Postgres: Setting Default Values for Fields Using Postgres Defaults

Introduction

You can set values of certain fields automatically when not explicitly passed to a fixed value, e.g. true for a boolean field, or output of a simple SQL function, e.g. now() for a timestamp field, by setting column default values in the table definition.

Note

The Postgres default value is ignored when a value is explicitly set to the field.

Example: Say we have a field created_at in a table article which we want to be set to the current timestamp whenever a new row is added to the table:

Step 1: Modify the table

Edit the created_at field and set its default value as the SQL function now().

Console

Open the Console and head to Data -> [article] -> Modify.

Click the Edit button next to the created_at field and add now() as a default value.

CLI

Create a migration manually and add the following SQL statement to the up.sql file:

ALTER TABLE ONLY "public"."article" ALTER COLUMN "created_at" SET DEFAULT now();

Add the following statement to the down.sql file in case you need to roll back the above statement:

ALTER TABLE article ALTER COLUMN created_at DROP DEFAULT;

Apply the migration by running:

hasura migrate apply

API

You can add a default value by using the run_sql schema API:

POST /v2/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "run_sql",
  "args": {
    "source": "<db_name>",
    "sql": "ALTER TABLE article ALTER COLUMN created_at SET DEFAULT now();"
  }
}

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.

Run the following SQL command to create a new sequence.

CREATE SEQUENCE roll_number_seq;

Now set the default value of the roll_number field as nextval('roll_number_seq').

Step 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.

GraphiQL

Query Variables

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Also see

• Postgres: Setting values of fields using SQL functions
• Postgres: Setting values for fields using role-based column presets