{"id":6221,"date":"2022-12-25T02:10:46","date_gmt":"2022-12-25T02:10:46","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=6221"},"modified":"2023-08-12T09:45:09","modified_gmt":"2023-08-12T09:45:09","slug":"django-one-to-many","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/django-tutorial\/django-one-to-many\/","title":{"rendered":"Django One-To-Many Relationship"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about one-to-many relationships in Django and how to use the ForeignKey to model them.<\/p>\n\n\n\n

Introduction to the Django one-to-many relationships #<\/a><\/h2>\n\n\n\n

In a one-to-many relationship, a row in a table is associated with one or more rows in another table. For example, a department may have one or more employees and each employee belongs to one department. <\/p>\n\n\n\n

The relationship between departments and employees is a one-to-many relationship. Conversely, the relationship between employees and departments is a many-to-one relationship.<\/p>\n\n\n\n

To create a one-to-many relationship in Django, you use ForeignKey<\/code>. For example, the following uses the ForeignKey<\/code> to create a one-to-many relationship between Department<\/code> and Employee<\/code> models:<\/p>\n\n\n

from<\/span> django.db import<\/span> models\n<\/span><\/span>\n<\/span><\/span>\n<\/span><\/span>class<\/span> Contact<\/span>(models.Model)<\/span>:<\/span>\n<\/span><\/span>    phone = models.CharField(max_length=50<\/span>, unique=True<\/span>)\n<\/span><\/span>    address = models.CharField(max_length=50<\/span>)\n<\/span><\/span>\n<\/span><\/span>    def<\/span> __str__<\/span>(self)<\/span>:<\/span>\n<\/span><\/span>        return<\/span> self.phone\n<\/span><\/span>\n<\/span><\/span>\n<\/span><\/span>class<\/span> Department<\/span>(models.Model)<\/span>:<\/span>\n<\/span><\/span>    name = models.CharField(max_length=255<\/span>)\n<\/span><\/span>    description = models.TextField(null=True<\/span>, blank=True<\/span>)\n<\/span><\/span>\n<\/span><\/span>    def<\/span> __str__<\/span>(self)<\/span>:<\/span>\n<\/span><\/span>        return<\/span> self.name\n<\/span><\/span>\n<\/span><\/span>\n<\/span><\/span>class<\/span> Employee<\/span>(models.Model)<\/span>:<\/span>\n<\/span><\/span>    first_name = models.CharField(max_length=100<\/span>)\n<\/span><\/span>    last_name = models.CharField(max_length=100<\/span>)\n<\/span><\/span>\n<\/span><\/span>    contact = models.OneToOneField(\n<\/span><\/span>        Contact,\n<\/span><\/span>        on_delete=models.CASCADE,\n<\/span><\/span>        null=True<\/span>\n<\/span><\/span>    )\n<\/span><\/span>\n<\/span><\/span>    department = models.ForeignKey(\n<\/span><\/mark>        Department,\n<\/span><\/mark>        on_delete=models.CASCADE\n<\/span><\/mark>    )\n<\/span><\/mark>\n<\/span><\/span>    def<\/span> __str__<\/span>(self)<\/span>:<\/span>\n<\/span><\/span>        return<\/span> f'{self.first_name}<\/span> {self.last_name}<\/span>'<\/span>\n<\/span><\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, define the Department<\/code> model class.<\/p>\n\n\n\n
Second, modify the Employee<\/code> class by adding the one-to-many relationship using the ForeignKey<\/code>:<\/p>\n\n\n
department = models.ForeignKey(\n   Department,\n   on_delete=models.CASCADE\n)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In the ForeignKey<\/code>, we pass the Department<\/code> as the first argument and the on_delete<\/code> keyword argument as models.CASCADE<\/code>.<\/p>\n\n\n\n
The on_delete=models.CASCADE<\/code> indicates that if a department is deleted, all employees associated with the department are also deleted.<\/p>\n\n\n\n
Note that you define the ForeignKey<\/code> field in the many-side of the relationship.<\/p>\n\n\n\n
python manage.py makemigrations<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Django will issue the following message:<\/p>\n\n\n
It is impossible to add a non-nullable field 'department' to employee without specifying a default. This is because the database needs something to populate existing rows.\nPlease select a fix:\n 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)\n 2) Quit and manually define a default value in models.py.\nSelect an option:<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
In the Employee<\/code> model, we define the department<\/code> field as a non-nullable field. Therefore, Django needs a default value to update the department<\/code> field (or department_id<\/code> column) for the existing rows in the database table. <\/p>\n\n\n\n
Even if the hr_employees<\/code> table doesn’t have any rows, Django also requests you to provide a default value.<\/p>\n\n\n\n
