How do I delete a foreign key constraint in SQLAlchemy?

Question

I'm using SQLAlchemy Migrate to keep track of database changes and I'm running into an issue with removing a foreign key. I have two tables, t_new is a new table, and t_exists is an existing table. I need to add t_new, then add a foreign key to t_exists. Then I need to be able to reverse the operation (which is where I'm having trouble).

t_new = sa.Table("new", meta.metadata,
    sa.Column("new_id", sa.types.Integer, primary_key=True)
)
t_exists = sa.Table("exists", meta.metadata,
    sa.Column("exists_id", sa.types.Integer, primary_key=True),
    sa.Column(
        "new_id", 
        sa.types.Integer,
        sa.ForeignKey("new.new_id", onupdate="CASCADE", ondelete="CASCADE"),
        nullable=False
    )
)

This works fine:

t_new.create()
t_exists.c.new_id.create()

But this does not:

t_exists.c.new_id.drop()
t_new.drop()

Trying to drop the foreign key column gives an error: 1025, "Error on rename of '.\\my_db_name\\#sql-1b0_2e6' to '.\\my_db_name\\exists' (errno: 150)"

If I do this with raw SQL, i can remove the foreign key manually then remove the column, but I haven't been able to figure out how to remove the foreign key with SQLAlchemy? How can I remove the foreign key, and then the column?

Answer 1

You can do it with sqlalchemy.migrate.

In order to make it work, I have had to create the foreign key constraint explicitly rather than implicitely with Column('fk', ForeignKey('fk_table.field')):

Alas, instead of doing this:

p2 = Table('tablename',
            metadata,
            Column('id', Integer, primary_key=True),
            Column('fk', ForeignKey('fk_table.field')),
            mysql_engine='InnoDB',
           )

do that:

p2 = Table('tablename',
            metadata,
            Column('id', Integer, primary_key=True),
            Column('fk', Integer, index=True),
            mysql_engine='InnoDB',
            )
ForeignKeyConstraint(columns=[p2.c.fk], refcolumns=[p3.c.id]).create()

Then the deletion process looks like this:

def downgrade(migrate_engine):
     # First drop the constraint
     ForeignKeyConstraint(columns=[p2.c.fk], refcolumns=[p3.c.id]).drop()
     # Then drop the table
     p2.drop()

Answer 2

I was able to accomplish this by creating a separate metadata instance and using Session.execute() to run raw SQL. Ideally, there would be a solution that uses sqlalchemy exclusively, so I wouldn't have to use MySQL-specific solutions. But as of now, I am not aware of such a solution.

Answer 3

I believe you can achieve this with SQLAlchemy-Migrate. Note that a ForeignKey is on an isolated column. A ForeignKeyConstraint is at the table level and relates the columns together. If you look at the ForeignKey object on the column you will see that it references a ForeignKeyConstraint.

I would unable to test this idea because of the two databases I use MS SQL isn't supported by SqlAlchemy-Migrate and sqlite doesn't support "alter table" for constraints. I did get SQLAlchemy to try to remove a FK via a drop on the references constraint on a sqlite table so it was looking good. YMMV.

Answer 4

Well, you can achieve this in sqlalchemy: just drop() the all the constraints before you drop() the column (theoretically, you might have multiple constraints):

def drop_column(column):
    for fk in column.table.foreign_keys:
        if fk.column == column:
            print 'deleting fk ', fk
            fk.drop()
    column.drop()

drop_column(t_exists.c.new_id)