{"id":6232,"date":"2022-12-25T09:00:56","date_gmt":"2022-12-25T09:00:56","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=6232"},"modified":"2023-01-18T03:22:19","modified_gmt":"2023-01-18T03:22:19","slug":"django-many-to-many","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/django-tutorial\/django-many-to-many\/","title":{"rendered":"Django Many-to-Many Relationship"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use ManyToManyField<\/code> to model a Many-to-Many relationship in Django.<\/p>\n\n\n\n

Introduction to the Django Many-to-Many relationship #<\/a><\/h2>\n\n\n\n

In a many-to-many relationship, multiple rows in a table are associated with multiple rows in another table.<\/p>\n\n\n\n

For example, an employee may have multiple compensation programs and a compensation program may belong to multiple employees.<\/p>\n\n\n\n

Therefore, multiple rows in the employee table are associated with multiple rows in the compensation table. Hence, the relationship between employees and compensation programs is a many-to-many relationship.<\/p>\n\n\n\n

Typically, relational databases do not implement a direct many-to-many relationship between two tables. Instead, it uses a third table, the join table, to establish two one-to-many relationships<\/a> between the two tables and the join table.<\/p>\n\n\n\n

The following diagram illustrates the many-to-many relationships in the database between the hr_employee and hr_compensation tables:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

The hr_employee_compensations<\/code> table is a join table. It has two foreign keys<\/a> employee_id<\/code> and compensation_id<\/code>.<\/p>\n\n\n\n

The employee_id<\/code> foreign key references the id<\/code> of the hr_employee<\/code> table and the compensation_id<\/code> foreign key references the id<\/code> in the hr_compensation<\/code> table.<\/p>\n\n\n\n

Typically, you don’t need the id<\/code> column in the hr_employee_compensations<\/code> table as a primary key<\/a> and use both employee_id<\/code> and compensation_id<\/code> as a composite primary key. However, Django always creates the id<\/code> column as a primary key for the join table. <\/p>\n\n\n\n

Also, Django creates a unique constraint<\/a> that includes the employee_id<\/code> and compensation_id<\/code> columns. In other words, there will be no duplicate pairs of employee_id<\/code> and compensation_id<\/code> values in the hr_employee_compensations<\/code> table.<\/p>\n\n\n\n

