Hello,
This mail is the continuation of this question:
https://groups.google.com/forum/#!topic/sqlalchemy/Ba7urbeZBqQ
The problem being different, I prefer to open a new topic.
I am currently meeting a problem regarding generic associations and the use
of MutableList associated to the array type of postgres.
I coded two versions of the same case:
- the first one uses a generic association. The attribute "text" should be
a MutableList but suddenly becomes a "List" during the first commit. The
next changes are therefore not longer detected and the data are not
actualized in the database
- the second example is a classical relationship. In this case, the
attribute "text" is a MutableList and don't changes during the commits.
Everything works as it should and the data are actualized in the database
It will be easier to see the problem with the two examples and their output.
The examples are attached.
*Example 1 - Generic Association Code (don't works as it should):*
Output:
>>>
Text before commit 1: ['Rex is a', 'good dog...']
Type before commit 1: <class 'sqlalchemy.ext.mutable.MutableList'>
Commit 1
Type after commit 1: <class 'list'>
We append duck sentence : ['Rex is a', 'good dog...', 'but I prefer ducks.'
]
Commit 2
Text after commit 2 : ['Rex is a', 'good dog...']
>>>
*Example 2 - Classical relationship (works fine):*
Output:
>>>
Text before commit 1: ['Rex is a', 'good dog...']
Type before commit 1: <class 'sqlalchemy.ext.mutable.MutableList'>
Commit 1
Type after commit 1: <class 'sqlalchemy.ext.mutable.MutableList'>
We append duck sentence : ['Rex is a', 'good dog...', 'but I prefer ducks.'
]
Commit 2
Text after commit 2 : ['Rex is a', 'good dog...', 'but I prefer ducks.']
>>>
I don't know if it helps but I noticed that the __init__ method of the
class Description doesn't have exactly the same behavior in the two
examples:
- in the Generic Association Code example, the "self.text = MutableList()"
in the __init__ method of the class Description is not really important. It
could also be "self.text = []" instead. In all cases, text is a MutableList.
- in the Classical relationship example, if the __init__ method of the
class Description contains "self.text = MutableList()", the attribute text
will be a MutableList and if the __init__ method contains "self.text = []",
it will be a List.
Does someone have an explanation ?
Thank you !
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import as_declarative, declared_attr
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.mutable import MutableList
@as_declarative()
class Base(object):
"""Base class which provides automated table name
and surrogate primary key column.
"""
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
id = Column(Integer, primary_key=True)
class Description(Base):
text = Column(MutableList.as_mutable(postgresql.ARRAY(String)), nullable = False)
parent_id = Column(Integer, ForeignKey('animal.id'))
parent = relationship("Animal", back_populates="description")
def __init__(self, name):
self.text = MutableList()
self.text.append(name)
def __repr__(self):
return self.text
class Animal(Base):
name = Column(String)
description = relationship("Description", uselist=False, back_populates="parent")
def __init__(self, name):
self.name = name
engine = create_engine('postgresql://LOGIN:PASSWORD@localhost/DATABASE_NAME')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
rex = Animal("Rex")
d1 = Description("Rex is a")
session.add(rex)
session.add(d1)
rex.description = d1
rex.description.text.append("good dog...")
print("Text before commit 1: ", rex.description.text)
print("Type before commit 1: ", type(rex.description.text))
print("Commit 1")
session.commit()
print("Type after commit 1: ", type(rex.description.text))
print("We append duck sentence : ", rex.description.text)
rex.description.text.append("but I prefer ducks.")
print("Commit 2")
session.commit()
print("Text after commit 2 : ", rex.description.text)
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import as_declarative, declared_attr
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.mutable import MutableList
@as_declarative()
class Base(object):
"""Base class which provides automated table name
and surrogate primary key column.
"""
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
id = Column(Integer, primary_key=True)
class Description():
text = Column(MutableList.as_mutable(postgresql.ARRAY(String)), nullable = False)
def __init__(self, text):
self.text = MutableList()
self.text.append(text)
def __repr__(self):
return self.text
class HasDescription(object):
@declared_attr
def description(cls):
cls.Description = type(
"%sDescription" % cls.__name__,
(Description, Base,),
dict(
__tablename__="%s_description" %
cls.__tablename__,
parent_id=Column(Integer,
ForeignKey("%s.id" % cls.__tablename__)),
parent=relationship(cls, back_populates="description")
)
)
return relationship(cls.Description, uselist=False, back_populates="parent")
class Animal(HasDescription, Base):
name = Column(String)
def __init__(self, name):
self.name = name
engine = create_engine('postgresql://LOGIN:PASSWORD@localhost/DATABASE_NAME')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
rex = Animal("Rex")
d1 = Animal.Description("Rex is a")
session.add(rex)
session.add(d1)
rex.description = d1
rex.description.text.append("good dog...")
print("Text before commit 1: ", rex.description.text)
print("Type before commit 1: ", type(rex.description.text))
print("Commit 1")
session.commit()
print("Type after commit 1: ", type(rex.description.text))
print("We append duck sentence : ", rex.description.text)
rex.description.text.append("but I prefer ducks.")
print("Commit 2")
session.commit()
print("Text after commit 2 : ", rex.description.text)
