Custom Compilation

[Soumaya Mauthoor]

Hello

I have two uses cases:

(1) drop cascade as option

I know I can use custom compilation to add cascade for postgres databases using this example:

https://stackoverflow.com/questions/38678336/sqlalchemy-how-to-implement-drop-table-cascade

Is it possible to use custom compilation to add cascade as an optional keyword argument to drop()? 

(2) overwriting func functions

Is it possible to overwrite func.greatest to return func.max for SQLite?

I'm trying to learn more about custom compilation in general. This page is very helpful:  

http://docs.sqlalchemy.org/en/latest/core/compiler.html

Are there other resources you recommend?

Soumaya

[Mike Bayer]

On Wed, Mar 18, 2020, at 1:23 PM, Soumaya Mauthoor wrote:

Hello

I have two uses cases:

(1) drop cascade as option

I know I can use custom compilation to add cascade for postgres databases using this example:

https://stackoverflow.com/questions/38678336/sqlalchemy-how-to-implement-drop-table-cascade

Is it possible to use custom compilation to add cascade as an optional keyword argument to drop()? 

drop() doesn't accept any optional keyword arguments so you're best off making your own subclass of DropTable  called DropTableCascade and just executing it:   conn.execute(DropTableCascade(mytable))

(2) overwriting func functions

Is it possible to overwrite func.greatest to return func.max for SQLite?

generic functions are used for this.  Since "max" is a multiple-typed function you can use a helper class called ReturnTypeFromArgs:

from sqlalchemy.sql.functions import ReturnTypeFromArgs

from sqlalchemy.ext.compiler import compiles

from sqlalchemy.sql import literal_column

from sqlalchemy.sql import func

from sqlalchemy.dialects import sqlite

class greatest(ReturnTypeFromArgs):

    pass

@compiles(greatest, "sqlite")

def _render_max(elem, compiler, **kw):

    return "max(%s)" % compiler.process(elem.clauses)

print(func.greatest(literal_column("foo")))

print(func.greatest(literal_column("foo")).compile(dialect=sqlite.dialect()))

I'm trying to learn more about custom compilation in general. This page is very helpful:  

http://docs.sqlalchemy.org/en/latest/core/compiler.html

Are there other resources you recommend?

Soumaya

--

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To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. See http://stackoverflow.com/help/mcve for a full description.

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[sumau]

Thanks Mike that worked perfectly :-)