Django - using multiple foreign key to the same model

Question

I'm trying to use the same foreign key for two fields in the same model and am getting an error.

Im trying to have a primary and secondary on call user, but am not sure how to format the relationship after receiving the errors below

class ManualRotas(models.Model):
    rota_name = models.CharField(max_length=200,choices=settings.ONCALL_ROTAS)
    primary_user = models.ForeignKey(User, unique=True, verbose_name="Primary OnCall Engineer")
    p_start_time = models.DateTimeField(verbose_name="Start Time")
    p_end_time = models.DateTimeField(verbose_name="End Time")
    secondary_user = models.ForeignKey(User, verbose_name="Backup OnCall Engineer", unique=True,blank=True,null=True)
    s_start_time = models.DateTimeField(blank=True,null=True, verbose_name="Start Time")
    s_end_time = models.DateTimeField(blank=True,null=True,verbose_name="Start Time")


ERRORS:
oncall.ManualRotas.primary_user: (fields.E304) Reverse accessor for 'ManualRotas.primary_user' clashes with reverse accessor for 'ManualRotas.secondary_user'.
        HINT: Add or change a related_name argument to the definition for 'ManualRotas.primary_user' or 'ManualRotas.secondary_user'.
oncall.ManualRotas.secondary_user: (fields.E304) Reverse accessor for 'ManualRotas.secondary_user' clashes with reverse accessor for 'ManualRotas.primary_user'.
        HINT: Add or change a related_name argument to the definition for 'ManualRotas.secondary_user' or 'ManualRotas.primary_user'.

WARNINGS:
oncall.ManualRotas.primary_user: (fields.W342) Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.
        HINT: ForeignKey(unique=True) is usually better served by a OneToOneField.
oncall.ManualRotas.secondary_user: (fields.W342) Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.
        HINT: ForeignKey(unique=True) is usually better served by a OneToOneField.

System check identified 4 issues (0 silenced).

Answer 1

You have to define different related_name to both the ForeignKeys columns. For example:

class ManualRotas(models.Model):
    primary_user = models.ForeignKey(User, related_name='related_primary_manual_roats', unique=True, verbose_name="Primary OnCall Engineer")
    #                            related names ^ v
    secondary_user = models.ForeignKey(User, related_name='related_secondary_manual_roats', verbose_name="Backup OnCall Engineer", unique=True,blank=True,null=True)
    # .... Other columns

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Please also refer ForeignKey.related_name document, which says:

The name to use for the relation from the related object back to this one. It’s also the default value for related_query_name (the name to use for the reverse filter name from the target model). See the related objects documentation for a full explanation and example. Note that you must set this value when defining relations on abstract models; and when you do so some special syntax is available.

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