syntax to query another table using relationship in ORM? - database

Question

The model is like this(in SQLAlchemy):
Class Cell(Base):
__tablename__ = "cell"
id = Column(Integer)
name = Column(String)
Class Sample(Base):
__tablename__ = "cell"
id = Column(Integer)
factor_id = Column(Integer, ForeignKey("cell.id"))
cell = relationship(Cell, backref = 'sample', order_by = "Cell.id")
When I execute the query like this:
DBSession.query(Sample).filter(Sample.cell.name == "a_string")
It throws an exception like this:
File "build/bdist.linux-x86_64/egg/sqlalchemy/orm/attributes.py", line 139, in __getattr__
key)
AttributeError: Neither 'InstrumentedAttribute' object nor 'Comparator' object has an attribute 'name'
It seems that the cell field in Sample class doesn't have a field called name. Then how can I query Cell.name in Sample class with cell field? Does anyone have ideas about this?
Thanks!

Answer 1

There are various ways to achieve that:
1. use join(...) - I would opt for this one in your case
qry = session.query(Sample).join(Cell).filter(Cell.name == "a_string")
>> SELECT sample.id AS sample_id, sample.factor_id AS sample_factor_id
>> FROM sample JOIN cell ON cell.id = sample.factor_id
>> WHERE cell.name = :name_1
2. use any/has(...) - this will use a sub-query
qry = session.query(Sample).filter(Sample.cell.has(Cell.name == "a_string"))
>> SELECT sample.id AS sample_id, sample.factor_id AS sample_factor_id
>> FROM sample
>> WHERE EXISTS (SELECT 1
>> FROM cell
>> WHERE cell.id = sample.factor_id AND cell.name = :name_1)