Update relationship is failing half of the time

[pkernevez]

CKAN version

2.9.7

Describe the bug

When updating a relationship between 2 dataset the first call fails with a 500 error, resending the same request succeeds.
First call response:

{
    "help": "http://localhost:5001/api/3/action/help_show?name=package_relationship_update",
    "error": {
        "__type": "Internal Server Error",
        "message": "Internal Server Error"
    },
    "success": false
}

Response of the Same request just after:
{
"help": "http://localhost:5001/api/3/action/help_show?name=package_relationship_update",
"success": true,
"result": {
"subject": "Proxy Recall",
"type": "derives_from",
"object": "Governance Ratings",
"comment": "Test comment"
}
}

Steps to reproduce

1. Create a link between 2 dataset
2. Update the same link with a change in the comment
3. The first time it fails
4. Redo the same request, it succeeds

Expected behaviour

Having the first request on success

Additional details

Trace on server:

ckan  | Traceback (most recent call last):
ckan  |   File "/srv/app/src/ckan/ckan/config/middleware/../../views/api.py", line 293, in action
ckan  |     result = function(context, request_data)
ckan  |   File "/srv/app/src/ckan/ckan/logic/__init__.py", line 504, in wrapped
ckan  |     result = _action(context, data_dict, **kw)
ckan  |   File "/srv/app/src/ckan/ckan/logic/action/update.py", line 642, in package_relationship_update
ckan  |     return _update_package_relationship(entity, comment, context)
ckan  |   File "/srv/app/src/ckan/ckan/logic/action/update.py", line 586, in _update_package_relationship
ckan  |     rel_dict = relationship.as_dict(package=relationship.subject,
ckan  |   File "/usr/lib/python3.8/site-packages/sqlalchemy/orm/attributes.py", line 282, in __get__
ckan  |     return self.impl.get(instance_state(instance), dict_)
ckan  |   File "/usr/lib/python3.8/site-packages/sqlalchemy/orm/attributes.py", line 699, in get
ckan  |     value = self.callable_(state, passive)
ckan  |   File "/usr/lib/python3.8/site-packages/sqlalchemy/orm/strategies.py", line 683, in _load_for_state
ckan  |     raise orm_exc.DetachedInstanceError(
ckan  | sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <PackageRelationship at 0x7f207ec4d220> is not bound to a Session; lazy load operation of attribute 'subject' cannot proceed (Background on this error at: http://sqlalche.me/e/bhk3)