Django dynamic filter based on variable (str)

Question

I am writing a function that can return table items based on a dynamically inserted model. So I need to refrain from using the model name in the function. Now I sometimes will have a many-to-many relationship with another model and want to select on that.

Say I have a projects = ManyToMany(Projects) field in my Files model. Now I can use:

filter(projects__in=[1])

to select for files related to projects with id 1, but I need the projects part of this filter to be dynamic, so I can use something like:

a_field = 'projects'
filter(a_field+'__in'=[1])

which obviously doesn't work. How should I do this?

I can see the similarity with the 'duplicate', but as a beginner, and related to many-to-many relationship, I think it's useful anyway. — Niels, Sep 19 '18 at 15:06
The answers being exactly what I was looking for by the way! — Niels, Sep 19 '18 at 15:06
No criticism at all :) the linked content of the dupe can be valuable for other users. — user2390182, Sep 19 '18 at 15:08

score 4 · Accepted Answer · answered Sep 19 '18 at 14:57

You can do this by using the **kwargs in Python: you construct a dictionary that maps string (the name of the parameters), to values, and then call the function with a dictionary with two asterisks, like:

a_field = 'projects'
params = {a_field+'__in': [1]}
filter(**params)

we of course need to construct the dictionary on a separate line:

a_field = 'projects'
filter(**{a_field+'__in': [1]})

If you thus have a dictionary like {'foo': 'bar', 'qux': 14}, then calling it with foo(**{'foo': 'bar', 'qux': 14}) is equivalent to foo(foo='bar', qux=14). This technique is not specific towards Django, but to Python. Python also has a way to do this with positional parameters: foo(*[1, 'a', 4]) is equivalent to foo(1, 'a', 4).

Django dynamic filter based on variable (str)

1 Answers1