Migration tries to delete already-deleted key from deleted column.

[sqlalchemy-bot]

Migrated issue, originally created by Andrew Magee (@amagee)

I'm using PostgreSQL. I deleted a non-nullable column from a table, which generated a migration like this:

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('cuser', 'username')
    op.drop_constraint('cuser_username_key', 'cuser')
    op.drop_index('cuser_username_key', table_name='cuser')
    ### end Alembic commands ###

But the second line, the drop_constraint, throws an exception because the "cuser_username_key" constraint no longer exists at that point. It seems that the drop_column call deletes it, because the constraint does actually exist if I inspect the table.

[sqlalchemy-bot]

Changes by Andrew Magee (@amagee):

• edited description

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

OK so two approaches, one is we get constraints to be ignored if an owning column is to be dropped, however, it might be better if we just put the constraint drops above the column and still include them; #232 seems to indicate that MySQL's drop column needs the constraints to be dropped explicitly in any case.

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

also, there is drop_constraint() and drop_index() here for the same name. the PG dialect is supposed to detect that kind of condition so it's....unexpected that it is generating both. but im going to ignore that here.

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

• A change in the ordering when columns and constraints are dropped;
  autogenerate will now place the "drop constraint" calls before
  the "drop column" calls, so that columns involved in those constraints
  still exist when the constraint is dropped.
  fixes Migration tries to delete already-deleted key from deleted column. #247

→ 2cdff3d

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

• Added a rule for Postgresql to not render a "drop unique" and "drop index"
  given the same name; for now it is assumed that the "index" is the
  implicit one Postgreql generates. Future integration with
  new SQLAlchemy 1.0 features will improve this to be more
  resilient.
  fixes Migration tries to delete already-deleted key from deleted column. #247

→ 360b8b9

[sqlalchemy-bot]

Changes by Michael Bayer (@zzzeek):

• changed status to closed

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

OK I found that one and i fixed that also.