Django Many-To-Many Relationship (category)

My goal is to add categories to my Post model. I would like to be able to later query all post by different and sometimes multiple categories.

models.py

class Category(models.Model):
    categories = (
        ('1', 'red'),
        ('2', 'blue'),
        ('3', 'black')
    )
    title = models.CharField(max_length=50, blank=True, choices=categories)

class Post(models.Model):
    text = models.CharField(max_length=500)
    category = models.ManyToManyField(Category)

forms.py

class Post_form(forms.ModelForm):

    categories = (
        ('1', 'red'),
        ('2', 'blue'),
        ('3', 'black')
    )
    category = forms.MultipleChoiceField(required=False, widget=forms.CheckboxSelectMultiple,
    choices=categories)

    class Meta:
        model = Post
        fields = (
            'text',
            'category'
        )

I'm confused on the logic of saving one obj before you can save another.

views.py

def post(request):
    if request.method == 'POST':
        form = Post_form(request.POST, request.FILES)
        if form.is_valid():
            post = form.save(commit=False)
            post.user = request.user

            p1 = Category(form.cleaned_data['category'])
            post.category.add(p1)

        return redirect('home:home')

    else:
        form = Post_form()
        args = {'form': form }
        return render(request, 'home/new_post.html', args)

error:

"<Post: Post object>" needs to have a value for field "id" before this many-to-many relationship can be used.

EDIT:

form.cleaned_data['category'] is returning a list causing the main issue. Now my problem is my choice isn't being saved into the title of the Category class, the selection number is being saved instead. Here's what i got.

def post(request):
    if request.method == 'POST':
        form = Post_form(request.POST, request.FILES)
        if form.is_valid():
            post = form.save(commit=False)
            post.user = request.user
            post.save()
            for k in form.cleaned_data['category']:
                p1 = Category(title=k)
                p1.save()
                post.category.add(p1)

SOLVED

views.py

def post(request):
    if request.method == 'POST':
        form = Post_form(request.POST, request.FILES)
        if form.is_valid():
            post = form.save(commit=False)
            post.user = request.user
            post.save()

            for k in form.cleaned_data['category']:
                selection = Category.objects.get(pk=k)
                post.category.add(selection)

            return redirect('home:home')