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96 lines
3.6 KiB
ReStructuredText
96 lines
3.6 KiB
ReStructuredText
.. meta::
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:description: Set default field values using role-based column presets
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:keywords: hasura, docs, schema, default value, role-based, column preset
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.. _column_presets:
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Setting default values for fields using role-based column presets
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=================================================================
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.. contents:: Table of contents
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:backlinks: none
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:depth: 1
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:local:
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Let's say you want certain fields to have their values set automatically when not explicitly passed using session
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variables or fixed values when a new row is created with a particular :ref:`user role <roles_variables>`.
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Hasura GraphQL engine's column presets let you define role-based default values for any field/column. These values
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can either be a session variable value or a static value.
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.. admonition:: Column preset restricts mutation access for configured role
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If a column has a preset defined for a given role, access to the column for mutations will be restricted for users
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with that role.
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**Example:** Say we have a field ``user_id`` in a table ``article`` which is to be set to the id of the user, from
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the value of the user's session variable whenever a new row is added to the ``article`` table.
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Step 1: Configure a column preset
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---------------------------------
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The column preset option is available under the ``Permissions`` tab of a table. Open the console and head to
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``Data -> article -> Permissions``:
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.. thumbnail:: /img/graphql/manual/schema/column-presets-option.png
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:alt: Add a column preset in the permissions tab
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Enable the column preset option to define presets for one or more columns. For each column, you can pick between
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setting the preset using a static value or from a session variable.
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.. thumbnail:: /img/graphql/manual/schema/column-presets-value-options.png
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:alt: Configure the column preset
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For our chosen example, we'll use the ``from session variable`` option and configure the ``user_id`` column to be
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automatically populated based on the value of the ``X-Hasura-User-Id`` session variable.
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.. note::
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To set a column preset for a nested object's column, simply set the corresponding column preset in the remote
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table.
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Step 2: Run an insert mutation
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------------------------------
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Head to the GraphiQL interface in the console and try making an insert mutation on the ``article`` table with the
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following headers (*to run through this example, don't forget to also grant the* ``user`` *role sufficient permissions
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to select from the* ``article`` *table*):
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- ``X-Hasura-Role`` --> ``user`` (*to test the behaviour for the configured role*)
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- ``X-Hasura-User-Id`` --> ``1`` (*this is the value we should expect in the* ``user_id`` *field*)
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As mentioned earlier, you'll notice when you add the ``X-Hasura-Role`` header that the field, ``user_id``, is no longer
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available as the mutation type's field:
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.. thumbnail:: /img/graphql/manual/schema/column-preset-schema-change-for-role.png
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:alt: Write an insert mutation
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Now, if we run the following insert mutation, we'll see that the ``user_id`` field is indeed being set with the value
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passed in the ``X-Hasura-User-Id`` variable:
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.. thumbnail:: /img/graphql/manual/schema/column-preset-mutation-result.png
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:alt: Run the insert mutation
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.. note::
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Not passing the configured header will result in a run-time error:
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.. code-block:: JSON
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{
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"errors": [
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{
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"path": "$",
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"error": "\"x-hasura-user-id\" header is expected but not found",
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"code": "not-found"
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}
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]
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}
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Also see
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--------
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- :ref:`postgres_defaults`
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- :ref:`sql_functions_as_default`
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