SQLAlchemy.declarative and deferred column loading

Question

Hi everybody, is it possible to specify some columns in the SQLAlchemy to be deferred-loading? I am using the sqlalchemy.ext.declarative module to define my mapping, example:

from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class SomeClass(Base):
    __tablename__ = 'some_table'
    id = Column(Integer, primary_key=True)
    name =  Column(String(50))

I want for example the column name be lazy loaded, how can I accomplish that?

Thank you Jan

Answer 1

Do not define the mapping for the columns which you want to load on demand. Then configure those as described in the Deferred Column Loading using mapper object. Modified code here:

from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class SomeClass(Base):
    __tablename__ = 'some_table'
    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    #big_name = Column(String(500))

SomeClass.__table__.append_column(Column('big_name', String(500)))
SomeClass.__mapper__.add_property('big_name', deferred(SomeClass.__table__.c.big_name))

Running this test code:

c = session.query(SomeClass).first()
# here SQL is loading all configured properties, but big_name
print "c: ", c
# only here another SQL request is made to load the property
print "big_name: ", c.big_name

produces log extract:

... INFO sqlalchemy.engine.base.Engine.0x...77d0 SELECT some_table.id AS some_table_id, some_table.name AS some_table_name 
FROM some_table 
LIMIT 1 OFFSET 0

... INFO sqlalchemy.engine.base.Engine.0x...77d0 SELECT some_table.big_name AS some_table_big_name 
FROM some_table 
WHERE some_table.id = ?

Answer 2

Just add deferred() around the column declaration:

class SomeClass(Base):
    __tablename__ = 'some_table'
    id = Column(Integer, primary_key=True)
    name =  deferred(Column(String(50)))