Django QuerySet API (Detailed Explain)
Tags: django, webdev, tutorial, python
Published: 2023-01-28 18:29:37

In Django, a QuerySet is a collection of objects from a database model. It can be used to retrieve, update, and delete records from the database. The QuerySet API provides a set of methods for working with the data.

Some common methods of the QuerySet API include:

1. filter(): filters the queryset based on the provided keyword arguments
2. exclude(): filters the queryset to exclude items that match the provided keyword arguments
3. all(): returns all objects in the queryset
4. get(): retrieves a single object that matches the provided keyword arguments, or raises an exception if no match is found
5. create(): creates a new object and saves it to the database
6. update(): updates one or more fields on all objects in the queryset
7. delete(): deletes all objects in the queryset
8. first(): returns the first object in the queryset
9. last(): returns the last object in the queryset
10. count(): returns the number of objects in the queryset
11. exists(): returns a Boolean indicating if the queryset contains any objects.
You can chain these methods together to create more complex queries.

----

**filter()**

In Django, the filter() method of a QuerySet is used to filter the queryset based on the provided keyword arguments. The resulting queryset will contain only the objects that match the specified filters.

Here is an example of how you can use the filter() method to retrieve all objects from a database model where the published field is True:

```
from myapp.models import MyModel

# Retrieve all objects from the MyModel where the published field is True
queryset = MyModel.objects.filter(published=True)
```

You can chain multiple filters together by chaining multiple filter() methods, like this:

```
queryset = MyModel.objects.filter(published=True).filter(author='John Smith')
```

This will return all objects from the MyModel where the published field is True and the author field is John Smith.

You can also use the Q object to create more complex queries using OR or AND statements like this:

```
from django.db.models import Q

queryset = MyModel.objects.filter(Q(published=True) | Q(author='John Smith'))
```

This will return all objects from the MyModel where the published field is True or the author field is John Smith

**exclude()**

In Django, the exclude() method of a QuerySet is used to exclude objects from the queryset that match the provided keyword arguments. The resulting queryset will contain only the objects that do not match the specified filters.

Here is an example of how you can use the exclude() method to retrieve all objects from a database model where the published field is False:

```
from myapp.models import MyModel

# Retrieve all objects from the MyModel where the published field is False
queryset = MyModel.objects.exclude(published=True)
```
You can chain multiple exclude() methods together to exclude multiple conditions, like this:

```
queryset = MyModel.objects.exclude(published=True).exclude(author='John Smith')
```

This will return all objects from the MyModel where the published field is False and the author field is not John Smith.

You can also use the Q object to create more complex queries using OR or AND statements like this:

```
from django.db.models import Q

queryset = MyModel.objects.exclude(Q(published=True) | Q(author='John Smith'))
```

This will return all objects from the MyModel where the 'published' field is False or the 'author' field is not 'John Smith'


**all()**

In Django, the all() method of a QuerySet is used to retrieve all objects from a database model. It returns a queryset containing all objects in the database table that corresponds to the model.
```
from myapp.models import MyModel

# Retrieve all objects from the MyModel
queryset = MyModel.objects.all()
```

This method does not take any arguments, and it is often the starting point for creating more complex queries using other QuerySet methods such as filter(), exclude(), order_by(), limit(), etc.

Here is an example of how you can chain multiple QuerySet methods to retrieve all objects from a database model where the published field is True and order them by date_published:
```
queryset = MyModel.objects.filter(published=True).order_by('-date_published')
```

**get()**

In Django, the get() method of a QuerySet is used to retrieve a single object from a database model that matches the provided keyword arguments. The method will raise a DoesNotExist exception if no matching object is found, and a MultipleObjectsReturned exception if multiple objects are found.

