在django-fsm中配置外键类型FSM字段失败：状态存为字符串

希望将Task模型的工作流状态字段设置为WorkFlowState模型的外键，同时通过Transition模型定义FSM状态转换规则，但配置后task_state字段被存储为字符串，而非预期的外键关联实例。

import datetime

from django.db import models
from django_fsm import FSMField, transition


class WorkFlowState(models.Model):
    name = models.CharField(max_length=64, unique=True)
    preceding = models.ForeignKey(
        'self', on_delete=models.CASCADE,
        blank=True, null=True)
    # postAction = models.CharField(max_length=64, null=True, blank=True)
    default_warn1 = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(hours=12))
    default_warn2 = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(days=1))
    default_duration = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(days=1))

    def __str__(self):
        return self.name


class Transition(models.Model):
    name = models.CharField(max_length=64)
    source_state = models.ForeignKey(WorkFlowState, on_delete=models.CASCADE, related_name='transitions_from')
    target_state = models.ForeignKey(WorkFlowState, on_delete=models.CASCADE, related_name='transitions_to')

    def __str__(self):
        return self.name


class Task(models.Model):
    name = models.CharField(max_length=100)
    assigned_on = models.DateTimeField(auto_now=True)
    created_on = models.DateTimeField(auto_now_add=True)
    task_state = FSMField(models.ForeignKey(WorkFlowState, on_delete=models.PROTECT), protected=True)

    def __str__(self):
        return self.task_state

    def perform_transition(self, transition_name):
        try:
            transition = Transition.objects.get(name=transition_name, source_state=self.task_state)
            setattr(self, 'assigned_to', transition.target_state)
            self.save()
            return True
        except Transition.DoesNotExist:
            return False

核心问题 #

1. FSMField 使用错误：django_fsm的FSMField本质是字符串类型字段，不能直接将ForeignKey作为参数传入。原代码中错误地将外键字段类型传给FSMField，导致字段存储的是外键实例的字符串表示，而非外键关联。
2. 状态转换字段名错误：perform_transition方法中错误地将状态赋值给assigned_to字段，实际应更新的是task_state。
3. __str__方法类型错误：直接返回外键实例会触发类型错误，需转为字符串。

修复方案一：直接使用ForeignKey配合django_fsm #

适合需要数据库外键约束、直接关联WorkFlowState实例的场景：

import datetime

from django.db import models
from django_fsm import transition


class WorkFlowState(models.Model):
    name = models.CharField(max_length=64, unique=True)
    preceding = models.ForeignKey(
        'self', on_delete=models.CASCADE,
        blank=True, null=True)
    default_warn1 = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(hours=12))
    default_warn2 = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(days=1))
    default_duration = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(days=1))

    def __str__(self):
        return self.name


class Transition(models.Model):
    name = models.CharField(max_length=64)
    source_state = models.ForeignKey(WorkFlowState, on_delete=models.CASCADE, related_name='transitions_from')
    target_state = models.ForeignKey(WorkFlowState, on_delete=models.CASCADE, related_name='transitions_to')

    def __str__(self):
        return self.name


class Task(models.Model):
    name = models.CharField(max_length=100)
    assigned_on = models.DateTimeField(auto_now=True)
    created_on = models.DateTimeField(auto_now_add=True)
    # 改为ForeignKey，需提前创建默认的WorkFlowState实例并指定default
    task_state = models.ForeignKey(WorkFlowState, on_delete=models.PROTECT)

    def __str__(self):
        return f"{self.name} - {str(self.task_state)}"

    @transition(field='task_state', source='*', target=None)
    def perform_transition(self, transition_name):
        try:
            transition_obj = Transition.objects.get(name=transition_name, source_state=self.task_state)
            self.task_state = transition_obj.target_state
            self.save()
            return True
        except Transition.DoesNotExist:
            return False

修复方案二：FSMField存储状态标识关联实例 #

适合需要利用django_fsm状态保护、转换历史等功能的场景：

import datetime

from django.db import models
from django_fsm import FSMField


class WorkFlowState(models.Model):
    name = models.CharField(max_length=64, unique=True)
    preceding = models.ForeignKey(
        'self', on_delete=models.CASCADE,
        blank=True, null=True)
    default_warn1 = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(hours=12))
    default_warn2 = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(days=1))
    default_duration = models.DurationField(
        blank=False, null=False, default=datetime.timedelta(days=1))

    def __str__(self):
        return self.name


class Transition(models.Model):
    name = models.CharField(max_length=64)
    source_state = models.ForeignKey(WorkFlowState, on_delete=models.CASCADE, related_name='transitions_from')
    target_state = models.ForeignKey(WorkFlowState, on_delete=models.CASCADE, related_name='transitions_to')

    def __str__(self):
        return self.name


class Task(models.Model):
    name = models.CharField(max_length=100)
    assigned_on = models.DateTimeField(auto_now=True)
    created_on = models.DateTimeField(auto_now_add=True)
    # 用FSMField存储状态名称（依赖WorkFlowState的name唯一约束）
    task_state_name = FSMField(max_length=64, protected=True)

    @property
    def task_state(self):
        return WorkFlowState.objects.get(name=self.task_state_name)

    @task_state.setter
    def task_state(self, state):
        self.task_state_name = state.name

    def __str__(self):
        return f"{self.name} - {self.task_state_name}"

    def perform_transition(self, transition_name):
        try:
            transition_obj = Transition.objects.get(name=transition_name, source_state=self.task_state)
            self.task_state = transition_obj.target_state
            self.save()
            return True
        except (Transition.DoesNotExist, WorkFlowState.DoesNotExist):
            return False

内容的提问来源于stack exchange，提问作者Reza Teymoori