Semgrep rule to validate Django's ForeignKey related_name field

Question

I'm trying to create a rule to enforce the usage of related_name when defining a ForeignKey field in Django, as well as its format.

Enforcing the usage of the field is done by the following rule:

  - id: python.custom.foreign-key-must-set-related-name
    message: ForeignKey relationships must explicitly set the `related_name` property as `<model>_<field>_set`
    languages: [python]
    severity: ERROR
    patterns:
    - pattern-inside: |
        class $M(...):
          ...
    - pattern-not: $F = django.db.models.ForeignKey(..., related_name=..., ...)
    - pattern: $F = django.db.models.ForeignKey(...)

My goal now is to modify this rule to also be able to validate the format of the related_name, so that it is: <model>_<field>_set (or, following the variables in the rule: $M_$F_set)

I've been trying several combinations and haven't been lucky so far. Is this possible to do?

---

Edit:

As an example, look at the following model:

class MyCoolModel(models.Model):
    correct_field = models.ForeignKey(to='something', related_name='mycoolmodel_correct_field_set')
    incorrect_field = models.ForeignKey(to='something', related_name='something_set')