Close, but seems to be missing id column. I don't know exactly how ActiveRecord behaves regarding nullable/default, so I'm just guessing good defaults for some of this.
class Strain(db.Model):
__tablename__ = 'strains'
id = Column(Integer, primary_key=True)
name = Column(String, nullable=False, unique=True, index=True)
description = Column(String, nullable=False, default='')
Also, MySQL and some other dbs expect a length for String, and treat String different than Text. At least PostgreSQL and SQLite don't care though.