does anybody know what is the equivalent to SQL "INSERT OR REPLACE" clause in SQLAlchemy and its SQL expression language?
Many thanks -- honzas
What about Session.merge?
Session.merge(instance, load=True, **kw)
Copy the state an instance onto the persistent instance with the same identifier.
If there is no persistent instance currently associated with the session, it will be loaded. Return the persistent instance. If the given instance is unsaved, save a copy of and return it as a newly persistent instance. The given instance does not become associated with the session. This operation cascades to associated instances if the association is mapped with cascade="merge".
from http://www.sqlalchemy.org/docs/reference/orm/sessions.html
Session.save_or_update(model)
I don't think (correct me if I'm wrong) INSERT OR REPLACE is in any of the SQL standards; it's an SQLite-specific thing. There is MERGE, but that isn't supported by all dialects either. So it's not available in SQLAlchemy's general dialect.
The cleanest solution is to use Session, as suggested by M. Utku. You could also use SAVEPOINTs to save, try: an insert, except IntegrityError: then rollback and do an update instead. A third solution is to write your INSERT with an OUTER JOIN and a WHERE clause that filters on the rows with nulls.
You can use OR REPLACE as a so-called prefix in your SQLAlchemy Insert -- the documentation for how to place OR REPLACE between INSERT and INTO in your SQL statement is here
Related
I'm using Python to talk to a Postgres DBMS using psycopg2.
Is it safe to assume that if an INSERT returns without raising an exception, then that INSERT actually did store something new in the database?
Right now, I've been checking the 'rowcount' attribute of the database cursor, and if it's 0 then that means the INSERT failed. However, I'm starting to think that this isn't necessary.
Is it safe to assume that if an INSERT returns without raising an
exception, then that INSERT actually did store something new in the
database?
No.
The affected record count will be zero if:
You ran an INSERT INTO ... SELECT ..., and the query returned no rows
You ran an INSERT INTO ... ON CONFLICT DO NOTHING, and it encountered a conflict
You have a BEFORE INSERT trigger on your table, and the trigger function returned NULL
You have a rule defined which results in no records being affected (e.g. ... DO INSTEAD NOTHING)
(... and possibly more, though nothing comes to mind.)
The common thread is that it will only affect zero records if you told it to, one way or another. Whether you want to treat any of these as a "failure" is highly dependent on your application logic.
Anything which is unequivocally a "failure" (constraint violation, serialisation failure, out of disk space...) should throw an error, so checking the record count is generally unnecessary.
By default postgres will return None for a successful insert:
cursor.execute - The method returns None. If a query was executed, the returned values can be retrieved using fetch*() methods.
http://initd.org/psycopg/docs/cursor.html
If you want to know something about the insert, an easy/efficient option is to use RETURNING (which takes the same options as a SELECT):
INSERT INTO ... RETURNING id
found similar question here, How to check if value is inserted successfully or not?
they seem to use the row count method to check if the data was inserted correctly.
I am new to sqlalachemy in python and i have a question.
I need to port this query:
UPDATE user_list SET last_visit=now() WHERE name='"+username+"'
I have read a lot about the update query, but I see possibility to set the value of the last_visit to the value generated by the now() function of postgresql. How would I do that properly? Assume that I would use the update functions of sqlalachemty. I could generate this as sql string and execute() it, but then the statement could be subject of sql injection. How to do this right?
You can use func.now() (import func from sqlalchemy).
Reference: http://docs.sqlalchemy.org/en/latest/core/functions.html
I'm inserting/updating objects into a MySQL database using the peewee ORM for Python. I have a model like this:
class Person(Model):
person_id = CharField(primary_key=True)
name = CharField()
I create the objects/rows with a loop, and each time through the loop have a dictionary like:
pd = {"name":"Alice","person_id":"A123456"}
Then I try creating an object and saving it.
po = Person()
for key,value in pd.items():
setattr(po,key,value)
po.save()
This takes a while to execute, and runs without errors, but it doesn't save anything to the database -- no records are created.
This works:
Person.create(**pd)
But also throws an error (and terminates the script) when the primary key already exists. From reading the manual, I thought save() was the function I needed -- that peewee would perform the update or insert as required.
