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copied!<p>I believe <a href="http://docs.sqlalchemy.org/en/rel_0_8/core/types.html#sqlalchemy.types.Enum" rel="nofollow noreferrer">sqlalchemy.types.Enum</a> should handle what you need to handle a defined list of strings.</p> <blockquote> <p>The Enum type provides a set of possible string values which the column is constrained towards. By default, uses the backend’s native ENUM type if available, else uses VARCHAR + a CHECK constraint.</p> </blockquote> <p>But looking at it from an object constraint level I imagine you can handle it with a ForeignKey with a Check constraint, but it seems the check contraints just pass it directly through to the database, so it will differ depending on what dbms you're running. <a href="https://stackoverflow.com/questions/1036217/is-it-possible-to-add-a-logic-constraint-to-a-foreign-key">This question goes deeper into the SQL</a> which should be able to be paired up with <a href="http://docs.sqlalchemy.org/en/latest/core/schema.html#check-constraint" rel="nofollow noreferrer">SQLAlchemy's Constraints</a>.</p> <p>A cleaner, more portable way might be to build it into your constructor, or just have a function that can validate the data before the query is fired</p> <pre><code>## Using the Car constructor for validation def __init__(self,color,manufacturer,price=1000): if color in manufacturer.allowed_colors: self.color = color self.manufacturer else: raise LookupError() # or something more relevant. self.price = price ## Or just a generic function that can be called: def validate_car(color,manufacturer): if color in manufacturer.allowed_colors: return true return false if validate_car(color,manufacturer): car = Car(color,manufacturer) session.add(car) </code></pre> <p>If it were me, this validation would live on the outskirts of my application. Assuming a Web app, that would mean it would be validated on the form level-- only relevant car colours would be shown to the user, and then the server would validate the chosen colour to make sure no users are squeezing in bad data. So I would have a function to <code>validate_car(color,manufacturer)</code> but it would be called when the form was being processedm if the form checked out, then I'd create and add the Car object.</p> <p>Having the constraint within the dbms itself would be fun for cascading deletions, e.g. if Ford decided to stop selling Blue cars, then you could just delete Blue and all Blue Ford cars would magically disappear, again though, I think if you were to go the constraint route, it would almost be manual SQL to hook it all up (though I would enjoy to learn if it isn't!).</p>

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