Question

The situation is a little bit simplified. I have two migration files for sqlalchemy-migrate:

In First I create table volume_usage_cache, then autoload it, create copy of its columns and print it:

from sqlalchemy import Column, DateTime
from sqlalchemy import Boolean, BigInteger, MetaData, Integer, String, Table

def upgrade(migrate_engine):

    meta = MetaData()
    meta.bind = migrate_engine

    # Create new table
    volume_usage_cache = Table('volume_usage_cache', meta,
        Column('deleted', Boolean(create_constraint=True, name=None)),
        Column('id', Integer(), primary_key=True, nullable=False),
        Column('curr_write_bytes', BigInteger(), default=0),
        mysql_engine='InnoDB',
        mysql_charset='utf8'
    )

    volume_usage_cache.create()
    volume_usage_cache = Table('volume_usage_cache', meta, autoload=True)
    columns = []
    [columns.append(column.copy()) for column in volume_usage_cache.columns]
    print columns

And I get in log what I expected:

[Column('deleted', Boolean(), table=None), Column('id', Integer(), table=None,
primary_key=True, nullable=False), Column('curr_write_bytes', BigInteger(), 
table=None, default=ColumnDefault(0))]

But if I make a copy of columns in Second migration file (that is runed after First):

from sqlalchemy import MetaData, String, Integer, Boolean, Table, Column, Index
def upgrade(migrate_engine):
    meta = MetaData()
    meta.bind = migrate_engine

    table = Table("volume_usage_cache", meta, autoload=True) 
    columns = []
    for column in table.columns:
        columns.append(column.copy())
    print columns

I get a different result:

[Column('deleted', INTEGER(), table=None, default=ColumnDefault(0)), 
Column(u'id', INTEGER(), table=None, primary_key=True, nullable=False), 
Column(u'curr_write_bytes', NullType(), table=None)]

Why curr_write_bytes column has NullType?

Answer 1

The are two problems:

First:
In First file we are using old metadata that already contains all columns with need types
So if we create new MetaData instance, SqlAlchemy will load info about table from database and will get the same result as in Second file.

---

Second:
There is no support in sqlAlchemy for BigInteger column type (in sqlite). And Sqlite doesn’t support types of column at all. So we can create table with column BigInteger (and it will work), but after autoload type of such column will be automatically converted to NullType.