如何根据与另一个 model 的关系在模板视图中查询另一个 Django model

Question

I have these models

class Protocol(models.Model):
    name = models.CharField(max_length=200)

class Description(models.Model):
    name = models.CharField(max_length=200)
    protocol = models.ForeignKey(Protocol)

class Trait(models.Model):
    desc = models.CharField(max_length=200)
    protocol = models.ForeignKey(Protocol)

class State(models.Model):
    desc = models.CharField(max_length=200)
    trait = models.ForeignKey(Trait)

class Expression(models.Model):
    state = models.ForeignKey(State)
    description = models.ForeignKey(Description)

So a Protocol (eg protocol "A") consist of a certain amount of Traits (eg height, weight, color). Each Trait can have multiple States (eg low, medium, high). Then, a Description is a collection of Expressions related to a specific Protocol . For example Description #1 is made with Protocol "A" and consist of two Expressions: height -> low, weight -> high; but Trait color is not specified.

Basically, what i want to do is display in a template view all the Traits linked to the specific Protocol of the selected Description, and then the corresponding Expression which can also be empty for some Trait .

Like this:

| Trait  | Expression |
|--------+------------|
| height |            |
| weight | high       |
| color  | blue       |

Using the shell i can easily return what i need

# Select a Description
desc = Description.objects.first()

# Get the protocol
protocol = desc.protocol

# Get all traits in the selected protocol
all_traits = protocol.trait_set.all()

# Get all the expressions in this description
expressions = desc.expression_set.all()

# Print all traits within the selected protocol and their related expression (if any)
for trait in all_traits:
    print(trait.desc)
    expr = expressions.filter(state__trait_id = trait.id).first()
    print(expr.state.desc)

However, I can't figure out how to do it in a view, especially I would like continue using a DetailView, but it's not a strict requirement if not possible. Is there any way?

Thanks

Answer 1

I found a way to do this, even though I'm not completely satisfied because I feel it is not a very elegant solution.

I created a description_extras.py in a templatetags/ directory inside my app (do not forget the __init__.py to treat it as a package)

Inside description_extras.py I've created a custom filter, like this:

from django import template

register = template.Library()

@register.filter
def expr_trait(expression,trait_id):
    return expression.filter(state__trait_id = trait_id).first()

Then in the template, I load the custom filter with

{% load description_extras %}

And then I can call the filter like this:

{% for trait in description.protocol.trait_set.all %}
    <tr>
        <td>{{ trait.desc }}</td>
        <td>{{ description.expression_set.all|expr_trait:trait.id }}</td>
    </tr>
{% endfor %}