Q:

telling django to use an integer as a foreign key on demand

I'm new to Django and currently defining an existing database scheme as a Django model. 
I have a central repository to store values like: 
(some_qualifier_id_1, some_qualifier_id_2, measure_id, value)

The value is database-wise an integer. It can however refer to categorical data, in which case I want to link it to another table which gives me additional information like the string that's supposed to be displayed instead of the number, ordering information. 
Can I tell Django to create a link to a table using value as a foreign key sometimes?
Update: 
Using the int to get the category - yes, that's what I do. However the point of the model layer as far as I understand it now was being able to tell Django how the relations between tables are. Just using an int and using it to look up stuff would mean hacking it together without telling Django what I'm doing, which will probably mean I have to generate SQL manually instead of using the model layer at some point. Use case: Order category values by some ordering field in the Categories table.

A:

If the measure_id value is semantically a foreign key, so each value must be an index for something in the measure table, then declare it as such, and everythin will come together alone.
If the measure_id value is not a foreign key, you can annotate your resultset on demand, using
MyModel.objects.filter(**your_filters).extra(select={
  'measure_name': 
    'SELECT measure.name FROM measure WHERE mymodeltable.measure_id = measure.id'
})

Then your retrieved objects will have a 'measure_name' attribute with the joined column.

