pwiz creates incorrect code for composite foreign keys
Peewee does not support foreign keys to composite primary keys using any defined APIs. Edited issue to add syntax highlighting and proper formatting.
Given this (PostgreSQL):
```sql
CREATE TABLE tablea (
    keyparta character varying(4) NOT NULL,
    keypartb integer NOT NULL,
    whatever character varying(20)
);

CREATE TABLE tableb (
    keyparta character varying(4) NOT NULL,
    keypartb integer NOT NULL,
    keypartc character varying(16) NOT NULL,
    whatever character varying(12)
);

ALTER TABLE tablea ADD CONSTRAINT tablea_primary PRIMARY KEY (keyparta, keypartb);
ALTER TABLE tableb ADD CONSTRAINT tableb_primary PRIMARY KEY (keyparta, keypartb, keypartc);
ALTER TABLE tableb ADD CONSTRAINT tablea_keyparta_keypartb FOREIGN KEY (keyparta, keypartb) REFERENCES tablea(keyparta, keypartb);
```
pwiz creates a correct Tablea class, and a nonfunctioning Tableb class.
```python
class Tableb(BaseModel):
  keyparta = ForeignKeyField(db_column='keyparta', rel_model=Tablea, related_name='tablea_keyparta_set', to_field='keypartb')
  ....
```
I wouldn't know how to express this schema either but having part of a foreign key pointing at a different part of it in the target table definitely doesn't work.


Fredrik