Thanks Simon it's work !!!
2014-12-11 16:06 GMT+01:00 Simon King <si...@simonking.org.uk>:
> On Thu, Dec 11, 2014 at 2:44 PM, Boris Sabatier
> <sabatier.bo...@gmail.com> wrote:
>> Hi,
>>
>> I have a problem to do the model I need.
>> I have 3 types of object : Category, Picture and Video.
>>
>> All of these types have some common part, like a name.
>> Also I want to be able to select all object with a name like "bar"
>> (Category, Picture and Video)
>> So I use the Joined_table inheritance pattern. All work fine.
>>
>> But I want to add : "they all can be child of a Category"
>> So I done this model :
>>
>> class Item(Base):
>> __tablename__ = 'item'
>> uid = Column(Integer, primary_key=True)
>> name = Column(String(50))
>> item_type = Column(String(50))
>> parent_id = Column(Integer, ForeignKey('category.uid'), nullable=True)
>> __mapper_args__ = {
>> 'polymorphic_identity':'item',
>> 'polymorphic_on':item_type,
>> }
>> ###############################################################
>> class Picture(Item):
>> __tablename__ = 'picture'
>> uid = Column(Integer, ForeignKey('item.uid'), primary_key=True)
>> __mapper_args__ = {
>> 'polymorphic_identity':'picture',
>> }
>> ###############################################################
>> class Video(Item):
>> __tablename__ = 'picture'
>> uid = Column(Integer, ForeignKey('item.uid'), primary_key=True)
>> __mapper_args__ = {
>> 'polymorphic_identity':'video',
>> }
>> ###############################################################
>> class Category(Item):
>> __tablename__ = 'category'
>> uid = Column(Integer, ForeignKey('item.uid'), primary_key=True)
>> __mapper_args__ = {
>> 'inherit_condition': (uid==Item.uid),
>> 'polymorphic_identity': 'category'
>> }
>>
>> First I add parent_id Column in the Item class. But when I do this, I have
>> AmbiguousForeignKeysError exception:
>> sqlalchemy.exc.AmbiguousForeignKeysError: Can't determine join between
>> 'item' and 'category'; tables have more than one foreign key constraint
>> relationship between them. Please specify the 'onclause' of this join
>> explicitly.
>>
>> After some research on internet (documentation, stack overflow and this
>> maling list) I heard about the 'inherit_condition'.
>> So I add it in the __mapper_args__ of Category. But when I do that, I have a
>> CircularDependencyError:
>> sqlalchemy.exc.CircularDependencyError: Circular dependency detected.
>
> The problem is that you've got a foreign key from "category.uid" to
> "item.uid", and also a foreign key from "item.parent_id" to
> "category.uid". Normally, foreign keys are defined in the "CREATE
> TABLE" sql statement, but that can't work when you have a circular
> dependency, because whichever table you define first will try to refer
> to another table which doesn't exist yet.
>
> The solution is "use_alter", which is a flag you can pass when
> defining your ForeignKey:
>
> http://docs.sqlalchemy.org/en/rel_0_9/core/constraints.html#sqlalchemy.schema.ForeignKeyConstraint.params.use_alter
>
> When it is set, SQLAlchemy will create the table *without* the foreign
> key, then when all the tables have been defined it will run an "ALTER
> TABLE" statement to create the foreign key.
>
> Hope that helps,
>
> Simon
>
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