Django ORM - call queryset update() in json field when the key to update is not present
2022-05-31
PostgresDjango
2022-05-31
During the development of web applications, working with APIs is a common task. However, sometimes APIs might not be fully developed or available, which can hinder the development process. To overcome this challenge, developers often resort to mocking APIs to simulate their behavior. One popular tool for achieving this is json-server. In this article, we'll walk you through the process of mocking an API using json-server and provide an example of how to create a mock API with custom data.
json-server globally using npm:npm install -g json-server
db.json with the following content:{
"products": [
{
"id": 1,
"name": "Mock Product 1",
"price": 19.99
},
{
"id": 2,
"name": "Mock Product 2",
"price": 29.99
}
]
}
Save this file in your project directory.
db.json, and start the mock server by running:json-server --watch db.json
The mock server will start at http://localhost:3000.
http://localhost:3000/products.The example above showcased a simple mock API for products. You can customize the data and structure of the API according to your application's needs. Add more endpoints, nested data, or different types of data to match your requirements.
json-server is a valuable tool for developers to create mock APIs that simulate real API behavior. It allows you to continue developing frontend components without waiting for backend APIs to be fully implemented. By following the steps outlined in this article, you can quickly set up a mock API for testing and development purposes. This approach enhances productivity and ensures a smooth development process, even when working with incomplete or unavailable APIs. Happy mocking!
Django is a popular and powerful web framework for Python that has become the go-to choice for many web developers. However, one common performance problem that can occur when using Django is the "n+1" issue. This problem can arise when querying a large amount of data, leading to increased loading times and decreased performance. In this blog post, we'll explore what the n+1 issue is, why it occurs, and how to solve it.
The n+1 issue occurs when querying a large amount of data from a database. Consider the following example: you have a model that represents a blog post, and you want to retrieve a list of all blog posts along with the author of each post. To do this, you might run the following code:
posts = BlogPost.objects.all()
for post in posts:
author = post.author
While this code will retrieve all the information you need, it can be slow and inefficient when dealing with large amounts of data. The reason for this is that it will execute a separate SQL query for each post to retrieve the author information. This can lead to an excessive number of SQL queries and slow down your application.
The n+1 issue occurs because Django does not retrieve all the data for a relationship in a single query. Instead, it retrieves data for each relationship as it is needed. In the example above, this means that Django will execute a separate SQL query for each post to retrieve the author information. This can lead to a large number of SQL queries and slow down your application.
There are several ways to solve the n+1 issue in Django, and the best solution depends on your specific use case. Here are some common solutions:
select_related methodThe select_related method is a convenient way to retrieve related data in a single query. You can use it to retrieve the author information for all posts in a single query like this:
posts = BlogPost.objects.all().select_related('author')
for post in posts:
author = post.author
prefetch_related methodThe prefetch_related method is similar to select_related, but it is more flexible and can be used for more complex relationships. You can use it to retrieve the author information for all posts in a single query like this:
posts = BlogPost.objects.all().prefetch_related('author')
for post in posts:
author = post.author
The n+1 issue is a common performance problem that can occur when using Django. By understanding what the n+1 issue is, why it occurs, and how to solve it, you can ensure that your Django applications run smoothly and efficiently.
This content was generated by ChatGPT at 9th February 2023 with the following questions:
The original post had a solution number 3 that looked like this:
Use the in operator: The in operator can be used to retrieve multiple records in a single query. You can use it to retrieve the author information for all posts in a single query like this:
posts = BlogPost.objects.all()
author_ids = [post.author_id for post in posts]
authors = Author.objects.filter(id__in=author_ids)
I told ChatGPT that this is not a solution for the issue and it answered this:
"Thank you for pointing that out. You're correct, using the in operator as I described in Solution 3 is not a direct solution for the n+1 issue. It is more of a workaround that can be used to reduce the number of SQL queries, but it doesn't address the root cause of the problem."
I learnt today that this is not possible with the Django ORM to call update in a queryset json field when the key that you want to update is not present. For example:
queryset.update(json_data__key="something")
This will return FieldDoesNotExist: ModelClass has no field named 'json_data__key' because some objects does not have the key “key”.
I found in this stackoverflow post a great solution for this.
from django.db.models.expressions import Func
class JsonSetValue(Func):
function = "jsonb_set"
template = "%(function)s(%(expressions)s, '{\"%(keyname)s\"}','\"%(new_value)s\"', %(create_missing)s)"
arity = 1
def __init__(
self,
expression: str,
keyname: str,
new_value: str,
create_missing: bool = False,
**extra,
):
super().__init__(
expression,
keyname=keyname,
new_value=new_value,
create_missing="true" if create_missing else "false",
**extra,
)
With help of JsonSetValue you will be able to do the following:
queryset.update(
json_data=JsonSetValue(
"json_data",
keyname="key",
new_value="something",
create_missing=True,
)
)