Edit inline broken for models with multiple foreign keys to another model.

[k.kosciuszkiewicz at gmail dot com]

Test case:

class Publication(meta.Model):
    author = meta.CharField(maxlength=100)
    title = meta.CharField(maxlength=100)
    class META:
        admin = meta.Admin()

class Reference(meta.Model):
    position = meta.PositiveSmallIntegerField(core=True)
    cited    = meta.ForeignKey(Publication,
                    related_name="citation",
                    core=True)
    citing   = meta.ForeignKey(Publication,
                    related_name="reference",
                    edit_inline=meta.TABULAR)
    class META:
        admin = meta.Admin()
        ordering = ['position']

This should be perfectly valid but breaks in django/contrib/admin/templatetags/admin_modify.py in method EditInlineMode.render() with statement:

    klass = relation.field.rel.edit_inline

that returns False instead of class name. This is propagated further to result in attempt to call False(...) instead of ClassName(...) later in template rendering code.

[Gary Wilson <gary.wilson@…>]

Marking this a duplicate of #1939, as there is more discussion there and also some code.