add docs on setting default value for columns (#437)

docs/graphql/manual/mutations/index.rst

@@ -34,7 +34,6 @@ Let's use this reference author/article schema to look at different types of mut
   Update <update>
   Delete <delete>
   multiple-mutations
-  sql-functions
 
 
 

docs/graphql/manual/schema/default-values.rst

@@ -0,0 +1,51 @@
+Setting default values for fields
+=================================
+
+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.
+
+We can achieve this by setting a default value for the field which could either be a fixed value or a simple sql
+function.
+
+**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.
+
+1) Modify the table
+-------------------
+
+Edit the ``created_at`` field and set its Default value as the SQL function ``now()``.
+
+
+Open the console and head to ``Data -> article -> Modify``:
+
+.. image:: ../../../img/graphql/manual/schema/add-default-value.png
+
+.. admonition:: To set an auto-incrementing default value
+
+  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.
+
+  Let's say we have a field called ``roll_number`` which we would like to be set by default as an auto-incremented
+  integer.
+
+  Head to ``Data -> SQL`` and run the following SQL command to create a new sequence.
+
+  .. code-block:: SQL
+
+    CREATE SEQUENCE roll_number_seq;
+
+  Now set the default value of the ``roll_number`` field as ``nextval('roll_number_seq')``
+
+
+2) Run an insert mutation
+-------------------------
+
+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.
+
+.. image:: ../../../img/graphql/manual/schema/default-value-response.png
+
+.. note::
+
+  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`

docs/graphql/manual/schema/index.rst

@@ -23,6 +23,8 @@ See:
   Customise with schema stitching <schema-stitching>
   Adding custom resolvers <custom-resolvers>
   Enum type fields <enums>
+  Default field values <default-values>
+  Set values using SQL functions/stored procedures <sql-functions>
   Using an existing database <using-existing-database>
   Export GraphQL schema <export-graphql-schema>
   How schema generation works <how-it-works>

docs/graphql/manual/schema/sql-functions.rst

@@ -1,11 +1,12 @@
-Set values using SQL functions/stored procedures
-================================================
+Setting values using SQL functions/stored procedures
+====================================================
 
 Let's say you want to set the value of some fields as the output of some custom SQL functions or stored procedures.
 
 This can be achieved by:
 
-#. Modifying the table to allow the columns we want to be set by the SQL functions as nullable.
+#. Modifying the table to allow the columns we want to be set by the SQL functions as nullable (to allow the initial
+   insert before the SQL function is run).
 #. Creating an insert/update trigger on the table that calls your SQL function and sets the output values in the output
    columns.
 #. Making your mutation requests without setting the SQL function output columns.
@@ -18,7 +19,7 @@ to set the value of the ``output`` column as the uppercased value of the string
 
 Modify the table ``sql_function_table`` and make its ``output`` column nullable.
 
-Head to ``Data -> sql_function_table -> Modify``
+Open the console and head to ``Data -> sql_function_table -> Modify``
 
 .. image:: ../../../img/graphql/manual/schema/modify-sql-fn-table.png
 
@@ -78,3 +79,8 @@ value (output="YABBA DABBA DOO!") will be set automatically.
         }
       }
     }
+
+.. note::
+
+  This approach enforces the value set in the field to always be the result of the defined SQL function unlike
+  :doc:`setting a default value <default-values>` instead.