Setting values of fields using SQL functions¶
Table of contents
Introduction¶
Let’s say you want to set the value of some fields as the output of some custom SQL functions
(a.k.a. stored procedures). This is useful to set values of fields which depend on other fields passed in the input. E.g. set
submission_time
of an online quiz as 1 hour from the start_time
.
This can be achieved by:
- Modifying the table to allow the columns we want to be set by the SQL functions to be 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.
Note
This approach enforces the value set in the field to always be the result of the defined SQL function even if a value is explicitly passed in the insert object.
For example, say we have a table sql_function_table
with columns input
and output
and we would like
to set the value of the output
column as the uppercased value of the string received in the input
field.
Step 1: Modify the table¶
Modify the table sql_function_table
and make its output
column nullable.
Create a migration manually and add the following SQL statement to the up.sql
file:
ALTER TABLE "public"."sql_function_table" ALTER COLUMN "output" DROP NOT NULL;
Add the following statement to the down.sql
file in case you need to roll back the above statement:
ALTER TABLE "public"."sql_function_table" ALTER COLUMN "output" SET NOT NULL;
Apply the migration by running:
hasura migrate apply
You can modify a table column by using the run_sql metadata API:
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin
{
"type": "run_sql",
"args": {
"sql": "ALTER TABLE sql_function_table ALTER COLUMN output DROP NOT NULL;"
}
}
Step 2: Create a trigger¶
The below SQL defines a trigger
which will simply uppercase the value passed in the input
field and set it to
the output
field whenever an insert or update is made to the sql_function_table
:
CREATE FUNCTION test_func() RETURNS trigger AS $emp_stamp$
BEGIN
NEW.output := UPPER(NEW.input);
RETURN NEW;
END;
$emp_stamp$ LANGUAGE plpgsql;
CREATE TRIGGER test_trigger BEFORE INSERT OR UPDATE ON sql_function_table
FOR EACH ROW EXECUTE PROCEDURE test_func();
Create a migration manually and add the above SQL to the up.sql
file. Also, add a statement to revert the previous statement to the down.sql
.
Apply the migration by running:
hasura migrate apply
You can create a trigger by using the run_sql metadata API:
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin
{
"type": "run_sql",
"args": {
"sql": "<above SQL>"
}
}
Step 3: Run an insert mutation¶
Run a mutation to insert an object with (input = “yabba dabba doo!”, output=null) and you’ll see the output value (output=”YABBA DABBA DOO!”) will be set automatically.
mutation {
insert_sql_function_table (
objects: [
{input: "yabba dabba doo!"}
]
) {
returning {
input
output
}
}
}