Hello,
I have two tables in PostgreSQL where one is INHERITed from the other:
CREATE TABLE a
(
pk serial,
type_pk integer,
value numeric,
CONSTRAINT a_pkey PRIMARY KEY (pk),
CONSTRAINT a_type_fk FOREIGN KEY (a_type_pk)
REFERENCES a_type (pk) MATCH SIMPLE
ON UPDATE CASCADE
ON DELETE RESTRICT
);
CREATE TABLE type
(
pk serial,
label text
);
CREATE TABLE b
(
pk ,
type_pk ,
value ,
b_value numeric
)
INHERITS (a);
I have my SQLAlchemy classes defined like this:
class Type(Base):
__tablename__ = 'a_type'
__table_args__ = {'autoload':True, 'schema':'public'}
class A(Base):
__tablename__ = 'a'
__table_args__ = {'autoload':True, 'schema':'public'}
type_pk = Column("type_pk", Integer, ForeignKey(Type.pk))
type = relationship(Type,
primaryjoin=Type.pk==type_pk,
backref="as")
__mapper_args__ = {
'polymorphic_identity':"a",
'polymorphic_on':case([
(type_pk == 1, "b")
], else_="a")
}
class B(A):
__tablename__ = 'b'
__table_args__ = {'autoload':True, 'schema':'public'}
pk = Column(name='pk', type_=Integer, primary_key=True, autoincrement=
True)
__mapper_args__ = {
'polymorphic_identity': "b",
'inherit_condition':pk==A.pk,
'inherit_foreign_keys':['type_pk']
}
This very nearly works and I get the polymorphism I am looking for. When I
create a new B() object in code and look at the SQL generated, it triggers
two inserts - one for the 'a' table, and one for the 'b' table. How can I
suppress the INSERT into 'a'? With PostgreSQL's INHERIT, I only need the
'b' INSERT. This works very well otherwise.
I get a few of these warnings. Is there a configuration I need to specify
to tell SQLAlchemy not to worry about it?
/usr/local/anaconda/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py:1766:
> SAWarning: Implicitly combining column a.pk with column b.pk under
> attribute 'pk'. Please configure one or more attributes for these
> same-named columns explicitly.
Finally, is there a way I can automatically set a default value of 1 for
the 'type_pk' propoerty when a new B() is created? I don't know if I can do
this in PostgreSQL since the field is inherited.
Thanks!
Demitri
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The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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