Difference between filter and filter_by in SQLAlchemy
Question:
Could anyone explain the difference between filter and filter_by functions in SQLAlchemy?
Which one should I be using?
Answers:
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'))
filter_by uses keyword arguments, whereas filter allows pythonic filtering arguments like filter(User.name=="john")
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.
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)
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.
Could anyone explain the difference between filter and filter_by functions in SQLAlchemy?
Which one should I be using?
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'))
filter_by uses keyword arguments, whereas filter allows pythonic filtering arguments like filter(User.name=="john")
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.
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)
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.