Not sure what I need to do here -- try getting each record first? Catch errors and try updating a record if it can't be created? I'm new to peewee, and would normally just write INSERT ... ON DUPLICATE KEY UPDATE or even REPLACE.
Person.save(force_insert=True)
It's documented: http://docs.peewee-orm.com/en/latest/peewee/models.html#non-integer-primary-keys-composite-keys-and-other-tricks
I've had a chance to re-test my answer, and I think it should be replaced. Here's the pattern I can now recommend; first, use get_or_create() on the model, which will create the database row if it doesn't exist. Then, if it is not created (object is retrieved from db instead), set all the attributes from the data dictionary and save the object.
po, created = Person.get_or_create(person_id=pd["person_id"],defaults=pd)
if created is False:
for key in pd:
setattr(fa,key,pd[key])
po.save()
As before, I should mention that these are two distinct transactions, so this should not be used with multi-user databases requiring a true upsert in one transaction.
I think you might try get_or_create()? http://peewee.readthedocs.org/en/latest/peewee/querying.html#get-or-create
You may do something like:
po = Person()
for key,value in pd.items():
setattr(po,key,value)
updated = po.save()
if not updated:
po.save(force_insert=True)
I have probably not grasped the use of #hybrid_property fully. But what I try to do is to make it easy to access a calculated value based on a column in another table and thus a join is required.
So what I have is something like this (which works but is awkward and feels wrong):
class Item():
:
#hybrid_property
def days_ago(self):
# Can I even write a python version of this ?
pass
#days_ago.expression
def days_ago(cls):
return func.datediff(func.NOW(), func.MAX(Event.date_started))
This requires me to add the join on the Action table by the caller when I need to use the days_ago property. Is the hybrid_property even the correct approach to simplifying my queries where I need to get hold of the days_ago value ?
One way or another you need to load or access Action rows either via join or via lazy load (note here it's not clear what Event vs. Action is, I'm assuming you have just Item.actions -> Action).
The non-"expression" version of days_ago intends to function against Action objects that are relevant only to the current instance. Normally within a hybrid, this means just iterating through Item.actions and performing the operation in Python against loaded Action objects. Though in this case you're looking for a simple aggregate you could instead opt to run a query, but again it would be local to self so this is like object_session(self).query(func.datediff(...)).select_from(Action).with_parent(self).scalar().
The expression version of the hybrid when formed against another table typically requires that the query in which it is used already have the correct FROM clauses set up, so it would look like session.query(Item).join(Item.actions).filter(Item.days_ago == xyz). This is explained at Join-Dependent Relationship Hybrid.
your expression here might be better produced as a column_property, if you can afford using a correlated subquery. See that at http://docs.sqlalchemy.org/en/latest/orm/mapping_columns.html#using-column-property-for-column-level-options.
I store the MySQL Compress function to insert compressed blob data to the database.
In a previous question I was instructed to to use
func.compress
( mysql Compress() with sqlalchemy )
The problem now is that I want to read also the data from the database.
In mysql I would have done
SELECT UNCOMPRESS(text) FROM ...
probably I should use a getter in the class.
I tried to do somethin like:
get_html(self):
return func.uncompress(self.text)
but this does not work. It returns an sqlalchemy.sql.expression.Function and not the string.
Moreover I could not find which functions contains sqlalchemy's func.
Any ideas on how i could write a getter in the object so I get back the uncompressed data.
func is actually a really fancy factory object for special function objects which are rendered to SQL at query time - you cannot evaluate them in Python since Python would not know how your database implements compress(). That's why it doesn't work.
SQLAlchemy lets you map SQL expressions to mapped class attributes. If you're using the declarative syntax, extend your class like so (it's untested, but I'm confident this is the way to go):
from sqlalchemy.orm import column_property
class Demo(...):
data_uncompressed = column_property(func.uncompress(data))
Now whenever SQLAlchemy loads an instance from the database, the SELECT query will contain SELECT ..., UNCOMPRESS(demotable.data), ... FROM demotable.
Edit by Giorgos Komninos:
I used the
http://docs.sqlalchemy.org/en/rel_0_7/orm/mapper_config.html#using-a-plain-descriptor
and it worked.