I need to import some sqlite databases to a postgresql data base unfortunately in some of the sqlite databases are some columns missing.
I'd like to use the same model for all the databases and if one column is missing, it should just initialize the variable corresponding to it to None.
I'd imagine the syntax would be like this:
import sqlalchemy as sql
import sqlalchemy.orm
TableBase = sql.orm.declarative_base()
class ExampleTbl(TableBase):
__tablename__ = "example"
# ... other columns
optional = sql.Column(sql.Text, if_not_exsits=None) # optional should be None if the column does not exists
# ... other columns
It probably isn't as easy as setting a flag but how could I achieve this?
EDIT:
I can actually find out if the column is in the table before querying it. So if sqlalchemy doesn't support it out of the box I could probably write a function that generates the required table that would match the one in the database but I would prefer something easier.
EDIT 2:
I'm going to do the approach that I suggested in the first edit, I've run in some problems so I'm going make a new question.
Related
Is there a way to bulk-insert/update values into a Microsoft SQLserver Database using Engine?
I have read several (very) old posts regarding this, and it seems not very easy to do (back then).
E.g in some examples we need to create a class, add those classes to a session and at last commit the session.
Isn't there a way like (pseudo) this:
from sqlalchemy import String, Integer, Float
values= [(1,"hello",2.5),(2,"world",10.5)] #values to insert
table = "my_schema.my_table" #Table name
col = ["id","statement","ratio"] #Name of the columns in the database
type = [Integer,String,Float] #Type of each value
engine = sqlalchemy.create_engine(connection_string)
with engine.session():
try:
engine.bulk_insert(table,values,col,type)
except:
engine.rollback()
or something else, instead of looping over engine.execute("INSERT INTO ...")?
I know I can use pandas.DataFrame.to_sql but since I want to be able to roll-back in case of errors etc. I won't use that
I am working on a open source persistance layer for a MQTT-Broker https://github.com/volkerjaenisch/amqtt_db
Incoming MQTT messages are irregular blobs of data so usually the DB-Backend is some kind of object storage.
I do it the hard way and deserialize the blobs into typed data colums and store them into a fast relational database. My finally target will be timescaleDB but first I go via SQLAlchemy to access a wide bunch of DBs with one API.
MQTT messages are volatile (think not always complete) so the DB scheme has to adjust dynamically e.g. adding new columns for new information.
First Message:
Time: 1234
Temperature : 23.4
Second Message:
Time: 1245
Temperature : 23.6
Rel Hum : 87 %
I have used SQLalchemy ORM for more than a decade but always for quite static databases. So I am new to work dynmically.
Utilizing the ORM to build DB tables dynamically from the structure of incoming MQTT-Messages was quite doable and worked out perfect.
But currently I am stuck with the case of additional information in the MQTT-Packages that extends the tables with new columns.
What I did so far:
Utilizing sqlalchemy-migration it was quite easy to dynamically add new columns to the existing table in the DB. In the code "topic_cls" is the declarative class and "column_def" a col_name - type mapping.
from migrate.versioning.schema import Table as MiTable, Column as MiColumn
def add_new_colums(self, topic_cls, column_def):
table_name = str(topic_cls.__table__.name)
table = MiTable(table_name, self.metadata)
for col_name, col_type in column_def.items():
col = MiColumn(col_name, col_type)
col.create(table)
Works like a charm. But how to get this changes to the DB reflected back into declarative classes? I tried to get a new inspection of the table:
new_table = Table(topic_cls.__table__.name, self.metadata, autoload_with=self.engine)
This also works but it gives me a new table but not a declarative base.
So my stupid questions are:
Is this the right way to achive my goal?
How can I get a declarative class by inspecting an already existing table in a DB?
"Drop the ORM and use SQL" is not the answer I am looking for.
Cheers,
Volker
Found a solution but it is a bit of a hack.
new_table = Table("test/topic_growth", Base.metadata, autoload_with=self.engine)
Base.metadata.remove(topic_cls.__table__)
new_dcl = type(str(table_name), (Base,), {'__table__': new_table})
Base.metadata._add_table(table_name, None, new_table)
After you obtained the new table via inspection, remove the old table entry from the metadata.
Then generate a new declarative base with the new table and same table name.
At last add the new table to the metadata.
I'm somewhat new to SQLAlchemy ORM, and I'm trying to select and then store data from a column within a view that has a forward slash in the name of the column.
The databases are mapped using the following:
source_engine = create_engine("...")
base = automap_base()
base.prepare(source_engine, reflect=True)
metadata = MetaData(self.engine)
table_1 = Table("table_1", self.metadata, autoload=True)
The second destination table is mapped the same way.
Then, I connect to this database, and I'm trying to select information from columns to copy into a different database:
source_table_session = Session(source_engine)
dest_table_session = Session(dest_engine)
table_1_data = table_1_session.query(table_1)
for instance in table_1_data:
newrow = dest_table.base.classes.dest_table()
newrow.Column1 = instance.Column1 # This works fine, column has normal name
But then, the problem is that there's a column in the view with the name "Slot/Port"
With a direct query, you can do:
select "Slot/Port" from source_database;
But in ORM, you can't just type:
newrow.Slot/Port = instance.Slot/Port
or
newrow.'Slot/Port' = instance.'Slot/Port'
That isn't going to be correct, and the following doesn't work either:
newrow.SlotPort = instance.SlotPort
AttributeError: 'result' object has no attribute 'SlotPort'
I have no control over how columns are named in the source database.
