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?
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.
3) What is the purpose of the ... in the visit_alter_column method?
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.
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)
Regards
Soumaya
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