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/
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