Wolfgang WaltherandWolfgang Walther
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98 lines
2.9 KiB
ReStructuredText
98 lines
2.9 KiB
ReStructuredText
Casting a type to a custom JSON object
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======================================
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:author: `steve-chavez <https://github.com/steve-chavez>`_
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While using PostgREST you might have noticed that certain PostgreSQL types translate to JSON strings when you would
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have expected a JSON object or array. For example, let's see the case of `range types <https://www.postgresql.org/docs/current/rangetypes.html>`_.
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.. code-block:: postgres
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-- example taken from https://www.postgresql.org/docs/current/rangetypes.html#RANGETYPES-EXAMPLES
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create table reservations (
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room int
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, during tsrange
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);
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insert into
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reservations
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values
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(1108, tsrange('2010-01-01 14:30', '2010-01-01 15:30'));
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Here we have a column named **during** as a ``tsrange`` type, we would like to get it as JSON through PostgREST.
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.. code-block:: bash
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curl "http://localhost:3000/reservations"
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Result:
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.. code-block:: json
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[
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{
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"room":1108,
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"during":"[\"2010-01-01 14:30:00\",\"2010-01-01 15:30:00\")"
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}
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]
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The **during** value is probably not the in the format you want. We get a JSON string because by default PostgreSQL casts
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the type to JSON by using its ``text`` representation. We can change this representation to a custom JSON object by `creating a CAST <https://www.postgresql.org/docs/current/sql-createcast.html>`_ .
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To do this, first we'll define the function that will do the conversion from ``tsrange`` to ``json``.
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.. code-block:: postgres
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create or replace function tsrange_to_json(tsrange) returns json as $$
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select json_build_object(
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'lower', lower($1)
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, 'upper', upper($1)
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, 'lower_inc', lower_inc($1)
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, 'upper_inc', upper_inc($1)
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);
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$$ language sql;
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Using this function we'll create the CAST.
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.. code-block:: postgres
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create cast (tsrange as json) with function tsrange_to_json(tsrange) as assignment;
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And we'll do the request and :ref:`cast the column <casting_columns>`.
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.. code-block:: bash
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curl "http://localhost:3000/reservations?select=room,during::json"
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The result now is:
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.. code-block:: json
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[
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{
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"room":1108,
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"during":{
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"lower" : "2010-01-01T14:30:00",
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"upper" : "2010-01-01T15:30:00",
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"lower_inc" : true,
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"upper_inc" : false
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}
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}
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]
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You can use the same idea for creating custom casts for different types.
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.. note::
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If you don't want to modify casts for built-in types, an option would be to `create a custom type <https://www.postgresql.org/docs/current/sql-createtype.html>`_
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for your own ``tsrange`` and add its own cast.
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.. code-block:: postgres
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create type mytsrange as range (subtype = timestamp, subtype_diff = tsrange_subdiff);
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-- define column types and casting function analoguously to the above example
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-- ...
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create cast (mytsrange as json) with function mytsrange_to_json(mytsrange) as assignment;
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