Here is an example of how you can use the get() method to retrieve an object from a database model where the id field is 1:

```
from myapp.models import MyModel

# Retrieve an object from the MyModel where the id field is 1
obj = MyModel.objects.get(id=1)
```

You can also use the get() method to retrieve an object by its primary key, which is the default field named id:

```
obj = MyModel.objects.get(pk=1)
```

You can also use the get() method to retrieve an object by other fields, like this:

```
obj = MyModel.objects.get(name='John Smith')
```

It's important to note that you should use the get() method only if you know that the query will return only one object. If your query might return multiple objects, you should use filter() instead, which will return a queryset containing all matching objects.



**create()**

In Django, the create() method of a Manager (the default manager of a model) is used to create a new object and save it to the database in one step. It's similar to instantiating a model object, setting its properties, and then calling save() on it.

Here is an example of how you can use the create() method to create a new object of a model and save it to the database:

```
from myapp.models import MyModel

# create a new object of MyModel and save it to the database
obj = MyModel.objects.create(name='John Smith', age=30)
```

The create() method takes keyword arguments that correspond to the fields of the model, and it returns the new object that has been saved to the database.

The create() method also accepts a defaults parameter which is a dictionary containing default values for fields that aren't specified in the keyword arguments.

```
obj = MyModel.objects.create(name='Jane Smith', defaults={'age':25})
```

This will create an object of the model with name='Jane Smith' and age=25 and will save it to the database.

Please note that the create() method will raise a IntegrityError exception if the object could not be saved due to a violation of a database constraint, such as a unique constraint.


**update()**

In Django, the update() method of a QuerySet is used to update multiple objects in the database at once. It updates the fields of the objects that match the provided keyword arguments.

Here is an example of how you can use the update() method to update all objects in the MyModel table where the published field is False and set it to True:

```
from myapp.models import MyModel

# update the 'published' field of all objects in the MyModel table where the published field is False
MyModel.objects.filter(published=False).update(published=True)
```
The update() method takes keyword arguments that correspond to the fields of the model, and it updates the fields of all objects that match the provided keyword arguments.

You can also use the update() method to update fields of the object by mathematical operations, like this:

```
MyModel.objects.filter(published=True).update(views=F('views')+1)
```

This will increment the views field of all objects in the MyModel table where the published field is True.

Please note that the update() method doesn't call the save() method on the objects, so any custom logic you have in the save() method will not be executed when using update(). Also the update() method doesn't call the pre_save or post_save signals, so if you're depending on these signals, you should use the save() method instead.



**delete()**

In Django, the delete() method of a QuerySet is used to delete multiple objects from the database at once. It deletes all objects that match the provided keyword arguments.

Here is an example of how you can use the delete() method to delete all objects in the MyModel table where the published field is False:

```
from myapp.models import MyModel

# delete all objects in the MyModel table where the published field is False
MyModel.objects.filter(published=False).delete()
```

You can also use the delete() method on a single object, like this:

```
obj = MyModel.objects.get(pk=1)
obj.delete()
```
Please note that the delete() method doesn't call the delete() method on the objects, so any custom logic you have in the delete() method will not be executed when using delete(). Also the delete() method doesn't call the pre_delete or post_delete signals, so if you're depending on these signals, you should use the delete() method on the object instead.

Also the delete() method will raise a ProtectedError if you're trying to delete an object that is related to another object and the related object has protect=True on the foreign key, like this:

```
class Parent(models.Model):
    name = models.CharField(max_length=32)

class Child(models.Model):
    name = models.CharField(max_length=32)
    parent = models.ForeignKey(Parent, on_delete=models.PROTECT)
```

In this example, if you try to delete a Parent object that has related Child objects, it will raise a ProtectedError


**first()**

In Django, the first() method of a QuerySet is used to retrieve the first object that matches the provided filters.

Here is an example of how you can use the first() method to retrieve the first object in the MyModel table where the published field is True:

```
from myapp.models import MyModel

# retrieve the first object in the MyModel table where the published field is True
first_obj = MyModel.objects.filter(published=True).first()
```

The first() method returns the first object that matches the filters, or None if no matching object is found. If you are trying to retrieve the first object and you are not sure whether the queryset will return any results or not, you can use the first() method and check if it returns None or not.

