How to Display City, State, and Country Details from Multiple Tables Using select_related() in Django?

Joining Multiple Tables to Show City, State, and Country in Django

Introduction

Database queries often involve joining multiple tables to access related data. In Django, such joins can be achieved using the select_related() method. This article demonstrates how to use select_related() to perform an inner join in Django and display the name of the city, state, and country associated with a publication.

Connecting Tables in Models

The models.py file defines the database model classes for the different tables:

<code class="python">class Country(models.Model):
    country_name = models.CharField(max_length=200, null=True)

class CountryState(models.Model):
    state_name = models.CharField(max_length=200, null=True)
    country = models.ForeignKey(Country, on_delete=models.CASCADE, null=True)

class City(models.Model):
    city_name = models.CharField(max_length=200, null=True)
    countrystate = models.ForeignKey(CountryState, on_delete=models.CASCADE, null=True)

class Publication(models.Model):
    country = models.ForeignKey(Country, on_delete=models.CASCADE, null=True)
    countrystate = models.ForeignKey(CountryState, on_delete=models.CASCADE, null=True)
    city = models.ForeignKey(City, on_delete=models.CASCADE, null=True)</code>

Fetching Publications in a View

The publications() view in views.py fetches publications:

<code class="python">def publications(request):
    mypublications = publication.objects.filter(user_id=request.session['account_id'])
    return render(request, 'blog/mypublications.html', {'plist': mypublications})</code>

Performing Inner Join with select_related()

To perform the inner join, you can use the select_related() method as follows:

<code class="python">pubs = publication.objects.select_related('country', 'country_state', 'city')</code>

This line joins the Publication table with the Country, CountryState, and City tables and pre-loads them for later use.

Accessing Related Data

Once the join has been performed, you can access the related data through the model objects. For example, in a template:

<code class="html">{% for p in pubs %}
     {{ p.city.city_name}}  # p.city has been populated in the initial query
     # ...
{% endfor %}</code>

Benefits of Using select_related()

• Reduced number of database queries.
• Improved performance for related object access.
• Simplicity and readability compared to raw SQL joins.

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