graphql-engine/docs/graphql/manual/schema/relationships/database-modelling/one-to-one.rst

Modelling one-to-one table relationships
========================================

A ``one-to-one`` relationship between two tables can be established via a **unique foreign-key constraint**.

Say we have the following two tables in our database schema:

.. code-block:: sql

  author (
    id INT PRIMARY KEY,
    name TEXT
  )

  passport_info (
    id INT PRIMARY KEY,
    owner_id INT
    passport_number TEXT
    ...
  )

These two tables are related via a ``one-to-one`` relationship. i.e:

- an ``author`` can have one ``passport_info``
- a ``passport_info`` has one ``owner``

This ``one-to-one`` relationship can be established in the database by:

1. Adding a **foreign-key constraint** from the ``passport_info`` table to the ``author`` table using the ``owner_id``
   and ``id`` columns of the tables respectively
2. Adding a **unique constraint** to the ``owner_id`` column for the ``passport_info`` table


This will ensure that the value of ``owner_id`` column in ``passport_info`` table  is present in the ``id`` column of
the ``author`` table and there will be only one row with a particular ``owner_id``.

To access the nested objects via the GraphQL API, :doc:`create the following relationships <../create>`:

- Object relationship, ``passport_info`` from ``author`` table using  ``id -> passport_info :: owner_id``
- Object relationship, ``owner`` from ``passport_info`` table using ``owner_id -> author :: id``

.. note::

  Currently the console suggests an array relationship from ``author`` to ``passport_info`` table as it doesn't
  detect the unique constraint on ``owner_id``. This will be fixed soon. For now, you can :ref:`create the
  object relationship manually <relationships-without-fkey>` without using the foreign-key.

We can now:

- fetch a list of authors with their ``passport_info``:

  .. graphiql::
    :view_only:
    :query:
        query {
          author {
            id
            name
            passport_info {
              id
              passport_number
            }
          }
        }
    :response:
      {
        "data": {
          "author": [
            {
              "id": 1,
              "name": "Justin",
              "passport_info": {
                "id": 1,
                "passport_number": "987456234"
              }
            },
            {
              "id": 2,
              "name": "Beltran",
              "passport_info": {
                "id": 2,
                "passport_number": "F0004586"
              }
            }
          ]
        }
      }


- fetch a list of passport_info with their ``owner``:

  .. graphiql::
    :view_only:
    :query:
        query {
          passport_info {
            id
            passport_number
            owner {
              id
              name
            }
          }
        }
    :response:
      {
        "data": {
          "passport_info": [
            {
              "id": 1,
              "passport_number": "987456234",
              "owner": {
                "id": 1,
                "name": "Justin"
              }
            },
            {
              "id": 2,
              "passport_number": "F0004586",
              "owner": {
                "id": 2,
                "name": "Beltran"
              }
            }
          ]
        }
      }