Alembic is a great tool for keeping track of schema changes in python applications. I am using it to manage DB migrations for braindump along with Flask SQL Alchemy as my ORM. One challenge is managing proper foreign key constraints. By default if you define a foreign key relationship in your schema definition it will not generate the proper migration code.
For example, in braindump we have a one to many relationship between users and notes.
class User(UserMixin, db.Model): __tablename__ = 'users' id = db.Column(db.Integer, primary_key=True) email = db.Column(db.String(254), unique=True, index=True) password_hash = db.Column(db.String(256)) confirmed = db.Column(db.Boolean, default=False) avatar_hash = db.Column(db.String(32)) created_date = db.Column(db.DateTime(), default=datetime.utcnow) updated_date = db.Column(db.DateTime(), default=datetime.utcnow) notes = db.relationship( 'Note', backref='author', lazy='dynamic', cascade="all, delete-orphan")
Even though we define the cascade behavior using SQLAlchemy. When we generate the migration with alembic we get something like this:
sa.ForeignKeyConstraint(['author_id'], ['users.id'], ),
Notice how we are missing the
ondelete action. What we actually want is something like this:
sa.ForeignKeyConstraint(['author_id'], ['users.id'], ondelete='CASCADE')
Running the default migration will not create the proper relationship in your database and in our case we are not able to delete a user until we have deleted all of the related records as well. There are two ways to fix this. If you catch this before running your migration adding
ondelete='CASCADE' will create the proper relationship. If you are like me, and do not catch this, then you will need to run a second migration to remove and recreate these keys. The migration code to do this is shown below:
from alembic import op import sqlalchemy as sa def upgrade(): with op.batch_alter_table("notes") as batch_op: batch_op.drop_constraint( "notes_author_id_fkey", type_="foreignkey") op.create_foreign_key( "notes_author_id_fkey", "notes", "users", ["author_id"], ["id"], ondelete="CASCADE")
Now you have the proper foreign key constraints and the CASCADE action exists in the DB.
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