Many-to-many relationships

A many-to-many relationship is a type of cardinality that refers to the relationship between two tables A and B in which A may contain a parent instance for which there are many children in B and vice versa.

The best example of a many-to-many relationship is the relationship between students and courses. A student could have enrolled in multiple courses and a course could have multiple students enrolled.

../../../../_images/many-to-many-example.png

Lets try to create a many to many relationship between the student table and the course table:

Creating a many-to-many relationship

  1. Create a table named enrollment with student_id and course_id fields.
  2. Create a many-to-one relationship from the student and course tables to the enrollment table.
  3. Create a one-to-many relationship from the enrollment table to the student and course tables.

Fetching over a many-to-many relationship

To fetch the list of all students along with the courses they are enrolled in, the query will look something like:

GraphQL
JSON API
query fetch_students {
   student {
       id
       student_enrollments {
           course_enrolled
       }
   }
}

POST data.<cluster-name>.hasura-app.io/v1/query HTTP/1.1
Content-Type: application/json
Authorization: Bearer <auth-token> # optional if cookie is set
X-Hasura-Role: admin

{
    "type" : "select",
    "args" : {
        "table" : "student",
        "columns": [
            "*",
            {
                "name": "student_enrollments",
                "columns": [
                    "*",
                    {
                        "name": "course_enrolled",
                        "columns": ["*"]
                    }
                ]
            }
        ]
    }
}
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