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add docs on setting default value for columns (#437)
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Setting default values for fields | ||
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Let's say you want certain fields to have their values set automatically by Postgres if | ||
not explicitly passed. For example, an auto-incrementing id, a created_at timestamp, etc. | ||
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We can achieve this by setting a default value for the field which could either be a fixed value or a simple sql | ||
function. | ||
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**Example:** Say we have a field ``created_at`` in a table ``article`` which we would want to be set to the current | ||
timestamp whenever a new row is added to the table. | ||
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1) Modify the table | ||
------------------- | ||
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Edit the ``created_at`` field and set its Default value as the SQL function ``now()``. | ||
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Open the console and head to ``Data -> article -> Modify``: | ||
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.. image:: ../../../img/graphql/manual/schema/add-default-value.png | ||
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.. admonition:: To set an auto-incrementing default value | ||
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To set a default value as an auto-incrementing integer you first need to set up a ``sequence`` which will be the | ||
source of our default value. | ||
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Let's say we have a field called ``roll_number`` which we would like to be set by default as an auto-incremented | ||
integer. | ||
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Head to ``Data -> SQL`` and run the following SQL command to create a new sequence. | ||
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.. code-block:: SQL | ||
CREATE SEQUENCE roll_number_seq; | ||
Now set the default value of the ``roll_number`` field as ``nextval('roll_number_seq')`` | ||
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2) Run an insert mutation | ||
------------------------- | ||
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Now if you do not pass the ``created_at`` field value while running an insert mutation on the ``article`` table, its | ||
value will be set automatically by Postgres. | ||
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.. image:: ../../../img/graphql/manual/schema/default-value-response.png | ||
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.. note:: | ||
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The default value is ignored when a value is explicity set to the field. To enforce the value set in a field is the | ||
result of the defined SQL function, see: :doc:`sql-functions` |
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