SQLAlchemy: Populate instance attributes from Database query

Question

I'm using SQLAlchemy with the below User model to track users on my site. I'm trying to get the __init__ method to populate all of the fields in the class instance once it's passed a valid user id, but it doesn't work like it's supposed to.

Here's my code:

class User(db.Model):
    # Fields
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(32))
    email = db.Column(db.String(255), index=True)

    # Methods
    def __init__(self, id):
        # Populate the fields
        u = User.query.filter_by(id=id).first()

        # Fill in the name, email fields from the result
        self = u # <-- Not working as expected
    # ...

The problem is that the attributes are not getting populated.

u = User(1)
u.id, u.name, u.email # <-- Output: None

How can I auto-populate all the class attributes from the user id without having to set them one by one?

Note: I've removed the validation bits from the above code to avoid clutter Note2: I'm a beginner in SQLAlchemy, and just recently started using Python.

Normally in SQLAlchemy, the constructor is used to create a new object, for example to insert into the database, not to load one from the database. Do you have a particular need to do things differently? If you're just looking for a convenient place to define a lookup function, see @Oleh's answer. — Dan Getz
– Dan Getz, Commented Jun 22, 2014 at 15:36
Maybe I should go further and say that in Python in general, a constructor is used for creating a new instance, not for looking up an existing one. Why use a constructor for looking up when you can write functions that are not methods on an already existing instance? (Normal functions, classmethods, staticmethods) — Dan Getz
– Dan Getz, Commented Jun 22, 2014 at 15:52
@DanGetz thanks Dan! So I just need to remove it from the constructor, and move it to a "normal" function. That makes perfect sense — FloatingRock
– FloatingRock, Commented Jun 22, 2014 at 16:06

Oleh Prypin · Accepted Answer · 2014-06-22 13:37:02Z

1

For this particular case you can use get which gets a row by its primary key:

u = User.get(1)
u.id, u.name, u.email

The problem with your code is that you're trying to use a bound method, even worse, the constructor (in which assigning to self does nothing at all) to get an existing, completely different instance.

I'll just show a reimplementation of the get method so you can reuse it for other cases.

class User(db.Model):
    # ...

    @classmethod
    def get_by_id(cls, id):
        return cls.query.filter_by(id=id).one()


u = User.get_by_id(1)

answered Jun 22, 2014 at 13:37

Oleh Prypin

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2 Comments

FloatingRock Over a year ago

Thank Oleh. I'm still unclear on how the model gets updated from within the same class?

Oleh Prypin Over a year ago

Nothing gets updated, thats the whole point. Read on classmethods.

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