Question

I am querying a proprietary database which is maintained by a third party. The database has many tables each with large numbers of fields.

My problem refers to three tables of interest, Tree, Site and Meter.

The tree table describes nodes in a simple tree structure. Along with other data it has a foreign key referencing its own primary key. It also has an Object_Type field and an Object_ID field. The Site and Meter tables each have many fields.

A tree node has a one-to-one relationship with either be a meter or a site. If the Object_Type field is 1 then the Object_ID field refers to the primary key in the Site table. If it is 2 then it refers to the primary key in the Meter table.

following this example https://bitbucket.org/sqlalchemy/sqlalchemy/src/408388e5faf4/examples/declarative_reflection/declarative_reflection.py

I am using reflection to load the table structures like so

Base = declarative_base(cls=DeclarativeReflectedBase)

class Meter(Base):
    __tablename__ = 'Meter'

class Site(Base):
    __tablename__ = 'Site'

class Tree(Base):
    __tablename__ = 'Tree'
    Parent_Node_ID = Column(Integer, ForeignKey('Tree.Node_ID'))
    Node_ID = Column(Integer, primary_key=True)
    children = relationship("Tree", backref=backref('parent', remote_side=[Node_ID]))

Base.prepare(engine)

I have included the self-referential relationship and that works perfectly. How can I add the two relationships using Object_ID as the foreign key, with the appropriate check on the Object_Type field?

Answer 1

for tree.object_id to be a foreign key that can refer either to Site or Meter, you can either have Site and Meter descend from a common base table, that is, joined table inheritance, or be mapped to the same table, that is, single table inheritance, or as someone said have Tree be mapped to two different tables as well as a common base table. This last suggestion goes well with the idea that TreeNode already has a “type” field.

The final alternative which might be easier is to use two foreign keys on TreeNode directly – site_id and meter_id, as well as two relationships, “meter” and “site”; then use a Python @property to return one or the other:

class TreeNode(Base):
   # ...

   @property
   def object(self):
       return self.meter or self.site