Resource Representation
#######################

PostgREST uses proper HTTP content negotiation (`RFC7231 <https://datatracker.ietf.org/doc/html/rfc7231#section-5.3>`_) to deliver a resource representation.
That is to say the same API endpoint can respond in different formats like JSON or CSV depending on the request.

.. _res_format:

Response Format
===============

Use the Accept request header to specify the acceptable format (or formats) for the response:

.. code-block:: bash

  curl "http://localhost:3000/people" \
    -H "Accept: application/json"

.. note::
  
  The ordering of columns in the response isn't guaranteed to align with the order specified in the ``select`` clause. For example, with resource embedding:

  .. code-block:: bash
    
    http://localhost:3000/films?select=directors(last_name,id),title
  
  We may get:

  .. code-block:: bash

    [
      {
        "title": "title",
        "directors": {
          "id": 5,
          "last_name": "name"
        }
      }
    ]

  This is in line with the `JSON schema spec <https://json-schema.org/draft/2020-12/json-schema-core#name-instance-data-model>`_:

    *"object: An unordered set of properties mapping a string to an instance"*

.. _builtin_media:

Builtin Media Type Handlers
===========================

Builtin handlers are offered for common standard media types.

* ``text/csv`` and ``application/json``, for all API endpoints. See :ref:`tables_views` and :ref:`functions`.
* ``application/openapi+json``, for the root endpoint. See :ref:`open-api`.
* ``application/geo+json``, see :ref:`ww_postgis`.
* ``*/*``, resolves to ``application/json`` for API endpoints and to ``application/openapi+json`` for the root endpoint.

The following vendor media types handlers are also supported.

* ``application/vnd.pgrst.plan``, see :ref:`explain_plan`.
* ``application/vnd.pgrst.object`` and ``application/vnd.pgrst.array``, see :ref:`singular_plural` and :ref:`stripped_nulls`.

Any unrecognized media type will throw an error.

.. code-block:: bash

  curl "http://localhost:3000/people" \
    -H "Accept: unknown/unknown"

.. code-block:: http

  HTTP/1.1 415 Unsupported Media Type

  {"code":"PGRST107","details":null,"hint":null,"message":"None of these media types are available: unknown/unknown"}

To extend the accepted media types, you can use :ref:`custom_media`.

.. _singular_plural:

Singular or Plural
------------------

By default PostgREST returns all JSON results in an array, even when there is only one item. For example, requesting :code:`/items?id=eq.1` returns

.. code:: json

  [
    { "id": 1 }
  ]

This can be inconvenient for client code. To return the first result as an object unenclosed by an array, specify :code:`vnd.pgrst.object` as part of the :code:`Accept` header

.. code-block:: bash

  curl "http://localhost:3000/items?id=eq.1" \
    -H "Accept: application/vnd.pgrst.object+json"

This returns

.. code:: json

  { "id": 1 }

When a singular response is requested but no entries are found, the server responds with an error message and 406 Not Acceptable status code rather than the usual empty array and 200 status:

.. code-block:: json

  {
    "code": "PGRST116",
    "message": "Cannot coerce the result to a single JSON object",
    "details": "The result contains 0 rows",
    "hint": null
  }

.. note::

  Many APIs distinguish plural and singular resources using a special nested URL convention e.g. `/stories` vs `/stories/1`. Why do we use `/stories?id=eq.1`? The answer is because a singular resource is (for us) a row determined by a primary key, and primary keys can be compound (meaning defined across more than one column). The more familiar nested urls consider only a degenerate case of simple and overwhelmingly numeric primary keys. These so-called artificial keys are often introduced automatically by Object Relational Mapping libraries.

  Admittedly PostgREST could detect when there is an equality condition holding on all columns constituting the primary key and automatically convert to singular. However this could lead to a surprising change of format that breaks unwary client code just by filtering on an extra column. Instead we allow manually specifying singular vs plural to decouple that choice from the URL format.

.. _stripped_nulls:

Stripped Nulls
--------------

By default PostgREST returns all JSON null values. For example, requesting ``/projects?id=gt.10`` returns

.. code:: json

  [
    { "id": 11, "name": "OSX",      "client_id": 1,    "another_col": "val" },
    { "id": 12, "name": "ProjectX", "client_id": null, "another_col": null },
    { "id": 13, "name": "Y",        "client_id": null, "another_col": null }
  ]

On large result sets, the unused keys with ``null`` values can waste bandwidth unnecessarily. To remove them, specify ``nulls=stripped`` as a parameter of ``application/vnd.pgrst.array``:

.. code-block:: bash

  curl "http://localhost:3000/projects?id=gt.10" \
    -H "Accept: application/vnd.pgrst.array+json;nulls=stripped"

This returns

.. code:: json

  [
    { "id": 11, "name": "OSX", "client_id": 1, "another_col": "val" },
    { "id": 12, "name": "ProjectX" },
    { "id": 13, "name": "Y"}
  ]

.. _req_body:

Request Body
============

The server handles the following request body media types:

* ``application/json``
* ``application/x-www-form-urlencoded``
* ``text/csv``

For :ref:`tables_views` this works on ``POST``, ``PATCH`` and ``PUT`` methods. For :ref:`functions`, it works on ``POST`` methods.

For functions there are three additional types:

* ``application/octet-stream``
* ``text/plain``
* ``text/xml``

See :ref:`function_single_unnamed`.