Some problem with modelform save

I use modelform. All works fine. Validation is ok and I want to save my form. Before saving I need to populate user_id into my modelform so i make something like this:

if form.is_valid():

obj = form.save(commit=False)

          obj.user = request.user

          obj.save()

It seems to be ok. Form is saved to DB BUT!: I use 

categories = models.ManyToManyField(Category)

When I use the method above - categories aren't saved into DB :(

When I do form.save() - categories are added but User object not :(

Am I something missing?

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