Django Complex queries joining tables with filters

Question

I am in need of some mentorship. I'm watching all of the Django videos and reading the instructions on models and queries, and I lack some understanding on joins.

Below is my model series. I am trying to write a query that returns a name from Member Name associated with a Position called "Commanding Officer", given a Unit. In short, I'm trying to get a Unit's Commanding Officer.

However, to get there, I need to bridge Member->MemberUnit->ClassPosition->Position.

Here's what I have:

class Member(models.Model):
    id = models.AutoField(primary_key=True)
    member_id = models.CharField(unique=True, max_length=30)
    first_name = models.CharField(max_length=30)
    middle_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)
    email = models.CharField(max_length=50)
    home_phone = PhoneNumberField(blank=True)
    cell_number = PhoneNumberField(blank=True)
    fax_number = PhoneNumberField(blank=True)
    Cell_phone = models.CharField(max_length=20)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)
    User = models.ForeignKey(User, blank=True, null=True)

    def __str__(self):              # __unicode__ on Python 2
        return self.first_name + " " + self.last_name

class Position(models.Model):
    Position_name = models.CharField(max_length=30, unique=True)
    Position_description = models.CharField(max_length=5000)
    Max_count = models.IntegerField()

    def __str__(self):
        return self.Position_name

class ClassPosition(models.Model):
    unit_class = models.ForeignKey(Class)
    unit_position = models.ForeignKey(Position)
    Date_effective = models.DateField()

    def __str__(self):
        return "Class: " + self.unit_class.class_name + "; Position: " + self.unit_position.Position_name

class MemberUnit(models.Model):
    Member = models.ForeignKey(Member)
    Unit = models.ForeignKey(Unit)
    Position = models.ForeignKey(ClassPosition)

    def __str__(self):
        return self.Unit.Unit_name + " " + self.Position.unit_position.Position_name + " ( " + self.Member.first_name + " " + self.Member.last_name + " )"

class Unit (models.Model):
    HType = (
        ('HQ', 'IFT Headquarters'),
        ('JTF', 'Joint Task Force'),
        ('GEO', 'Geographic Theater'),
        ('SHIP', 'Federation Vessel Chapter')
    )
    Unit_name = models.CharField(max_length=30)
    Hull_number = models.CharField(max_length=30)
    Hull_type = models.CharField(max_length=4, choices=HType)
    Parent_unit = models.ForeignKey("self", blank=True, null=True)
    Class = models.ForeignKey(Class)
    email = models.CharField(max_length=50, default='')
    Member = models.ManyToManyField(Member, through='MemberUnit')
    Address = models.CharField(max_length=500, default='')
    City = models.CharField(max_length=50, default='')
    State = models.CharField(max_length=2, default='')
    Zip = models.IntegerField()

    def __str__(self):
        return self.Unit_name

class Class(models.Model):
    class_type = models.CharField(max_length=30)
    class_name = models.CharField(max_length=20, unique=True)
    class_description = models.CharField(max_length=5000)

    def __str__(self):
        return self.class_name

If I can get an example of this kind of query, I believe I can roll with similar queries, using this by example. I would appreciate any help I can get for this.

Thank you.

Answer 1

I think this may be the situation you are searching. Try this; not tested;

unit_object = unit object which you already have;
member_unit = MemberUnit.objects.filter(unit=unit_object, Position__unit_position=Position.objects.get(Position_name='Commanding Officer'))

Here you can do a forloop and get all the members name;

for members in member_unit:
   print members.Member.first_name

or you can fetch a list of member names like;

member_names = MemberUnit.objects.filter(unit=unit_object, Position__unit_position=Position.objects.get(Position_name='Commanding Officer')).values_list('Member__first_name', flat=True)