How to fix ‘MultipleObjectsReturned’

This article presents an exploration of the ‘MultipleObjectsReturned‘ error, delineating the prevalent causes of this issue and offering prospective strategies for its resolution.

What is ‘MultipleObjectsReturned’?

In Django, this exception is raised by Django when a query returns more than one object, but only one object is expected. Django provides an MultipleObjectsReturned exception as an attribute of each class to identify the class of an object for which multiple objects were found. This allows us to catch exceptions for a particular model’s class.

The MultipleObjectsReturned exception in Django is a powerful tool for maintaining data integrity and handling exceptional cases in your application. It is commonly used when you expect a single result from a query but receive multiple records instead. By explicitly identifying the class associated with the exception, Django enables developers to pinpoint the source of the issue swiftly and apply targeted error handling.

This is a subclass of django.core.exceptions.MultipleObjectsReturned.

Example

django.models.Note.MultipleObjectsReturned: get() 
returned more than one 
Note -- it returned 3."

How to solve ‘MultipleObjectsReturned’?

Let’s go through some possible solutions that may solve this error. we have tried to cover all cases that will lead to this error.

Method 1. Make unique query

For instance, consider the following example where we have a ‘Note’ model with ‘author’ and ‘note_id’ fields. Our goal is to retrieve a single ‘Note’ object. Initially, we attempted to retrieve it using the ‘author’ field. However, since we didn’t define a unique constraint on this field, there could be multiple ‘Note’ objects with the same author name, leading to the potential of returning more than one object.

In contrast, when we retrieve ‘Note’ objects using the ‘note_id’ field, we have unique numbers assigned as primary keys, ensuring that only one object is returned. Therefore, it’s advisable to design your queries with such unique attributes to avoid encountering this issue.

Python3

class Note(models.Model):
    author = models.CharField(max_length=100)
    note_id = models.AutoField(primary_key=True, unique=True)

# May return more than one object.
Note.objects.filter(author="Suresh")
# Will only return one object.
Note.objects.get(note_id=1)

Alternatively, you can consider altering the model’s schema to ensure that each object possesses unique properties. This can be achieved by adding constraints such as NOT_NULL, primary_key, foreign_key, or check constraints. It’s important to note that ‘get()’ returns only one response, representing an exact object, while ‘filter()’ returns a queryset, potentially containing multiple objects in response.”.

Note: If you are not sure about filter and get. get( ) returns only one response i.e exact object. filter( ) returns queryset i.e multiple objects in response.

Method 2. Avoid use of duplicate names

To avoid encountering a ‘MultipleObjectsReturned‘ error when working with forms in your project, ensure that you don’t inadvertently define duplicate names for your input fields. If you assign the same name to multiple input fields, the form data will contain multiple values with that name, leading to the error. For instance:

Here, defining the same name ‘firstname‘ for two input fields will result in multiple values with the name ‘firstname‘ being sent to the backend, potentially causing issues. To prevent this error, always use unique names for your form input fields, ensuring data integrity in your project.

Python3

<form action="{% url 'some_url' %}" method="POST">
  <input name="firstname">
  <input name="firstname">
</form>

Method 3. Use methods on Query Set

The get() method is employed to retrieve a matched instance. It may return more than one object if an integrity error exists in your model. To address and resolve integrity errors, you can refer to the guidance provided in this article. However, for the purpose of this discussion, we’ll focus on achieving the desired result using the filter() method.

Here’s how you can use the filter() method to collect matching objects and retrieve the first object from the collection:

Python3

# Collects the matching objects and returns the first object from the collection
Note.objects.filter(author="Geeks").first()
                or
# Collects the matching objects and returns the last object from the collection
Note.objects.filter(author="Geeks").last()
                  or
# Here it search for the exact match.
Note.objects.get(author="Geeks")

Conclusion

It’s crucial to maintain data consistency by implementing database constraints, such as NOT_NULL, foreign keys, and check constraints, to ensure each object’s uniqueness. These measures can help prevent the ‘MultipleObjectsReturned‘ error from occurring in the first place. Always consider the specific requirements of your project and the integrity of your data model when implementing these solutions. Additionally, referring to Django’s official documentation and seeking assistance from the developer community can provide valuable insights and best practices for handling such errors effectively.