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How to create element in another model with django

I have a small question. That is actually making me scratch my head.
So in my Database, I have the following models:
Activity
Bill
Clients
I think you are all seeing the relationship I am trying to create :
A Bill has one client and one or more activities. Here is the trick to make this whole thing user-friendly I am trying to create Bills (with the url: Bill/new) that can be edited manually. So the user is sent to an HTML page with the basic Bill template and he has a table that can add some rows with the activity the time spent and its cost.
There are three things I am trying to achieve.
Generate automatically the ID of the Bill (it should be pk of Bill) but it seems it's not generated until I have pressed on save.
When I save a Bill I want to save also the activities I have entered manually.
When I save the Bill I would like to save it as a Word or PDF document in the database.
Are these possible?
Thanks all for reading and helping I am banging my head to figure out how to do all of this and I am quite a newbie so any help is welcome.
Thanks in advance.

One way would be for the Bill objects to have a boolean field in_preparation. There would be a sequence of forms involved. The first would create a minimal Bill object with in_preparation=True. Then the related objects could be created and linked to this Bill. The final stage would be to display the entire bill and related objects for checking, with options to go back and edit, or "Confirm and issue to customer". This latter would set in_preparation=False and generate the pdf and word files.
If all the necessary information is already available and you are just asking how to use a ModelForm in this circumstance, the answer is obj = form.save( commit=False). It's then up to you to call obj.save() once you have finished updating it or its related objects More information here
If you are saving a bunch of objects at once and want to be sure it's all-or-nothing (the latter if something throws an exception), you need a Django transaction.

Generate automatically the ID of the Bill (it should be pk of Bill) but it seems it's not generated until I have pressed on save.
For bills and any other "real world"-like documents you'd probably better define some custom number generator to detach number from the auto-id and to be able to generate numbers using more complex patterns (eg AB-12345/321).
But in any case whether you choose to use auto-id or a custom generator the most simple way to garanty existence and uniqueness of that id/number is to save bill instance first. Also this approach has some additional pros.
When I save a Bill I want to save also the activities I have entered manually.
You can use django formsets for this (assuming that your bill and activity models have fk or m2m relation)
https://docs.djangoproject.com/en/3.2/topics/forms/formsets/
When I save the Bill I would like to save it as a Word or PDF document in the database.
I'm not really sure if it is a good idea to store pdf or word files in db somehow.
In my opinion there are better ways
Generate pdf (word, excel etc) docs from data on request
Store as files in filesystem.
In this case files shouldn't be stored in publicly accessible
dirs (like media and static) and should be served instead with
FileResponce with proper access checks in the view

How to make a model Field take 2 values from two different drop down lists in Django

I am designing the models for a tutor page and I was wondering how to make a model field take two values from two different drop down lists and make them into one and the save that to the database.
My objective is to make an availability model field which takes the day that a tutor is available to work from one dropdown list, and the available time that he is able to work on that day from the second dropdown list, and also the ability to add more than one day that the tutor is available. Something that would return this in shell.
>>>tutor1.available_time
(('Monday','after 4pm'),('Tuesday','In the Morning'),('Friday','after 2pm'),)
>>>tutor2.available_time
(('AnyDay','after 12pm'))

If you really just want to use a single field, use a text field and store JSON encoded lists in it.
Alternatively, create a new model with two fields and make a foreign key from tutors to the new model.

Django - how to validate custom form for adding many similar entries at once

I have a model called "occurrence" which tracks occurrences of species at different sites. The model has 4 fields.
refID (foreign key to the reference source of data)
siteID (foreign key to site)
speciesID (foreign key to species)
abundance (integer)
I know I could create a model-form to add an entry. But modelforms would be tedious because, most of the time I want to enter data for dozens or hundreds of species with the same combination of siteID and refID. I have created my own data entry form in the template to select a refID and siteID, and use jQuery to add new lines for speciesID and abundance. Thus, I have a single refID + siteID combination, with many speciesIDs + abundance lines. Then, the idea is to iterate over all the added lines and save all the occurrences in the view.
The problem is that validation of this form is quite difficult, as I have to do everything "manually" in the view. This seems like it might be a common problem, so I wonder.....
Am I missing a pre-existing Django solution here?

Probably you're, as far as I understand your question.
have a look at Formsets

The simple solution to this problem turned out to be using a modelform to save a single instance at a time. I made the refID and siteID fields "sticky" by passing back the saved values for those fields to the reloaded form using the "initial" argument to the modelform. This way, I can use all the built in form validation.

Processing forms that generate many rows in DB

I'm wondering what the best approach to take here is. I've got a form that people use to register for a class and a lot of times the manager of a company will register multiple people for the class at the same time. Presently, they'd have to go through the registration process multiple times and resubmit the form once for every person they want to register.
What I want to do is give the user a form that has a single <input/> for one person to register with, along with all the other fields they'll need to fill out (Email, phone number, etc); if they want to add more people, they'll be able to press a button and a new <input/> will be generated. This part I know how to do, but I'm including it to best describe what I'm aiming to do.
The part I don't know how to approach is processing that data the form submits, I need some way of making a new row in the Registrant table for every <input/> that's added and include the same contact information (phone, email, etc) as the first row with that row. For the record, I'm using the Django framework for my back-end code.
What's the best approach here? Should it just POST the form x times for x people, or is there a less "brute force" way of handling this?

Django includes FormSet for dealing with exactly these challenges. Using a FormSet you can create multiple forms for creating or updating information. There's even possible to generate the FormSets from a Model. http://docs.djangoproject.com/en/dev/topics/forms/formsets/ and http://docs.djangoproject.com/en/dev/topics/forms/modelforms/#id1 are great resources.
Now, for creating more forms on the fly, you need some javascript magic. I've done this on work projects using jQuery which made it a lot simpler. The basic idea is create a new form with the correct inputs and change the hidden metadata in the formset form so it will now how many forms to process. The admin implements this when using multiple inline forms so I suggest looking there for code as it is a bit tricky to get right.

Django best practice for displaying mostly read-only form, one field writeable

I have a requirement where one user creates an 'instance' of an object via a ModelForm. Another user of a different group has access to read all of the fields of the form, but has to update only one field. Think of a student who creates an exam object. Then a teach pulls up the exam and just needs to put in a grade, the rest of the exam is read only.
What's the best way to do that? Should I just query for the object, and display each field individually, then create a form (not a ModelForm?) for just the one field?

Should I just query for the object, and display each field individually, then create a form (not a ModelForm?) for just the one field?
This is probably the best way to go about it. Note you can use a ModelForm for the teacher form, see the Django documentation on using a subset of fields on a model form. You will have to display all the other fields manually in your template, but you should probably have a separate template for this view (I would use separate views as well).
You could find some code for a read only field on Django Snippets, but generally it's better to be explicit about what fields you are updating from each view. This is likely to be more trouble than it's worth.