如何从Django中的models.py查询数据库项目？

Question

I have different model. Choices of Multiselctfield of one model is dependent on another model.So , database has to be queried inside model.py While doing so, this causes problem in migration. (Table doesn't exist error)

class Invigilator(models.Model):
    ---

# this method queries Shift objects and Examroom
def get_invigilator_assignment_list ():
    assignment = []
    shifts = Shift.objects.all()
    for shift in shifts:
        rooms= ExamRoom.objects.all()
        for room in rooms:
            assign = str (shift.shiftName)+ " " +str (room.name) 
            assignment.append (assign)
    return assignment
assignment_choice = []
assign = get_invigilator_assignment_list()
i = 0
for assignm in assign:
    datatuple = (i,assignm)
    assignment_choice.append(datatuple) 
    i= i+1
ASSIGNMENT_CHOICE = tuple(assignment_choice) 

assignment =MultiSelectField (choices = ASSIGNMENT_CHOICE, blank = True, verbose_name="Assignments")

Answer 1

You cannot add dynamic choices because they are all stored in the migration files and table info. If Django lets you do that, this means that everytime someone adds a record to those 2 models, a new migration should be created and the db should be changed. You must approach this problem differently.

As far as I know django-smart-selects has a ChainedManyToMany field which can do the trick.

Here is an example from the repo.

from smart_selects.db_fields import ChainedManyToManyField

class Publication(models.Model):
    name = models.CharField(max_length=255)

class Writer(models.Model):
    name = models.CharField(max_length=255)
    publications = models.ManyToManyField('Publication', blank=True, null=True)

class Book(models.Model):
    publication = models.ForeignKey(Publication)
    writer = ChainedManyToManyField(
        Writer,
        chained_field="publication",
        chained_model_field="publications")
    name = models.CharField(max_length=255)

Answer 2

This cannot be done in the model and doesn't make sense. It's like you're trying to create a column in a table with a certain fixed set of choices (what is MultiSelecField anyway?), but when someone later adds a new row in the Shift or ExamRoom table, the initial column choices have to change again.

You can

• either make your assignment column a simple CharField and create the choices dynamically when creating the form
• or you can try to model your relationships differently. For example, since it looks like assignment is a combination of Shift and ExamRoom , I would create a through relationship:

shifts = models.ManyToManyField(Shift, through=Assignment)

class Assignment(Model):
    room = ForeignKey(ExamRoom)
    shift = ForeignKey(Shift)
    invigilator = ForeignKey(Invigilator)

When creating the relationship, you'd have to pick a Shift and a Room which would create the Assignment object. Then you can query things like invigilator.shifts.all() or invigilator.assignment_set.first().room .