SQLAlchemy allow null as ForeignKey

Question

Let's say I have two models

class EntityModel(DB.Model):     id = DB.Column(DB.Unicode(37), primary_key=True)  class DocumentModel(DB.Model):     id = DB.Column(DB.Unicode(37), primary_key=True)     entity_id = DB.Column(DB.Unicode(37), DB.ForeignKey('entity.id',   ondelete='cascade'), nullable=True)     entity = DB.relationship('EntityModel') 

I can create new Document with entity_id NULL. But once I set valid entity_id to that document I can no more null it back. I get error:

Cannot add or update a child row: a foreign key constraint fails

How can I set null to entity_id in some document, if it has valid entity_id?

Answer 1

your ORM definition looks fine, DocumentModel.entity_id is nullable indeed. What I would do in this case is check the actual table definition in MySQL and make sure it corresponds with your ORM definition, e.g.

-- make sure DocumentModel.entity_id is nullable : SHOW CREATE TABLE DocumentModel\G  UPDATE DocumentModel.entity_id SET entity_id = NULL WHERE entity_id = XXX;