not create FK if exists

[tinogis]

If we are trying to create a fk constraint and it exists, fails.

We can put a constraint existence test like:

select constraint_name from information_schema.constraint_column_usage where table_name = [t_name] and constraint_name = [c_name]

or drop and create:

ALTER TABLE [t_name] DROP CONSTRAINT IF EXISTS [c_name]

in https://github.com/gisce/oopgrade/blob/661689373ae565a68f0d5400307a383b244d1109/oopgrade/oopgrade.py#L297C28-L297C29

@ecarreras @polsala

[ecarreras]

The intention of add_columns_fk is only add not check if it must be added, for that exists get_foreign_keys which should be called in the script to check if foreign_key must be created or not

[ecarreras]

We can use get_foreign_keys in the method to check if foreign exists or not with the following order:

• Get the foreign keys
• If exists and is with the same foreign key and foreign table don't do anything
• If exists but is with other foreign key or other foreign table raise an exception
• If the foreign key doesn't exist create it.