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Queryset API #

• get(pk=<num>) will return one object of the model instance
• filter, exclude will return querysets
• Querysets are sliceable like <ModelName>.objects.all()[:5]
• update takes keyword args to update a model
• All queryset methods, update, save, etc. except delete are also exposed to the model manager to act on all objects
• selected_related() follows foreign key relationships
• values_list returns the values of a query as a list of tuples (unless called with one argument and flat=true then it will be a normal list)
• values returns a queryset that returns dictionaries instead of objects
• Both values and values_list are used when you do not need the overhead of creating model instances
• using(<alias>) controls what databases the queryset is executed against

Field Lookups #

These are predefined keyword arguments to queryset filter methods to help filter the data. They generally take the form field_name__lookuptype=value e.g. Posts.objects.all().filter(pub_date__lte='2016-01-01') for less than or equal to.

Alternatively if you don’t have a lookup type you can just do field_name=value. This is just synatactic sugar for the default field_name__exact=value.

Relationship spanning #

Filtering can also span relationships. For example, consider an Entry model with a foreign key to a Blog model with a name field. You can filter with Entry.objects.all().filter(blog__name=value)

Related Name #

Related names are set on the model defining a foreign key field with related_name=<related_name>. This can be then used to essentially reverse query from the original model all the model instances with foreign keys references to it. Take this example:

class Department(models.model):
    description = models.TextField()


class Employee(models.model):
    name = models.charField()
    department = models.ForeignKey(
        'Department',
        on_delete=models.CASCADE,
        related_name="employees"
    )

• With related_name you can reverse the foreign key relationships, accessing Department.employees
• Without related_name the default reverse accessor is<Model>.<lowercase_fk_model>_set. E.g. department.employee_set

select_related #

Select related is a way to pre-populate foreign key relationship data in one query instead of grabbing the initial model, then getting the foreign key representation of it. This accomplishes the same thing as normal indexing but in one less database hit. While that does not make a ton of sense, an example will clear it up

post = Post.objects.get(id=2) # Hits database to get post
user = post.user # Hits database again to get user

post = Post.objects.select_related('users').get(id=2) #
user = post.user # Doesn't hit the database because the post model is already populated with user because of select_related

From a department instance you can get all employees with a foreign key reference to that department with <department_model>.employees.all(). Without setting the related field you would have to do <department_model>.employee_set.all().

Q expressions #

• To do more complex filtering (with and or or) you can use Q expressions
  • | indicats an or, & an and