You can also use the first() method with no parameters and it will return the first object in the table, like this:

```
first_obj = MyModel.objects.first()
```

Please note that the first() method is equivalent to [queryset.all()[0]] or [queryset[0]] but is more readable and can be more efficient with large querysets because it stops querying the database as soon as it finds the first result.

**last()**

matches the provided filters. The last() method works similarly to the first() method, but instead of returning the first object, it returns the last object.

Here is an example of how you can use the last() method to retrieve the last object in the MyModel table where the published field is True:

```
from myapp.models import MyModel

# retrieve the last object in the MyModel table where the published field is True
last_obj = MyModel.objects.filter(published=True).last()
```

The last() method returns the last object that matches the filters, or None if no matching object is found. If you are trying to retrieve the last object and you are not sure whether the queryset will return any results or not, you can use the last() method and check if it returns None or not.

You can also use the last() method with no parameters and it will return the last object in the table, like this:

```
last_obj = MyModel.objects.last()
```

Please note that the last() method is equivalent to [queryset.all()[-1]] or [queryset[-1]] but is more readable and can be more efficient with large querysets because it stops querying the database as soon as it finds the last result.

**count()**

In Django, the count() method of a QuerySet is used to retrieve the number of objects that match the provided filters.

Here is an example of how you can use the count() method to retrieve the number of objects in the MyModel table where the published field is True:

```
from myapp.models import MyModel

# retrieve the count of objects in the MyModel table where the published field is True
count = MyModel.objects.filter(published=True).count()
```

The count() method returns an integer representing the number of objects that match the filters.

You can also use the count() method with no parameters and it will return the count of all objects in the table, like this:

```
count = MyModel.objects.count()
```

Please note that the count() method is more efficient than loading all objects into memory and then counting them using Python's len() function, especially when working with large datasets. Also, it may be more efficient than using SQL COUNT() function, because it will use COUNT(*) and stop querying as soon as the count is known.

 **exists()**

In Django, the exists() method of a QuerySet is used to check if any object exists in the database that matches the provided filters.

Here is an example of how you can use the exists() method to check if any objects exist in the MyModel table where the published field is True:

```
from myapp.models import MyModel

# check if any objects exist in the MyModel table where the published field is True
exists = MyModel.objects.filter(published=True).exists()
```

The exists() method returns a Boolean indicating whether any objects exist that match the filters. It returns True if any objects exist and False if no objects exist.

You can also use the exists() method with no parameters and it will return True if there are any objects in the table, like this:

```
exists = MyModel.objects.exists()
```

Please note that the exists() method is more efficient than loading all objects into memory and then checking if the queryset is empty, especially when working with large datasets. Also, it may be more efficient than using SQL EXISTS() function, because it will use LIMIT 1 and stop querying as soon as the first object is found.


**conclusion**

In conclusion, the QuerySet API in Django is a powerful tool that allows you to retrieve, create, update and delete objects in a database. The QuerySet API provides several methods such as _filter()_, _exclude()_, _all()_, _get()_, _create()_, _update()_, _delete()_, _first()_, _last()_, _count()_, _exists()_ that can be used to perform various operations on the data.

_filter()_ method is used to retrieve objects that match specific filters, _exclude()_ method is used to retrieve objects that do not match specific filters, _all()_ method is used to retrieve all objects in the table, _get()_ method is used to retrieve a single object that matches the filters, _create()_ method is used to create a new object in the table, _update()_ method is used to update existing objects in the table, _delete()_ method is used to delete existing objects in the table. first() method is used to retrieve the first object that matches the filters, _last()_ method is used to retrieve the last object that matches the filters, _count()_ method is used to retrieve the number of objects that match the filters, _exists()_ method is used to check if any objects exist that match the filters.

These methods can be used in combination to perform more complex operations on the data. Additionally, you can chain these methods together to perform multiple operations in a single query. These methods are efficient and can be used to work with large datasets.