CODEWITHSUNDEEP
pythondatabasedjangomodelmany-to-many
ClaudioA
ClaudioA

Reputation: 317

Django ManyToMany Field

I need to make a smart menu, for which I need a ManyToMany relation.

My model is:

from django.db import models

    class Health_plan(models.Model):
        a = models.IntegerField ()
        b = models.IntegerField ()

   class Doctors_list(models.Model):

        name = models.CharField(max_length=30)
        hp_id = models.ManyToManyField(Health_plan)

        def __unicode__(self):
            return self.name

How do I make this relation in the database ? I was thinking in puting the health_plans (a,b) as columns, and the doctors as rows, with 0s and 1s to identify their covered health_plans.

Someone told me this was a misuse of a ManyToManyField, I don't know wich step to take.

Help appreciated

Upvotes: 1

Views: 5793

Answers (4)

allcaps
allcaps

Reputation: 11248

Django creates the tabels for you. In your project folder run:

python manage.py syncdb

Health_plan and Doctors_list are both tables. 'a' and 'b' are columns in Health_plan. 'Name' and 'hp_id' are columns in Doctors_list. Django will create a column for id in each table. Django will also create a table "Doctor_list_Health_plan" to store the relation information.

Django models are Python classes, so the Python naming conventions apply. Use HealthPlan and Doctor (CapitalizeWord singular).

Your field names are a bit abstract. I suggest you use more descriptive names. Eg:

class HealthPlan(models.Model):
    name = models.CharField()
    extra_care = models.BooleanField()

Upvotes: 1

bruno desthuilliers
bruno desthuilliers

Reputation: 77932

Django's ORM already takes care of the intermediate table so you don't have to "make this relation(ship) in the database", but given your question you obviously need to learn about proper relational model normalisation - if you don't understand the relational model you won't get nowhere with Django's ORM, nor with any other sql stuff FWIW.

For the record, in the relational model, a many to many relationship is modeled as a relation ("table" in SQL) with foreign keys on both other tables, ie:

health_plan(#health_plan_id, name, ...)
doctor(#doctor_id, firstname, lastname, ...)
doctors_health_plans(#health_plan_id, #doctor_id)

So your django models should be:

class HealthPlan(models.Model):
    # no need to define an 'id' field,
    # the ORM provides one by default
    name = models.CharField(....)

class Doctor(models.Model):
    firstname = models.CharField(....)
    lastname = models.CharField(....)
    health_plans = models.ManyToManyField(HealthPlan, related_name="doctors")

Then you'll be able to get all HealthPlans for a Doctor :

  doc = Doctor.objects.get(pk=xxxx)
  doc.health_plans.all()

and all Doctors for an HealthPlan:

  plan = HealthPlan.objects.get(pk=xxxx)
  plan.doctors.all()

The FineManual(tm) is your friend as usual...

Upvotes: 5

Martin Maillard
Martin Maillard

Reputation: 2811

The approach of puting the health_plans as columns is not necessarily wrong, but it implies that you have a fixed number of health plans and that you will never add a new one.

The traditional approach for many-to-many relationships in relational databases is to introduce a table in the middle. This table will just contain the association between a doctor and a health plan.

If you have a Doctor table that contains:

id    name
1     foo
2     bar

And a HealthPlan table:

id    model
1     a
2     b

You then add a table Doctor_HealthPlan that is like:

doctor_id    healthplan_id
1            2
2            1
2            2

The ManyToMany field type in django will automatically create this table for you. Your code is correct, but you should probably rename hp_id to something like health_plans, since it is a proxy that allows you to access the list of health plans associated to a doctor.

Upvotes: 5

Bwire
Bwire

Reputation: 1201

You just need to save the two models first then add the healthplan instance to the doctors list. Django will handle the rest for you .

For example :

doctor_list = Doctors_list(name="Bwire")
health_plan.save()
doctor_list.save()

#Then add the plan to the doctors list.
doctor_list.hp_id.add(health_plan)

Upvotes: 1

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