Hello Mike
Thanks for the quick response. Would you mind helping me write a complete
example? It would help me understand compilation better :-) I would also be
happy to turn this into a merge request if you think other people will
benefit from it? I have also converted this into a sqlite example so using
add column instead of alter column, to make it more generic and follow the
sql expression tutorial. Also in the example I pass in the column type as a
string, how would I pass in the type as a sqlalchemy data type?
This is what I have so far:
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.schema import DDLElement
from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, String, MetaData
engine = create_engine('sqlite:///:memory:', echo=True)
metadata = MetaData()
users = Table('users', metadata,
Column('id', Integer),
)
metadata.create_all(engine)
class AddColumn(DDLElement):
def __init__(self, column_name, column_type):
self.column_name = column_name
self.column_type = column_type
@compiles(AddColumn)
def visit_add_column(element, column, compiler, **kw):
return "ALTER TABLE %s ADD COLUMN %s %s %s" % (element.table.name,
AddColumn.column_name, AddColumn.column_type)
s = AddColumn(users,'col1', 'int')
print(str(s))
with engine.connect() as conn:
conn.execute(AddColumn(users,'col1', 'int'))
print(conn.execute(users.select()).fetchall())
Regards
Soumaya
Le lun. 19 avr. 2021 à 23:24, Mike Bayer <mike...@zzzcomputing.com> a
écrit :
>
>
> On Mon, Apr 19, 2021, at 2:09 PM, sumau wrote:
>
> Hello
>
> I'm trying to understand compilation and working through the compiler
> tutorial: https://docs.sqlalchemy.org/en/14/core/compiler.html
>
> I was hoping for some clarification :
>
> 1) What is the difference between starting your method with visit_XXX
> instead of compile_XX?
>
>
> no difference. the compiler extension does not rely upon naming
> conventions, just that the decorator is there. you can name any function
> anything or even use a lambda.
>
>
> 2) self.cmd is used in the init of AlterColumn but I can't see where it is
> used in the visit_alter_column method.
>
>
> the example is incomplete and "cmd" would represent some kind of
> alteration to the column, in the SQL where you see the ellipses...this is
> not a fully real world example.
>
>
> 3) What is the purpose of the ... in the visit_alter_column method?
>
>
> in the example there it's a function. Assuming you mean the function,
> that's where you implement how to turn your AlterColumn object into a SQL
> string.
>
>
>
> 4) What is the example usage for AlterColumn? I tried this:
>
> from sqlalchemy import create_engine
> from sqlalchemy import Table, Column, Integer, String, MetaData
>
> engine = create_engine('postgresql://***:***@localhost:5432/postgres')
> conn = engine.connect()
>
> metadata = MetaData()
> users = Table('users', metadata,
> Column('id', Integer),
> Column('name', String),
> Column('fullname', String),
> )
>
> metadata.create_all(engine)
> users.AlterColumn(users.c.name, 'type int')
>
> without success.
>
>
> noting that this AlterColumn isn't "real" and the SQL it genreates isn't
> valid, you would execute it:
>
> with engine.connect() as conn:
> conn.execute(AlterColumn(...))
>
>
>
> 5) How would I create an AddColumn function? Something like this perhaps?
>
> class AddColumn(DDLElement):
> def __init__(self, column, column_type):
> self.column = column
> self.column_type = column_type
>
> @compiles(AddColumn)
> def visit_add_column(element, compiler, **kw):
> return "ALTER TABLE %s ADD COLUMN %s ..." % (element.table.name,
> element.column.name)
>
>
> AddColumn is a class but other than that, that's the idea sure!
>
>
>
>
>
> Regards
> Soumaya
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
> ---
> You received this message because you are subscribed to the Google Groups
> "sqlalchemy" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to sqlalchemy+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/sqlalchemy/0b3e9438-7631-46d8-bd3a-8f835a8c11c1n%40googlegroups.com
> <https://groups.google.com/d/msgid/sqlalchemy/0b3e9438-7631-46d8-bd3a-8f835a8c11c1n%40googlegroups.com?utm_medium=email&utm_source=footer>
> .
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
> ---
> You received this message because you are subscribed to the Google Groups
> "sqlalchemy" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to sqlalchemy+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/sqlalchemy/e1053691-8d35-4452-9826-c4f4df04dbda%40www.fastmail.com
> <https://groups.google.com/d/msgid/sqlalchemy/e1053691-8d35-4452-9826-c4f4df04dbda%40www.fastmail.com?utm_medium=email&utm_source=footer>
> .
>
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
---
You received this message because you are subscribed to the Google Groups
"sqlalchemy" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to sqlalchemy+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/sqlalchemy/CAN14jWTbx5tKqfTjkyuL-3KLY6Gyykzrq8Qyvur-AqDMUqFWfw%40mail.gmail.com.
