I'm using Peewee with Postgres database. I want to know how to update multiple records in a tabel at once?
We can perform this update in SQL using these commands, and I'm looking for a Peewee equivalent approach.
Yes, you can use the insert_many() function:
Insert multiple rows at once. The rows parameter must be an iterable
that yields dictionaries. As with insert(), fields that are not
specified in the dictionary will use their default value, if one
exists.
Example:
usernames = ['charlie', 'huey', 'peewee', 'mickey']
row_dicts = ({'username': username} for username in usernames)
# Insert 4 new rows.
User.insert_many(row_dicts).execute()
More details at: http://docs.peewee-orm.com/en/latest/peewee/api.html#Model.insert_many
ORMs usually dose not support bulk update and you have to use custom SQL, you can see samples in this link (db.excute_sql)
Related
Is there a way to do the following in django's ORM?
INSERT INTO mytable
VALUES (1,2,3)
ON DUPLICATE KEY
UPDATE field=4
I'm familiar with get_or_create, which takes default values, but that doesn't update the record if there are differences in the defaults. Usually I use the following approach, but it takes two queries instead of one:
item = Item(id=1)
item.update(**fields)
item.save()
Is there another way to do this?
I'm familiar with get_or_create, which takes default values, but that doesn't update the record if there are differences in the defaults.
update_or_create should provide the behavior you're looking for.
Item.objects.update_or_create(
id=1,
defaults=fields,
)
It returns the same (object, created) tuple as get_or_create.
Note that this will still perform two queries, but only in the event the record does not already exist (as is the case with get_or_create). If that is for some reason unacceptable, you will likely be stuck writing raw SQL to handle this, which would be unfortunate in terms of readability and maintainability.
I think get_or_create() is still the answer, but only specify the pk field(s).
item, _ = Item.objects.get_or_create(id=1)
item.update(**fields)
item.save()
Django 4.1 has added the support for INSERT...ON DUPLICATE KEY UPDATE query. It will update the fields in case the unique validation fails.
Example of above in a single query:
# Let's say we have an Item model with unique on key
items = [
Item(key='foobar', value=10),
Item(key='foobaz', value=20),
]
# this function will create 2 rows in a single SQL query
Item.objects.bulk_create(items)
# this time it will update the value for foobar
# and create new row for barbaz
# all in a single SQL query
items = [
Item(key='foobar', value=30),
Item(key='barbaz', value=50),
]
Item.objects.bulk_create(
items,
update_conflicts=True,
update_fields=['rate']
)
I have the following dict:
base = {}
base['id1'] = {'apple':2, 'banana':4,'coconut':1}
base['id2'] = {'apple':4, 'pear':8}
base['id3'] = {'banana':1, 'tomato':2}
....
base['idN'] = {'pineapple':1}
I want to create a SQL database to store it. I normally use sqlite but here the number of variables (features in the dict) is not the same for all ids and I do not know all of them thus I cannot use the standard procedure.
Does someone know an easy way to do it ?
ids will get duplicated if you use the sql i would suggest use postgres as it has a jsonfield ypu can put your data there corresponding to each key. Assuming you are not constrained to use SQL.
I have the query:
q = Session.query(func.array_agg(Order.col))
The compiled query will be:
SELECT array_agg(order.col) FROM orders
I want dynamically replace the existing column. After replacing query have to be:
SELECT group_concat(orders.col) FROM orders
I have to use Session and model. I don't have to use SQLAlchemy core. I don't have to use subqueries. And, of course, there can be some other columns, but I need to replace only one. I tried to replace objects in column_descriptions property, I tried to use q.selectable.replace (or something like this, sorry, but I don't remember right names) and I didn't get right result.
The right method:
q = Session.query(func.array_agg(Order.col))
q.with_entities(func.group_concat(Order.col))
SELECT group_concat(orders.col) FROM orders
Hello StackEx community.
I am implementing a relational database using SQLite interfaced with Python. My table consists of 5 attributes with around a million tuples.
To avoid large number of database queries, I wish to execute a single query that updates 2 attributes of multiple tuples. These updated values depend on the tuples' Primary Key value and so, are different for each tuple.
I am trying something like the following in Python 2.7:
stmt= 'UPDATE Users SET Userid (?,?), Neighbours (?,?) WHERE Username IN (?,?)'
cursor.execute(stmt, [(_id1, _Ngbr1, _name1), (_id2, _Ngbr2, _name2)])
In other words, I am trying to update the rows that have Primary Keys _name1 and _name2 by substituting the Neighbours and Userid columns with corresponding values. The execution of the two statements returns the following error:
OperationalError: near "(": syntax error
I am reluctant to use executemany() because I want to reduce the number of trips across the database.
I am struggling with this issue for a couple of hours now but couldn't figure out either the error or an alternate on the web. Please help.
Thanks in advance.
If the column that is used to look up the row to update is properly indexed, then executing multiple UPDATE statements would be likely to be more efficient than a single statement, because in the latter case the database would probably need to scan all rows.
Anyway, if you really want to do this, you can use CASE expressions (and explicitly numbered parameters, to avoid duplicates):
UPDATE Users
SET Userid = CASE Username
WHEN ?5 THEN ?1
WHEN ?6 THEN ?2
END,
Neighbours = CASE Username
WHEN ?5 THEN ?3
WHEN ?6 THEN ?4
END,
WHERE Username IN (?5, ?6);
There exists a table Users and in my code I have a big list of User objects. To insert them I can use :
session.add_all(user_list)
session.commit()
The problem is that there can be several duplicates which I want to update but the database wont allow to insert duplicate entries. For sure, I can iterate over user_list and try to insert user in the database and if it fails - update it :
for u in users:
q = session.query(T).filter(T.fullname==u.fullname).first()
if q:
session.query(T).filter_by(index=q.index).update({column: getattr(u,column) for column in Users.__table__.columns.keys() if column!='id'})
session.commit()
else:
session.add(u)
session.commit()
but I find this solution quiet ineffective : first, I am making several requests to retrieve object q, and instead of batch inserting of new items I insert them one per one. I wonder if there exists a better solution for this task.
UPD better version:
for u in users:
q = session.query(T).filter(Users.fullname==u.fullname).first()
if q:
for column in Users.__table__.columns.keys():
if not column=='index':
setattr(q,column,getattr(u,column))
session.add(q)
else:
session.add(u)
session.commit()
a better solution would be to use
INSERT ... ON DUPLICATE KEY UPDATE ...
bulk MySQL construct (I assume you're using MySQL because your post is tagged with 'mysql'). This way you're both inserting new entries and updating existing ones in one statement / transaction, see http://dev.mysql.com/doc/refman/5.6/en/insert-on-duplicate.html
It's not ideal if you have multiple unique indexes and, depending on your schema, you'll have to fill in all NOT NULL values (hence issuing one bulk SELECT before calling it), but it's definitely the most efficient option and we use it a lot. The bulk version will look something like (let's assume name is a unique key):
INSERT INTO User (name, phone, ...) VALUES
('ksmith', '111-11-11', ...),
('jford', '222-22,22', ...),
...,
ON DUPLICATE KEY UPDATE
phone = VALUES(phone),
... ;
Unfortunately, INSERT ... ON DUPLICATE KEY UPDATE ... is not supported natively by SQLa so you'll have to implement a little helper function which will build the query for you.