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

# ...<\/span>\n<\/span><\/span>class<\/span> Compensation<\/span>(models.Model)<\/span>:<\/span>\n<\/span><\/span>    name = models.CharField(max_length=255<\/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><\/span>        Department,\n<\/span><\/span>        on_delete=models.CASCADE\n<\/span><\/span>    )\n<\/span><\/span>\n<\/span><\/span>    compensations = models.ManyToManyField(Compensation)\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 a new Compensation<\/code> model class that extends the models.Model<\/code> class.<\/p>\n\n\n\n
Second, add the compensations<\/code> field to the Employee<\/code> class. The compensations<\/code> field uses the ManyToManyField<\/code> to establish the many-to-many relationship between the Employee<\/code> and Compensation<\/code> classes.<\/p>\n\n\n\n
To propagate the changes of the models to the database, you run the makemigrations<\/code> command:<\/p>\n\n\n
python<\/span> manage<\/span>.py<\/span> makemigrations<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
It’ll output something like this:<\/p>\n\n\n
Migrations for 'hr':\n  hr\\migrations\\0004_compensation_employee_compensations.py\n    - Create model Compensation\n    - Add field compensations to employee<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
And execute 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.0004_compensation_employee_compensations... OK  <\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Django created two new tables hr_compensation<\/code> and a join table hr_employee_compensations<\/code> as follows:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Creating data #<\/a><\/h2>\n\n\n\n
First, run the shell_plus<\/code> command:<\/p>\n\n\n
python<\/span> manage<\/span>.py<\/span> shell_plus<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create three compensation programs including Stock<\/code>, Bonuses<\/code>, and Profit Sharing<\/code>:<\/p>\n\n\n
>>> <\/span>c1 = Compensation(name='Stock'<\/span>)\n>>> <\/span>c1.save()\n>>> <\/span>c2 = Compensation(name='Bonuses'<\/span>) \n>>> <\/span>c2.save()\n>>> <\/span>c3 = Compensation(name='Profit Sharing'<\/span>)  \n>>> <\/span>c3.save()\n>>> <\/span>Compensation.objects.all()\n<QuerySet [<Compensation: Stock>, <Compensation: Bonuses>, <Compensation: Profit Sharing>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, get the employee with the first name John<\/code> and last name Doe<\/code>:<\/p>\n\n\n
>>> <\/span>e = Employee.objects.filter(first_name='John'<\/span>,last_name='Doe'<\/span>).first()\n>>> <\/span>e\n<Employee: John Doe><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Adding compensations to employees #<\/a><\/h2>\n\n\n\n
First, enroll John Doe<\/code> in the stock<\/code> (c1<\/code>) and bonuses <\/code>(c2<\/code>) compensation programs using the add()<\/code> method of the compensations<\/code> attribute and the save()<\/code> method of the Employee<\/code> object:<\/p>\n\n\n
>>> <\/span>e.compensations.add(c1)\n>>> <\/span>e.compensations.add(c2) \n>>> <\/span>e.save()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, access all compensations<\/code> program of John Doe<\/code> using the all()<\/code> method of the compensations<\/code> attribute:<\/p>\n\n\n
>>> <\/span>e.compensations.all()\n<QuerySet [<Compensation: Stock>, <Compensation: Bonuses>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
As clearly shown in the output, John Doe<\/code> has two compensation programs.<\/p>\n\n\n\n
Third, enroll Jane Doe<\/code> in three compensation programs including stock, bonuses, and profit sharing:<\/p>\n\n\n
>>> <\/span>e = Employee.objects.filter(first_name='Jane'<\/span>,last_name='Doe'<\/span>).first()\n>>> <\/span>e \n<Employee: Jane Doe>\n>>> <\/span>e.compensations.add(c1)\n>>> <\/span>e.compensations.add(c2) \n>>> <\/span>e.compensations.add(c3) \n>>> <\/span>e.save()\n>>> <\/span>e.compensations.all()\n<QuerySet [<Compensation: Stock>, <Compensation: Bonuses>, <Compensation: Profit Sharing>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Internally, Django inserted the ids of employees and compensations into the join table:<\/p>\n\n\n
 id | employee_id | compensation_id\n----+-------------+-----------------\n  1 |           5 |               1\n  2 |           5 |               2\n  3 |           6 |               1\n  4 |           6 |               2\n  5 |           6 |               3\n(5 rows)<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Fourth, find all employees who were enrolled in the stock compensation plan using the employee_set<\/code> attribute of the Compensation<\/code> object:<\/p>\n\n\n
>>> <\/span>c1\n<Compensation: Stock>\n>>> <\/span>c1.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
It returned two employees as expected.<\/p>\n\n\n\n
Fifth, you can use the employee_set<\/code> attribute to find all employees who have the profit-sharing compensation program:<\/p>\n\n\n
>>> <\/span>c3                   \n<Compensation: Profit Sharing>\n>>> <\/span>c3.employee_set.all()\n<QuerySet [<Employee: Jane Doe>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
It returned one employee.<\/p>\n\n\n\n
Django allows you to query across the relationship. For example, you can find all employees who have the  compensation with id 1:<\/p>\n\n\n
>>> <\/span>Employee.objects.filter(compensations__id=1<\/span>) \n<QuerySet [<Employee: John Doe>, <Employee: Jane Doe>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Or with the name \"Profit Sharing\"<\/code>:<\/p>\n\n\n
>>> <\/span>Employee.objects.filter(compensations__name=\"Profit Sharing\"<\/span>) \n<QuerySet [<Employee: Jane Doe>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Removing compensations from employees #<\/a><\/h2>\n\n\n\n
To remove a compensation program from an employee, you use the remove()<\/code> method of the compensations<\/code> attribute of the Employee<\/code> object. For example:<\/p>\n\n\n\n
First, get the employee whose name is Jane Doe<\/code>:<\/p>\n\n\n
>>> <\/span>e = Employee.objects.filter(first_name='Jane'<\/span>,last_name='Doe'<\/span>).first()\n>>> <\/span>e                                                                     \n<Employee: Jane Doe><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, remove the profit sharing<\/code> compensation (c3<\/code>) from Jane Doe<\/code> and save the changes to the database:<\/p>\n\n\n
>>> <\/span>e.compensations.remove(c3)\n>>> <\/span>e.save()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, get all the compensation programs of Jane Doe<\/code>:<\/p>\n\n\n
>>> <\/span>e.compensations.all()\n<QuerySet [<Compensation: Stock>, <Compensation: Bonuses>]><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, Jane Doe<\/code> has two compensation programs left.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • In a many-to-many relationship, multiple rows in a table are associated with multiple rows in another table.<\/li>\n\n\n\n
  • Relation databases use a join table to establish a many-to-many relationship between two tables.<\/li>\n\n\n\n
  • Use ManyToManyField<\/code> to model a many-to-many relationship between models in Django.<\/li>\n<\/ul>\n
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Download the Django many-to-many relationship source code<\/a>.<\/p>\n\n\n\n