Could anyone explain the difference between filter and filter_by functions in SQLAlchemy?
Which one should I be using?
Answers:
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Method 1
filter_by is used for simple queries on the column names using regular kwargs, like
db.users.filter_by(name='Joe')
The same can be accomplished with filter, not using kwargs, but instead using the ‘==’ equality operator, which has been overloaded on the db.users.name object:
db.users.filter(db.users.name=='Joe')
You can also write more powerful queries using filter, such as expressions like:
db.users.filter(or_(db.users.name=='Ryan', db.users.country=='England'))
Method 2
We actually had these merged together originally, i.e. there was a “filter”-like method that accepted *args and **kwargs, where you could pass a SQL expression or keyword arguments (or both). I actually find that a lot more convenient, but people were always confused by it, since they’re usually still getting over the difference between column == expression and keyword = expression. So we split them up.
Method 3
It is a syntax sugar for faster query writing. Its implementation in pseudocode:
def filter_by(self, **kwargs):
return self.filter(sql.and_(**kwargs))
For AND you can simply write:
session.query(db.users).filter_by(name='Joe', surname='Dodson')
btw
session.query(db.users).filter(or_(db.users.name=='Ryan', db.users.country=='England'))
can be written as
session.query(db.users).filter((db.users.name=='Ryan') | (db.users.country=='England'))
Also you can get object directly by PK via get method:
Users.query.get(123) # And even by a composite PK Users.query.get(123, 321)
When using get case its important that object can be returned without database request from identity map which can be used as cache(associated with transaction)
Method 4
filter_by uses keyword arguments, whereas filter allows pythonic filtering arguments like filter(User.name=="john")
Method 5
Apart from all the technical information posted before, there is a significant difference between filter() and filter_by() in its usability.
The second one, filter_by(), may be used only for filtering by something specifically stated – a string or some number value. So it’s usable only for category filtering, not for expression filtering.
On the other hand filter() allows using comparison expressions (==, <, >, etc.) so it’s helpful e.g. when ‘less/more than’ filtering is needed. But can be used like filter_by() as well (when == used).
Just to remember both functions have different syntax for argument typing.
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