How do I change column type on SQLAlchemy declarative model dynamically?

Question

I am running mysql in production but would like to run a simple tests in a sqlite in memory db. The legacy mysql db has tables with columns that are mysql specific types, Which are declared in declarative models (subclassing declarative_base). I would like to run some simple tests without going to mysql and so would need to swap out the columns of the model. How do I do this? I've tried writing a patcher/unpatcher to swap out table in my model, but when I run some tests, I get ``` OperationalError: (OperationalError) near ")": syntax error u'
CREATE TABLE my_table (
)

' () ``` Which makes my think that I am not patching the columns properly. Does anyone know how I can do this? What am I doing wrong? Currently, I create new columns and attach brand new Table object to `__table__` and save the old table. The DB is created, create_all() is and convert_columns is run in setUp. drop_all() and revert_columns is run during tearDown in my tests ``` mysql_sqlite_mapping = {INTEGER: Intege…

Accepted Answer

I hope the below might help you with a better way of dealing with these kind of modifications. But your error is weird as it has no columns in the `CREATE TABLE` clause, so I do not think this is a direct answer/solution to your question.

Anyways, instead of completely replacing the tables and columns, why don't you try to use the Custom SQL Constructs and Compilation Extension / Dialect-specific compilation rules. See the sample code that should generate a proper SQL statement for sqlite without changing the table definitions:

```
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.dialects.mysql import MEDIUMINT, INTEGER, TEXT

@compiles(MEDIUMINT, 'sqlite')
def compile_MEDIUMINT(element, compiler, **kw):
    """ Handles mysql MEDIUMINT datatype as Integer in sqlite.  """
    return compiler.visit_integer(element, **kw)

@compiles(INTEGER, 'sqlite')
def compile_INTEGER(element, compiler, **kw):
    """ Handles mysql INTEGER datatype as Integer in sqlite.  """
    return compiler.visit_integer(element, **kw)

@compiles(TEXT, 'sqlite')
def compile_TEXT(element, compiler, **kw):
    """ Handles mysql TEXT datatype as text in sqlite.  """
    return compiler.visit_text(element, **kw)
```

How do I change column type on SQLAlchemy declarative model dynamically?

Solution

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