I now is a string from the front page, and then I need to use the string to find TestCase foreign key associated data does not exist the inventory data, could you tell me how to deal with ModelForm? "Has prompted a valid option: the choice is not in the options available,", here is my code now, still hope you to help us have a look at, thank you

 
# model 
The class TestCase (BaseModel) : 
Number=models. ForeignKey (WorkTask, on_delete=models. The CASCADE, verbose_name='task number') 
Name=models. CharField (max_length=30, unique=True, verbose_name='use case name') # test case name is the only value 
Few=models. CharField (max_length=200, null=True, blank=True, verbose_name='remarks') 

The class Meta: 
Verbose_name='case information' 
Verbose_name_plural=verbose_name 

Def the __str__ (self) : 
Return the self. The name 

# form 
# test cases to add 
Class AddTestCaseForm (forms. ModelForm) : 

The class Meta: 
The model=TestCase 
Fields=[' number ', 'name', 'few'] 

Def the clean (self) : 
Number=self. Data. The get (' number ', ') # access number 
Try: 
Worktask=worktask. Objects. The get (number=number) # query whether there is a task number 
Self. Cleaned_data [' number ']=worktask # replace number for the model object 
Return self. Cleaned_data [' number '] # return cleaned_data 
Except the Exception as e: 
Raise forms. ValidationError (' task number errors') # if not find the corresponding model objects, capture and throw an error