cant change foreginkey from null=false to null=true in postgresql

[anonymous]

I'm using South 0.7.5 and PostgreSQL 9.0.4 on x86_64-apple-darwin11.1.0, compiled by GCC i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00), 64-bit. I am starting with a new database, and migrations which may have been created by prior versions of South (0.7.3???). I get an error when trying to

The tail end of the stack trace is the following:

File "/Users/chema/envs/cf/lib/python2.7/site-packages/south/db/generic.py", line 533, in alter_column

field.rel.to._meta.get_field(field.rel.field_name).column

File "/Users/chema/envs/cf/lib/python2.7/site-packages/south/db/generic.py", line 273, in execute

cursor.execute(sql, params)

File "/Users/chema/envs/cf/lib/python2.7/site-packages/django/db/backends/util.py", line 34, in execute

return self.cursor.execute(sql, params)

File "/Users/chema/envs/cf/lib/python2.7/site-packages/django/db/backends/postgresql_psycopg2/base.py", line 44, in execute

return self.cursor.execute(query, args)

django.db.utils.DatabaseError?: constraint "revoked_by_id_refs_id_4128367e0dc3da83" for relation "credits_transaction" already exists

I managed to fix this by changing south/db/generic.py to include the following:

# Drop all foreign key constraints
try:

self.delete_foreign_key(table_name, name)

except ValueError?:

# There weren't any
ignore_constraints=True #THIS IS THE LINE I ADDED
pass