Jinja2 UndefinedError - HTML Function call not working

Question

I am working on a Grocery-Store web application using Flask and SQLAlchemy I am trying to print the products as a card arranged under their respective categories But then I get this error. I can't seem to figure out what the error is. All the functions and variables seem to be declared perfectly.

Here's the block of code of app.py

return Product.query.filter_by(category_id=category_id).all()
@app.route('/')
def index():
categories = Category.query.all()
products = Product.query.order_by(Product.id.desc()).limit(10).all()
return render_template('index.html', categories=categories, products=products)

The block of code in index.html


    {% extends "base.html" %}
    
    {% block content %}
    <div class="container mt-5">
        {% for category in categories %}
        <h2>{{ category.name }}</h2>
        <div class="row">
            {% set category_products = get_products_by_category(category.id) %}
            {% for product in category_products %}
                        <div class="col-md-4 mb-4">
                        <div class="card">
                            <div class="card-body">
                                <h5 class="card-title">{{ product.name }}</h5>
                                <p class="card-text">Price: {{ product.price }}</p>
                                {% if 'user_id' in session %}
                                    <form action="{{ url_for('add_to_cart', product_id=product.id) }}" method="post">
                                        <button class="btn btn-primary" type="submit">+</button>
                                    </form>
                                    <form action="{{ url_for('remove_from_cart', product_id=product.id) }}" method="post">
                                        <button class="btn btn-danger" type="submit">-</button>
                                    </form>
                                {% else %}
                                    <p><a href="{{ url_for('login') }}">Log in</a> to add products to your cart</p>
                                {% endif %}
                            </div>
                        </div>
                    </div>
                {% endfor %}
            </div>
        {% endfor %}
    </div>
    
    {% endblock %}

This is the traceback:

return self.wsgi_app(environ, start_response)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\flask\app.py", line 2193, in wsgi_app
response = self.handle_exception(e)
           ^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\flask\app.py", line 2190, in wsgi_app
response = self.full_dispatch_request()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\flask\app.py", line 1486, in full_dispatch_request
rv = self.handle_user_exception(e)
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\flask\app.py", line 1484, in full_dispatch_request
rv = self.dispatch_request()
     ^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\flask\app.py", line 1469, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "c:\Users\Admin\Desktop\Ananyaa\MAD1\app.py", line 71, in index
return render_template('index.html', categories=categories, products=products)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\flask\templating.py", line 151, in render_template
return _render(app, template, context)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\flask\templating.py", line 132, in _render
rv = template.render(context)
     ^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\jinja2\environment.py", line 1301, in render
self.environment.handle_exception()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\jinja2\environment.py", line 936, in handle_exception
raise rewrite_traceback_stack(source=source)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "c:\Users\Admin\Desktop\Ananyaa\MAD1\template\index.html", line 11, in top-level template code
{% extends "base.html" %}
File "c:\Users\Admin\Desktop\Ananyaa\MAD1\template\base.html", line 28, in top-level template code
{% block content %}
File "c:\Users\Admin\Desktop\Ananyaa\MAD1\template\index.html", line 18, in block 'content'
{% set category_products = get_products_by_category(category.id) %}
File "C:\Users\Admin\AppData\Local\Programs\Python\Python311\Lib\site-packages\jinja2\utils.py", line 83, in from_obj
if hasattr(obj, "jinja_pass_arg"):
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
jinja2.exceptions.UndefinedError: 'get_products_by_category' is undefined```

Can someone help me point out the error?

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community, Aug 11 '23 at 18:50

Detlef · Answer 1 · 2023-08-11T22:23:11.270

The function get_products_by_category is not known within jinja.
It is not possible to use a defined function in Jinja without assigning it to the Jinja environment globals or passing it to the template via render_template.

app.jinja_env.globals.update(get_products_by_category=get_products_by_category)

In your example, however, I think it makes more sense to define a relationship between the two database models Category and Product beyond the use of a foreign key.
The following example shows you the possibility of using relationship in combination with the backref attribute to make the products of the category accessible.

class Product(db.Model):
    __tablename__ = 'products'

    id = db.Column(db.Integer, primary_key=True)
    
    # ...

    category_id = db.Column(db.Integer, db.ForeignKey('categories.id'))
    category = db.relationship('Category', backref='products')

class Category(db.Model):
    __tablename__ = 'categories'

    id = db.Column(db.Integer, primary_key=True)

    # ...

The iteration within the template would then look like this.

<div class="container mt-5">
    {% for category in categories -%}
    <!-- ... -->
    <div class="row">
        {% for product in category.products -%}
        {# Use the respective product of the category here. #}
        <!-- ... -->
        {% endfor -%}
    </div>
    {% endfor -%}
</div>

Jinja2 UndefinedError - HTML Function call not working

1 Answers1