Need help please... Admin add item Field value from another ForeignKey

Using Django The Admin
1

Dear Django Coders…

I’am a newbie at Django Coding…

i need help with the Django Admin - Add item Function…

i have this Model:

class MachineGroups(models.Model):
    group = models.CharField(verbose_name="Maschinengruppe", max_length=200, unique=True)
    added_date = models.DateTimeField("hinzugefügt am", auto_now_add=True)
    edited_date = models.DateTimeField("bearbeitet am", auto_now=True)

    def __str__(self):
        return self.group
    class Meta:
        verbose_name = "Maschinengruppe"
        verbose_name_plural = "Maschinengruppen"

class Machines(models.Model):
    name = models.CharField(verbose_name="Maschinenname", max_length=200, unique=True)
    group = models.ForeignKey(MachineGroups, verbose_name="Maschinengruppe", on_delete=models.PROTECT)
    added_date = models.DateTimeField("hinzugefügt am", auto_now_add=True)
    edited_date = models.DateTimeField("bearbeitet am", auto_now=True)

    def __str__(self):
        return self.name
    
  class Meta:
        verbose_name = "Maschine"
        verbose_name_plural = "Maschinen"

class MachineModules(models.Model):
    machine_module = models.CharField(verbose_name="Maschinenmodul", max_length=200, unique=True)
    machine_group = models.ForeignKey(MachineGroups, verbose_name="Maschinengruppe", on_delete=models.PROTECT)
    machine_name = models.ForeignKey(Machines, verbose_name="Maschine", on_delete=models.PROTECT)
    added_date = models.DateTimeField("hinzugefügt am", auto_now_add=True)
    edited_date = models.DateTimeField("bearbeitet am", auto_now=True)

    def __str__(self):
        return self.name
    class Meta:
        verbose_name = "Maschinenmodul"
        verbose_name_plural = "Maschinenmodule"

and this admin.py:

class MachinesAdmin(admin.ModelAdmin):
    fieldsets = [
        ("Maschinendaten", {"fields": ["name", "group"]}),
    ]
    list_display = ["name", "group", "added_date", "edited_date"]
    list_filter = ["group"]
    search_fields = ["name", "added_date", "edited_date"]

class MachineGroupsAdmin(admin.ModelAdmin):
    fieldsets = [
        ("Maschinendaten", {"fields": ["group"]}),
    ]
    list_display = ["group", "added_date", "edited_date"]
    search_fields = ["group", "added_date", "edited_date"]

class MachineModulesAdmin(admin.ModelAdmin):
    fieldsets = [
        ("Maschinendaten", {"fields": ["machine_name", "machine_module"]}),
    ]
    list_display = ["machine_group", "added_date", "edited_date"]
    search_fields = ["machine_group", "added_date", "edited_date"]

I can add items to Machines and MachineGroups normally. The MachineGroup is an select Field. That is fine.

Now i have implemented the MachineModules model.
If i want to add an MachineModel the Machine Field is an select field too. So far so good.
The only thing that i need is that the machine_group column gets the value from the assigned group from Machine because i don’t want to select the Group a second time.

If i add an MachineModule now i get the following Error:

(1048, "Column 'machine_group_id' cannot be null")

I know that the error is shown because an ForeignKey can not be null. But how can i assign the value from the group from the selected Machine to this field?

Can anyone help me please?

Many thanks in advance

2

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3

ok thank you for the information. i will do this now

4

Almost perfect. The lines of ``` must be their own lines, they can’t be part of any other line. (No worries, I fixed that for you.)

It sounds like you don’t need to have machine_group as a field in this model, if the machine_group is determined by the group attribute of the related Machines.

I’d remove that field.

5

Thank you for fixing that. Now it looks much better :+1::grinning:

Unfortunately I need that field in this model because of further models in my app that I can get the full Information of Modules.

Do you have any idea how I get this to work? Or is it not possible with Django?

Thanks in advance

6

You can, by following the foreign key link in the related Machines object. There’s no need to replicate that information.

7

Aha ok. Can you show me an example how I can follow the ForeignKey please?

Best Thanks

8

If you have an instance of Machines named machine, how would you access the name field of that machine?

9

With machine.name i think? But how I get the group name if I only have an instance of Modules in modules? modules.group_name?

:thinking:

10

Absolutely correct.

So, along the same lines, machine.group is a reference to the MachineGroups instance by the foreign key.

Continuing along this same line, if you have an instance of MachineGroups named machine_group, then the reference to the group attribute would be machine_group.group.

So following this along through to the conclusion, if you have an instance of MachineModules named machine_module, then:

machine_module.machine_name.group.group
^^^^^^^^^^^^^^ Instance of MachineModules
               ^^^^^^^^^^^^^ ForeignKey to Machines
                            ^^^^^ ForeignKey to MachineGroups
                                  ^^^^^ CharField
11

Ok, that’s really nice :grinning::+1::grinning:

Thank you very much for the detailed description

You are the best :+1: