SQLAlchemy + MYSQL unique string column gets truncated

Question

I would like to enforce the uniqueness of a column, but after I add an object to the database the string in this unique column gets cutoff. I have a model defined as follows:

class Topic(Base):
    __tablename__ = 'topic'
    id = Column(Integer(), primary_key=True)
    slug = Column(String(256), nullable=False, unique=True)
    name = Column(String(256))

Im using SQLAlchemy and MYSQL. When I inspect the table that gets created:

mysql> DESCRIBE topic;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| id       | int(11)      | NO   | PRI | NULL    | auto_increment |
| slug     | varchar(10)  | NO   | UNI | NULL    |                |
| name     | varchar(256) | YES  |     | NULL    |                |

How do I get the slug column to have type varchar(256) and Key UNI?

Haleemur Ali · Accepted Answer · 2015-03-02 16:40:36Z

1

SQLAlchemy doesn't support database migrations. For that, you'd need something like Alembic, which is authored by the same person who wrote SQLAlchemy.

Alternatively, you can issue a DDL statement directly on the MySQL server, and change the Table Definition in Python.

[on mysql]

ALTER TABLE topic MODIFY COLUMN slug VARCHAR(256);

[on python]

class Topic(Base):
    __tablename__ = 'topic'
    id = Column(Integer(), primary_key=True)
    slug = Column(String(256), nullable=False, unique=True)
    name = Column(String(256))

answered Mar 2, 2015 at 16:40

Haleemur Ali

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1 Comment

MarkAWard Over a year ago

Thank you for your response. I also discovered that there is a max key length of 767 bytes so String(256) will not fit. I was able to decrease the length and everything works fine.

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