[Answer]-Model.objects.only("columnname") doesn't work. It shows me everything

Using only or its counterpart defer does not prevent accessing the deferred attributes. It only delays retrieval of said attributes until they are accessed. So take the following:

for theme in Theme.objects.all():
    print theme.name
    print theme.other_attribute

This will execute a single query when the loop starts. Now consider the following:

for theme in Theme.objects.only('name'):
    print theme.name
    print theme.other_attribute

In this case, the other_attribute is not loaded in the initial query at the start of the loop. However, it is added to the model’s list of deferred attributes. When you try to access it, another query is executed to retrieve the value of other_attribute. In the second case, a total of n+1 queries is executed for n Theme objects.

The only and defer methods should only ever be used in advanced use-cases, after the need for optimization arises, and after proper analysing of your code. Even then, there are often workarounds that work better than deferring fields. Please read the note at the bottom of the defer documentation.