Alembic doesn't recognize False default value

Question

While maintaining a SQLAlchemy data model and utilizing alembic for version control, the following code change I made resulted in an empty revision:

some_column = Column(Boolean, nullable=False, default=False)

While previously it was:

some_column = Column(Boolean, nullable=False)

So adding a default value produces no changes in alembic, i.e. generates an empty revision. I tried other values offered by SQLAlchemy like false() and expression.false() instead of False, but the result is the same (empty alembic revision). Also tried server_default instead of default. The database in question is PostgreSQL.

By empty revision, of course I mean that alembic doesn't recognize any change being made in SQLAlchemy:

def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    pass
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    pass
    # ### end Alembic commands ###

Appreciate any help in this regard.

score 9 · Accepted Answer · edited Jan 05 '21 at 20:34

To do this automatically you have to turn on a setting to detect server default changes.

In your env.py, for the context.configure calls (online and offline migrations, so in 2 places), add a compare_server_default=True kwarg.

It is probably safer to just put in the alter_column yourself as well as definitely use server_default because default is just for python-side setting of the default(which is ok but sounds like not what you want).

Quoted from https://alembic.sqlalchemy.org/en/latest/autogenerate.html#what-does-autogenerate-detect-and-what-does-it-not-detect

Autogenerate can optionally detect:

...

Change of server default. This will occur if you set the EnvironmentContext.configure.compare_server_default parameter to True, or to a custom callable function. This feature works well for simple cases but cannot always produce accurate results.

...

score 6 · Answer 2 · edited Mar 19 '23 at 21:27

6

Just pass false() in server_default kwarg in Column.

ref: https://docs.sqlalchemy.org/en/14/core/sqlelement.html


from sqlalchemy.sql import false

some_column = Column(Boolean, server_default=false(), nullable=False)

edited Mar 19 '23 at 21:27

Henry

3,472
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answered Nov 10 '22 at 16:07

Daniel Bailo

127
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6

Mykola Černov · Answer 3 · 2023-05-03T19:37:29.347

0

Next part of code demonstrates how to define default value for new Boolean field in the table

from alembic import op
import sqlalchemy as sa

def upgrade():
    with op.batch_alter_table("jobs", schema=None) as batch_op:
        batch_op.add_column(
            sa.Column(
                "is_deleted",
                sa.Boolean(),
                server_default=sa.false(),
            )
        )

edited May 03 '23 at 19:37

answered Apr 26 '23 at 17:31

Mykola Černov

1
2

1

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Apr 28 '23 at 18:17
This doesnt seem to work if you say are trying to do a bulk COPY from one table to another, but excluding a boolean column. – mike01010 Jul 11 '23 at 03:57

Alembic doesn't recognize False default value

3 Answers3

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