[Sqlalchemy-tickets] Issue #3639: insert().select_from() ignores custom dialect-specific keywords (zzzeek/sqlalchemy)

[Drino <iss...@bi...>]

New issue 3639: insert().select_from() ignores custom dialect-specific keywords
https://bitbucket.org/zzzeek/sqlalchemy/issues/3639/insert-select_from-ignores-custom-dialect

Drino:

I use sqlalchemy 1.0.11 with postgresql database. 
I want to merge two tables and use "ON CONFLICT IGNORE" postgresql-specific keyword to do this. I want to do it the right way and probably want to use "ON CONFLICT" construct somewhere else in my code. So I write the following:

```
#!python

@compiles(Insert)
def postgresql_on_conflict(insert, compiler, **kw):
    s = compiler.visit_insert(insert, **kw)
    if insert.kwargs.get('postgresql_on_conflict') is not None:
        return s + " ON CONFLICT " + insert.kwargs['postgresql_on_conflict']
    return s

Insert.argument_for('postgresql', 'on_conflict', None)

print tableA.insert(postgresql_on_conflict='IGNORE').from_select(tableB.c, tableB.select()).compile()
```

And it produces the following output:

```
#!sql

INSERT INTO general."tablea" (field) SELECT field 
FROM general."tableb"

```

The postgresql_on_conflict keyword works fine by itself.