ValueError - Cannot assign: must be an instance

Question

I get the following error when I try to run an insert into one of my tables.

Cannot assign "1": "Team.department_id" must be a "Department" instance

Admittedly I'm slightly unsure if I'm using the foreign key concept correctly. The insert I'm trying to run and a snippet from my models.py are given below.

What I'm trying to do is that when someone wants to create a new team. They have to attach it to a department. Therefore the department ID should be in both sets of tables.

new_team = Team(
    nickname = team_name,
    employee_id = employee_id,
    department_id = int(Department.objects.get(
                        password = password,
                        department_name = department_name
                        ).department_id))

models.py

class Department(models.Model):  
    department_id = models.AutoField(auto_created=True,
                                     primary_key=True,
                                     default=1)  
    department_name = models.CharField(max_length=60)
    head_id = models.CharField(max_length=30)
    password = models.CharField(max_length=128)


class Team(models.Model):  
    team_id = models.AutoField(primary_key=True)
    department_id = models.ForeignKey('Department',
                                       related_name = 'Department_id')
    employee_id = models.CharField(max_length=30)
    nickname = models.CharField(max_length=60)
    team_image = models.ImageField(upload_to=get_image_path,
                                   blank=True, null=True)

score 74 · Accepted Answer · edited May 09 '23 at 00:11

74

You don't need to pass the department id, the instance itself is enough.

The following should work just fine:

new_team = Team(
    nickname = team_name,
    employee_id = employee_id,
    department_id = Department.objects.get(password = password,
                                           department_name = department_name))

Just a note: don't ever name your foreign fields something_id.

That something is enough. Django is meant to make things easy from the user's perspective and the _id suffix means you're thinking of the database layer. In fact, if you named your column department, django will automatically create department_id column in the database for you.

The way things are, you're making Django create department_id_id which is rather silly.

edited May 09 '23 at 00:11

ivanleoncz

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answered Jun 15 '16 at 15:43

Anonymous

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Thank you so much. It worked. But also both of your explanations really helped with my understanding. – rahimbah Jun 15 '16 at 16:01
don't forget to add this line of code: new_team.save() – Karam Qusai Jan 11 '22 at 08:49

score 13 · Answer 2 · answered Jul 29 '21 at 06:27

This came up first on a Google search so offering an alternative for newcomers. You can also do it this way in case you have handy access to the id and don't want to do another query:

new_team = Team(
    nickname = team_name,
    employee_id = employee_id,
    department_id_id = Department.objects.get(password = password, department_name = department_name).department_id
)

In short, {foreign_key_name}_id if you want to assign the id directly.

{foreign_key_name}_id was exactly what I was searching for. Thank you. — thiras, Jul 30 '21 at 01:17

score 0 · Answer 3 · answered Oct 20 '22 at 16:06

A different variant for @munsu 's answer. If you already have the id for the foreign field.

department_id = 2; #the department id for the new team

new_team = Team(
    nickname = team_name,
    employee_id = employee_id,
    department_id = Department(id=department_id)
)

new_team.save()

# but
new_team.department_id.id # 2 
new_team.department_id.department_name # None you cannot access any other fields

I don't know if this would be useful for any case, but here it is. :)

ValueError - Cannot assign: must be an instance

3 Answers3

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