Question

I am storing a bunch of patent data in a MySQL database and interacting with it via SQLAlchemy. I have a collection inside the Patent class that represents the list of assignees (the companies that were assigned the patent):

assignees = relationship('Company', secondary=patent_company_table, backref='patents')

I am processing some of the objects stored in the database and for a Patent object p, I want to delete some assignee a (a Company object) from p‘s assignee list. Based off of http://docs.sqlalchemy.org/en/latest/orm/session.html#deleting-from-collections , it seems that calling s.delete(a) will actually delete the Company object a. I simply want to remove assignee a from the list of assignees for p (i.e. remove a row in the patent_company_table), NOT actually delete the Company object, because a might be in another Patent object’s list of assignees.

I tried creating a new list new_assignees that only includes the assignees from p besides a and then called:

p.assignees = new_assignees
s.add(p)

This unfortunately does not actually mark p as dirty, so I assume it would not affect the database.

Do you have any suggestions for how to remove an object from the collection, deleting the row in the patent_company_table as opposed to deleting the object from the Company table?

Thank you.

UPDATE

Here is a snippet of the code:

assignees = patent.assignees
for assignee in assignees:
    if assignee in duplicate_company_to_default:
        patent.assignees.remove(assignee)
        default_company = duplicate_company_to_default[assignee]
        if default_company not in assignees:
            added_patent_count += 1
            patent.assignees.append(default_company)

After looping through all of the patents, added_patent_count = 983672 but there are no objects in session.dirty(). Do I need to add manually to the session after modifying via append or remove?

Answer 1

SQLAlchemy collections support list-like append/remove operations.

p.assignees.remove(c)

This should remove c form p.assignees without deleting c from database.