Mismatched fields in composite primary key ForeignObject example model

[Jacob Walls]

The field types on ​this example model are not correct; the primary key of Order is CharField, and Product is IntegerField:

class Foo(models.Model):
    item_order_id = models.IntegerField()
    item_product_id = models.CharField(max_length=20)
    item = models.ForeignObject(
        OrderLineItem,
        on_delete=models.CASCADE,
        from_fields=("item_order_id", "item_product_id"),
        to_fields=("order_id", "product_id"),
    )

It should be:

class Foo(models.Model):
    item_order_id = models.CharField(max_length=20)
    item_product_id = models.IntegerField()
    item = models.ForeignObject(
        OrderLineItem,
        on_delete=models.CASCADE,
        from_fields=("item_order_id", "item_product_id"),
        to_fields=("order_id", "product_id"),
    )

Found this while trying to write a test with these models, e.g. this fails:

In [28]: f = Foo.objects.first()

In [29]: f.item
Out[29]: <OrderLineItem: OrderLineItem object ((1, '1'))>

In [30]: Foo.objects.filter(item=item)
Out[30]: ---------------------------------------------------------------------------
UndefinedFunction                         Traceback (most recent call last)
File ~/django/django/db/backends/utils.py:105, in CursorWrapper._execute(self, sql, params, *ignored_wrapper_args)
    104 else:
--> 105     return self.cursor.execute(sql, params)

UndefinedFunction: operator does not exist: character varying = integer
LINE 1: ...."item_order_id", "models_foo"."item_product_id") = ('1', 1)...
                                                             ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.

---

While here, also suggest documenting inside the caveats about how ForeignObject differs from ForeignKey that the on_delete argument is ignored.

[Jacob Walls]

​PR