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How to access data across M2M tables in Django?

What is the 'best practice' way of accessing data across a 1 (or more) many-to-many tables?
This is incredibly difficult for me as I am not sure what I shuld be googling/looking up.
I have attached a diagram of my data model. I am able to query data for 'C' related ot a user, by utilizing serializers.
there has to be a simpler way of doing this (I'm hoping).
Doing it with serializers seems incredibly limiting. I'd like to access a user's 'B' and 'C' and transform the object to only have a custom structure and possible unique values.
Any direction is much appreciated. Pretty new to Django, so I apologize for this newb type of question.

Here is an example of M2M relation using Django:
class User(models.Model):
name = models.CharField(...)
class Song(models.Model)
title = models.CharField(...)
users_that_like_me = models.ManyToManyField('User', ..., related_name='songs_that_i_like')
So a User can like many Songs and a Song can be liked by many Users.
To see all the songs a user liked, we can do:
user = User.objects.get(id='<the-user-id>')
liked_songs = user.songs_that_i_like.all()
And to see all the users who like a particular song we can similarly do:
song = Song.objects.get(id='<the-song-id>')
users_that_like_this_song = song.users_that_like_me.all()
Both liked_songs and users_that_like_this_song are actually querysets, meaning we can do some Django magic on them.
For example, to find all users named Jon that liked this song we can do:
users_that_like_this_song.filter(name='Jon')
We can also add some property shortcuts to our Models to help with some common tasks, for example:
class User(models.Model):
...
#property
def number_of_liked_songs(self):
return self.songs_that_i_like.count()
Then we can do:
user = User.objects.get(id='<the-user-id>')
number_of_songs_i_like = user.number_of_liked_songs
There's much more we can do with Django - if you're looking for something specific let us know.

Django: prefetch_related() with m2m through relationship v2

I know there is already a similar question, but I think my case is a bit more complicated because I have a different entry point.
These are my models:
class m_Interaction(models.Model):
fk_ip = models.ForeignKey('m_IP', related_name="interactions")
class m_User(models.Model):
name = models.CharField(max_length=200)
class m_IP(models.Model):
fk_user = models.ForeignKey('m_User', related_name="ips" )
class m_Feature(models.Model):
name = models.CharField(max_length=200)
m2m_interaction = models.ManyToManyField(m_Interaction, related_name='features', through='m_Featurescore')
class m_Featurescore(models.Model):
score = models.FloatField(null=False)
fk_interaction = models.ForeignKey(m_Interaction, related_name='featurescore')
fk_feature = models.ForeignKey(m_Feature, related_name='featurescore')
I start with m_User, follow the reverse relationship over m_IP to the Interactions (m_Interaction). Then I want to get every m_Featurescore.score for each Interaction for a specific instance of m_Feature.
My working query to access at least all interactions in a performant way:
m_User.objects.all().prefetch_related('ips__interactions')
But I can't figure out the correct 'prefetch_related'-statement to access the m_Featurescore.score like this
db_obj_interaction.featurescore.get(fk_feature=db_obj_feature).score
without making a lot of queries.
I already tried almost all combinations of the following:
'ips__interactions__features__featurescore'
Any suggestions?

I found the answer to my own question with the help of noamk in the comments:
I didn't consider that the get()-method in db_obj_interaction.featurescore.get(fk_feature=db_obj_feature).score will issue a new query everytime it's called (it's kinda obvious now).
Therefore I simply restructured my code and now I don't need get() anymore and can use the benefit of the prefetch.
If somebody still needs to filter the Prefetch()-object should be used as suggested by noamk

Django creating multiple tables/model classes from same base class with factory function

I have been trying to figure out the best way to automate creating multiple SQL tables based on separate but identical models, all based on the same base class. I'm basically creating pseudo message boards or walls with different Groups, and I wanted each Group to have its own db_table of Posts, each Post containing the user id, timestamp, etc.
My first thought was to have one base class of Posts and just include a field for Group name, but I thought this would be bad practice. My rationale was that one table containing every Post for all Groups would get really big (in theory anyway) and slow down filtering, and also that the extra field for group name would in the long run be a waste of memory when I could have separate tables per group and skip this field.
I've also considered using a ForeignKey with a Many-to-One relationship, but as far as I can tell this has the same drawbacks. Am I wrong to think that? Or are these size concerns not really an issue?
So my next idea was to make Posts an abstract class, and then create subclasses based on each Group. This is ultimately what I did. However, I found myself having to copy and paste the code over and over and change the class name each time. This felt very unPythonic to me. It was something like:
class Posts(models.Model):
timestamp = models.DateTimeField(auto_now_add=True, unique=False)
user_id = ...
#etc.
#
class Meta:
abstract = True
class GroupA(Posts):
class Meta(Posts.Meta):
db_table = 'groupa_board'
class GroupB(Posts):
class Meta(Posts.Meta):
db_table = 'groupb_board'
class GroupC...etc.
What I really was looking for was a factory function to do this for me. I tried this sort of thing:
def makeBoard(group):
class Board(Posts):
class Meta(Posts.Meta):
db_table = group
return board #note I tried with and without this line
And then I ran a simple for loop using a list of groups.
for group in groups:
makeBoard(group)
I found myself hitting a RuntimeError: conflicting models in application, and I probably deserved it. So then I figured what I need is something like:
def makeBoard(group):
class group(Posts): #***group here being a variable, not the class name
class Meta(Posts.Meta):
db_table = '%s' % group #maybe issues here too, but the table
return group #name is not that important if the class
#name works
But I couldn't figure out how to make this work! Is there a way to pass a variable from a list to a class name?
Anyway if you're still with me I appreciate it. I've been on stackoverflow all day and while I've found guides for creating abstract base classes and subclasses to solve similar issues, I didn't see a way to create a function to do this for me. I ultimately punted here and just make a subclass for each group by hand. If there is a way to automate this process, I'd love to hear it.
Also, if I'm being stupid for not just going with one db table containing every post, I'd like to know that too, and why! Or if there's a better way to implement this kind of system altogether. I apologize if this has been answered before, I really couldn't find it.
Thank you!

