When trying to add columns to a table using alembic, I ran into the following error:
File "/path/to/site-packages/sqlalchemy/sql/elements.py", line 460, in __bool__ raise TypeError("Boolean value of this clause is not defined")
Here’s the code that I was using:
def upgrade(): op.add_column('table', sa.Column('field_one', sa.String(50), nullable=False), sa.Column('field_two', sa.String(50), nullable=True) )
I’d copied and pasted it from somewhere online and just added the second column as I wanted to add multiple columns. This is what broke SQLAlchemy.
Unfortunately, the error message isn’t very descriptive. It sent me on a wild goose chase through the SQLAlchemy docs before I finally decided just to delete code until it worked. After realising that the second item was causing issues, I tried using
op.add_columns rather than
op.add_column but was greeted by the following error:
AttributeError: 'module' object has no attribute 'add_columns'
To get it working, you have to add the columns one at a time:
def upgrade(): op.add_column('table', sa.Column('field_one', sa.String(50), nullable=False)) op.add_column('table', sa.Column('field_two', sa.String(50), nullable=True))
It’s worth noting that the same applies to dropping columns too:
def downgrade(): op.drop_column("table", "field_one") op.drop_column("table", "field_two")
Michael is a polyglot software engineer, committed to reducing complexity in systems and making them more predictable. Working with a variety of languages and tools, he shares his technical expertise to audiences all around the world at user groups and conferences. You can follow @mheap on Twitter