As clearly shown in the message, Django provides you with two options. You need to enter 1 or 2 to select the corresponding option.<\/p>\n\n\n\n
In the first option, Django requests a one-off default value. If you enter 1, then Django will show the Python command line:<\/p>\n\n\n
>>><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
>>> <\/span>None<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Once you provide a default value, Django will make the migrations like this:<\/p>\n\n\n
Migrations for 'hr':\n  hr\\migrations\\0002_department_employee_department.py\n    - Create model Department\n    - Add field department to employee<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
\"\"<\/figure>\n\n\n\n
In the database, Django creates the hr_department<\/code> and adds the department_id<\/code> column to the hr_employee<\/code> table. The department_id<\/code> column of the hr_employee<\/code> table links to the id<\/code> column of the hr_department<\/code> table.<\/p>\n\n\n\n
If you select the second option by entering the number 2, Django will allow you to manually define the default value for the department field. In this case, you can add a default value to the department<\/code> field in the models.py<\/code> like this:<\/p>\n\n\n
department = models.ForeignKey(\n<\/span><\/span>   Department,\n<\/span><\/span>   on_delete=models.CASCADE,\n<\/span><\/span>   default=None<\/span>\n<\/span><\/mark>)\n<\/span><\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
After that, you can make the migrations using the makemigrations<\/code> command:<\/p>\n\n\n
python manage.py makemigrations<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
It’ll show the following output:<\/p>\n\n\n
Migrations for 'hr':\n  hr\\migrations\\0002_department_employee_department.py\n    - Create model Department\n    - Add field department to employee<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
If the hr_employee<\/code> table has any rows, you need to remove all of them before migrating the new migrations:<\/p>\n\n\n
python<\/span> manage<\/span>.py<\/span> shell_plus<\/span>\n>>> Employee<\/span>.objects<\/span>.all<\/span>().delete<\/span>()<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
Then you can make the changes to the database using the migrate<\/code> command:<\/p>\n\n\n
python manage.py migrate<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Operations to perform:\n  Apply all migrations: admin, auth, contenttypes, hr, sessions\nRunning migrations:\n  Applying hr.0002_department_employee_department... OK<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Another way to deal with this is to reset migrations that we will cover in the reset migrations tutorial.<\/p>\n\n\n\n
Interacting with models #<\/a><\/h2>\n\n\n\n
First, run the shell_plus<\/code> command:<\/p>\n\n\n
python manage.py shell_plus<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create a new department with the name IT<\/code>:<\/p>\n\n\n
>>> <\/span>d = Department(name='IT'<\/span>,description='Information Technology'<\/span>)\n>>> <\/span>d.save()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, create two employees and assign them to the IT<\/code> department:<\/p>\n\n\n
>>> <\/span>e = Employee(first_name='John'<\/span>,last_name='Doe'<\/span>,department=d)\n>>> <\/span>e.save()\n>>> <\/span>e = Employee(first_name='Jane'<\/span>,last_name='Doe'<\/span>,department=d)\n>>> <\/span>e.save()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Fourth, access the department<\/code> object from the employee<\/code> object:<\/p>\n\n\n
>>> <\/span>e.department \n<Department: IT>\n>>> <\/span>e.department.description\n'Information Technology'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Fif,  get all employees of a department using use the employee_set<\/code> attribute like this:<\/p>\n\n\n
>>> <\/span>d.employee_set.all()\n<QuerySet [<Employee: John Doe>, <Employee: Jane Doe>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that we did not define the employee_set<\/code> property in the Department<\/code> model. Internally, Django automatically added the employee_set<\/code> property to the Department<\/code> model when we defined the one-to-many relationship using the ForeignKey<\/code>.<\/p>\n\n\n\n
The all()<\/code> method of the employee_set<\/code> returns a QuerySet<\/code> that contains all employees who belong to the department.<\/p>\n\n\n\n
In this case, you can use any valid value in Python e.g., None<\/a><\/code>:<\/p>\n\n\n
Third, make migrations<\/a> using the makemigrations<\/code> command:<\/p>\n\n\n