Using a single table would not be bad practice. The extra memory is minimal, on modern systems that shouldn't be a problem. You shouldn't worry about performance either, premature optimization (not including the actual system design) is considered bad practice, but if you run into performance problems you can always specify an index on the group column:
group = models.CharField(max_length=100, db_index=True)
That's not to say that it is the best option, or that your method isn't good. Also, it is entirely possible to dynamically create models, using the type() built-in function. The only difference with dynamically creating models and creating other classes is that you must specifically pass the __module__ attribute. You can create subclasses for Posts in the following way:
def fabric(names, baseclass=Posts):
for name in names:
class Meta:
db_table = '%s_table' % name.lower()
attrs = {'__module__': baseclass.__module__, 'Meta': Meta}
# specify any other class attributes here. E.g. you can specify extra fields:
attrs.update({'my_field': models.CharField(max_length=100)})
newclass = type(str(name), (baseclass,), attrs)
globals()[name] = newclass
fabric(['GroupA', 'GroupB', 'GroupC', etc...])
Put that code in your models.py after your Posts class, and all classes will be created for you. They can be used in any way normal classes can be used: Django doesn't even know you dynamically created this class. Though your Meta class doesn't inherit from Posts.Meta, your meta settings should still be preserved.
Tested with Django 1.4.

Try smth like this
import app.models as group_models
from django.db.models.base import ModelBase
def fabric(group):
for item in dir(group_models):
c = getattr(group_models, item)
if type(c) is ModelBase:
if c._meta.db_table == '%s_table' % group:
return c
return None

Creating a multi-model in Django

I'd like to create a directed graph in Django, but each node could be a separate model, with separate fields, etc.
Here's what I've got so far:
from bannergraph.apps.banners.models import *
class Node(models.Model):
uuid = UUIDField(db_index=True, auto=True)
class Meta:
abstract = True
class FirstNode(Node):
field_name = models.CharField(max_length=100)
next_node = UUIDField()
class SecondNode(Node):
is_something = models.BooleanField(default=False)
first_choice = UUIDField()
second_choice = UUIDField()
(obviously FirstNode and SecondNode are placeholders for the more domain-specific models, but hopefully you get the point.)
So what I'd like to do is query all the subclasses at once, returning all of the ones that match. I'm not quite sure how to do this efficiently.
Things I've tried:
Iterating over the subclasses with queries - I don't like this, as it could get quite heavy with the number of queries.
Making Node concrete. Apparently I have to still check for each subclass, which goes back to #1.
Things I've considered:
Making Node the class, and sticking a JSON blob in it. I don't like this.
Storing pointers in an external table or system. This would mean 2 queries per UUID, where I'd ideally want to have 1, but it would probably do OK in a pinch.
So, am I approaching this wrong, or forgetting about some neat feature of Django? I'd rather not use a schemaless DB if I don't have to (the Django admin is almost essential for this project). Any ideas?

The InheritanceManager from django-model-utils is what you are looking for.
You can iterate over all your Nodes with:
nodes = Node.objects.filter(foo="bar").select_subclasses()
for node in nodes:
#logic

Django filter many-to-many with contains

I am trying to filter a bunch of objects through a many-to-many relation. Because the trigger_roles field may contain multiple entries I tried the contains filter. But as that is designed to be used with strings I'm pretty much helpless how i should filter this relation (you can ignore the values_list() atm.).
This function is attached to the user profile:
def getVisiblePackages(self):
visiblePackages = {}
for product in self.products.all():
moduleDict = {}
for module in product.module_set.all():
pkgList = []
involvedStatus = module.workflow_set.filter(trigger_roles__contains=self.role.id,allowed=True).values_list('current_state', flat=True)
My workflow model looks like this (simplified):
class Workflow(models.Model):
module = models.ForeignKey(Module)
current_state = models.ForeignKey(Status)
next_state = models.ForeignKey(Status)
allowed = models.BooleanField(default=False)
involved_roles = models.ManyToManyField(Role, blank=True, null=True)
trigger_roles = models.ManyToManyField(Role, blank=True, null=True)
Though the solution might be quiet simple, my brain won't tell me.
Thanks for your help.

Have you tried something like this:
module.workflow_set.filter(trigger_roles__in=[self.role], allowed=True)
or just if self.role.id is not a list of pks:
module.workflow_set.filter(trigger_roles__id__exact=self.role.id, allowed=True)

The simplest approach to achieve this would be checking for equalty over the whole instance (instead of the id) in the ManyToManyField. That looks if the instance is inside the many to many relationship. Example:
module.workflow_set.filter(trigger_roles=self.role, allowed=True)

I know this is an old question, but it looks like the OP never quite got the answer he was looking for. If you have two sets of ManyToManyFields you want to compare, the trick is to use the __in operator, not contains. So for example if you have an "Event" model with a ManyToMany to "Group" on field eventgroups, and your User model (obviously) attaches to Group, you can query like this:
Event.objects.filter(eventgroups__in=u.groups.all())

singularity is almost right with the first example. You just need to make sure it's a list. The second example, checking the trigger_roles__id__exact is a better solution though.
module.workflow_set.filter(trigger_roles__in=[self.role.id],allowed=True)