I find the SQLAlchemy documentation to be generally fragmented (only showing small snippets of code) and confusing, so I'm not sure if this is kind of thing is addressed or not. Is there a way to get around this limitation, or perhaps if the columns are already mapped to a valid name without a slash or a way to do so?
Thanks to #DeepSpace for helping me find the answer.
Instead of
newrow.whatever = instance.whatever
I needed:
setattr(newrow, 'Slot/Port', getattr(instance, 'Slot/Port'))
I am building a sqlite browser in Python/sqlalchemy.
Here is my requirement.
I want to do insert operation on the table.
I need to pass a table name to a function. It should return all columns along with the respective types.
Can anyone tell me how to do this in sqlalchemy ?
You can access all columns of a Table like this:
my_table.c
Which returns a type that behaves similar to a dictionary, i.e. it has values method and so on:
columns = [(item.name, item.type) for item in my_table.c.values()]
You can play around with that to see what you can get from that. Using the declarative extension you can access the table through the class' __table__ attribute. Furthermore, you might find the Runtime Inspection API helpful.
I would like to be able to full text search across several text fields of one of my SQLAlchemy mapped objects. I would also like my mapped object to support foreign keys and transactions.
I plan to use MySQL to run the full text search. However, I understand that MySQL can only run full text search on a MyISAM table, which does not support transactions and foreign keys.
In order to accomplish my objective I plan to create two tables. My code will look something like this:
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
name = Column(String(50))
description = Column(Text)
users_myisam = Table('users_myisam', Base.metadata,
Column('id', Integer),
Column('name', String(50)),
Column('description', Text),
mysql_engine='MyISAM')
conn = Base.metadata.bind.connect()
conn.execute("CREATE FULLTEXT INDEX idx_users_ftxt \
on users_myisam (name, description)")
Then, to search I will run this:
q = 'monkey'
ft_search = users_myisam.select("MATCH (name,description) AGAINST ('%s')" % q)
result = ft_search.execute()
for row in result: print row
This seems to work, but I have a few questions:
Is my approach of creating two tables to solve my problem reasonable? Is there a standard/better/cleaner way to do this?
Is there a SQLAlchemy way to create the fulltext index, or am I best to just directly execute "CREATE FULLTEXT INDEX ..." as I did above?
Looks like I have a SQL injection problem in my search/match against query. How can I do the select the "SQLAlchemy way" to fix this?
Is there a clean way to join the users_myisam select/match against right back to my user table and return actual User instances, since this is what I really want?
In order to keep my users_myisam table in sync with my mapped object user table, does it make sense for me to use a MapperExtension on my User class, and set the before_insert, before_update, and before_delete methods to update the users_myisam table appropriately, or is there some better way to accomplish this?
Thanks,
Michael
Is my approach of creating two tables to solve my problem reasonable?
Is there a standard/better/cleaner way to do this?
I've not seen this use case attempted before, as developers who value transactions and constraints tend to use Postgresql in the first place. I understand that may not be possible in your specific scenario.
Is there a SQLAlchemy way to create the fulltext index, or am I best
to just directly execute "CREATE FULLTEXT INDEX ..." as I did above?
conn.execute() is fine though if you want something slightly more integrated you can use the DDL() construct, read through http://docs.sqlalchemy.org/en/rel_0_8/core/schema.html?highlight=ddl#customizing-ddl for details
Looks like I have a SQL injection problem in my search/match against query. How can I do the
select the "SQLAlchemy way" to fix this?
note: this recipe is only for MATCH against multiple columns simultaneously - if you have just one column, use the match() operator more simply.
most basically you could use the text() construct:
from sqlalchemy import text, bindparam
users_myisam.select(
text("MATCH (name,description) AGAINST (:value)",
bindparams=[bindparam('value', q)])
)
more comprehensively you could define a custom construct:
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import ClauseElement
from sqlalchemy import literal
class Match(ClauseElement):
def __init__(self, columns, value):
self.columns = columns
self.value = literal(value)
#compiles(Match)
def _match(element, compiler, **kw):
return "MATCH (%s) AGAINST (%s)" % (
", ".join(compiler.process(c, **kw) for c in element.columns),
compiler.process(element.value)
)
my_table.select(Match([my_table.c.a, my_table.c.b], "some value"))
docs:
http://docs.sqlalchemy.org/en/rel_0_8/core/compiler.html
Is there a clean way to join the users_myisam select/match against right back
to my user table and return actual User instances, since this is what I really want?
you should probably create a UserMyISAM class, map it just like User, then use relationship() to link the two classes together, then simple operations like this are possible:
query(User).join(User.search_table).\
filter(Match([UserSearch.x, UserSearch.y], "some value"))
In order to keep my users_myisam table in sync with my mapped object
user table, does it make sense for me to use a MapperExtension on my
User class, and set the before_insert, before_update, and
before_delete methods to update the users_myisam table appropriately,
or is there some better way to accomplish this?
MapperExtensions are deprecated, so you'd at least use the event API, and in most cases we want to try applying object mutations outside of the flush process. In this case, I'd be using the constructor for User, or alternatively the init event, as well as a basic #validates decorator which will receive values for the target attributes on User and copy those values into User.search_table.
Overall, if you've been learning SQLAlchemy from another source (like the Oreilly book), its really out of date by many years, and I'd be focusing on the current online documentation.