CODEWITHSUNDEEP
djangodjango-formsdjango-orm
sunprophit
sunprophit

Reputation: 1689

Django. Many-To-Many Field for form, but not for model

I have DB that should have one field with type Many-To-Many, but it is not and I can't change this.

For example I have a list of students and a list of subjects. Subject should be many-to-many field in students table, but as i said it is not. Students table doesn't have this field at all. But there is still another table students-subjects that contains subject_id-student_id items.

How can I embed in student form some kind of subject field that could change students-subjects data when I save a student in DB? The problem is that I can't change the DB structure so need to make it only with help of Django.

Upvotes: 1

Views: 1373

Answers (1)

j0ker
j0ker

Reputation: 4139

There is no such thing as a 'Many-to-many field' for a database-table like you might know it of foreign keys. In database representation many-to-many relationships are realized by using an extra table that links the primary keys (usually the ids) of the records you want to set in relation. In your case that is the table students-subjects. Django uses this table when you define a many-to-many relationship in the model. You do not have to change your database structure at all, it will be working perfectly as it is now.

See the documentation: ManyToManyField

You'll have to set the db_table option with the name of your intermediary table (i.e. students-subjects). Then everything should work fine.

EDIT:
Considering your comment, the problem is that Django expects a certain naming convention (i.e. MODELNAME_id) which isn't provided by your table. Since you say that you cannot change the table itself, you have to try something else.
You have to create an extra Model for your intermediary table (students-subjects) and define the field 'students' as a foreign key to the students model and the field 'subjects' as a foreign key to the subjects model. Then for the many-to-many field you specifiy the option 'through' with the name of your intermediary table. Set the options 'db_column' to let Django know which names you'd like to use for the databse columns. 'db_table' in the meta class is needed to specify your database table name.

You get something like:

class StudentsSubjects(models.Model):
    student = models.ForeignKey(Student, db_column='student')
    subject = models.ForeignKey(Subject, db_column='subject')

    class Meta:
        db_table = 'students-subjects'    

class Student(models.Model):
    ...
    subjects = models.ManyToManyField(Subject, through='StudentsSubjects')
    ...

class Subject(models.Model):
    ...

I hope that will help you.
For more detail see: Extra fields on many-to-many relationship.

Upvotes: 1

Related Questions