Question

I am currently wanting to run a SQL statement that will filter my database but it accesses multiple tables. From the other examples I have read on this website, they all use foreign keys not on the primary key; however, that is my current setup. I am having two issues, the first being the SQL filter. These are my models:

class UserProfile(models.Model):
        user = models.ForeignKey(User, unique=True);

        isATutor = models.BooleanField();
        timeAvailable = models.CharField(max_length=3);

class TutorProfile(models.Model):
        user = models.ForeignKey(User);

        language = models.CharField(max_length=30);
        unique_together = (("user", "language"), );

class Tutor(models.Model):
        user = models.ForeignKey(User);

        subject = models.CharField(max_length=30);
        unique_together = (("user", "subject"), );

I am using raw SQL at the moment.

def sqlQuery(avail, lang, sub):
    from django.db import connection, transaction
    cursor = connection.cursor();

    cursor.execute("SELECT a.first_name, a.last_name, a.email FROM auth_user a, books_tutor b, books_tutorprofile c, books_userprofile d WHERE a.id = b.user_id AND a.id = c.user_id AND a.id = d.user_id AND b.subject=%s AND c.language=%s AND d.timeAvailable=%s", [sub, lang, avail]);
    row = cursor.fetchall();

    return row;

timeAvailable is taking a three character string is in the form ‘MAE’ where M = morning, A=Afternoon, E=Evening and if they are not needed, we’ll replace with – e.g. ‘M–‘ is only available in morning.

So the first issue is I’d love to be able to keep the above SQL as a django model. The second issue is I need a good way to query for MAE. I’m thinking I may have to do 8 different SQL parts and UNION them depending on what is chosen.

Thanks,

Steve

Answer 1

why not use subclassing?

class UserProfile(User):
    time_available = models.CharField(max_length=3);

class Tutor(UserProfile):
    subject = models.CharField(max_length=30);

class TutorProfile(Tutor):
    language = models.CharField(max_length=30);
    class Meta:
        unique_together = ['username','subject','language'] # <--- username field from User base class.

• UserProfile has access to all User fields, plus time_available.
• Tutor has access to all User, UserProfile fields, plus subject.
• TutorProfile has access to all User, UserProfile and Tutor fields, plus language.

in this way, you can achieve the join simply by looking up an instance of TutorProfile.

I should mention that when using subclassing, a OneToOneField is automatically created from the subclass to the superclass.