How to defer foreign key constraint in Postgres

I tried to set a foreign key constraint deferrable so that it won't be checked when I insert into a lookup/pivot table until end of transaction. However, it woks in psql shell but its just not working in the code. same as in the psql shell, I start a transaction with begin in code as well.

This is the sql:

create table campaign_r_company (
  campaign_id         uuid            not null references campaign(id) on delete cascade deferrable initially deferred,
  company_id          varchar(32)     not null,
  primary key (campaign_id, company_id)
);

Here's the code:

  tx, err := d.Begin()
  if err != nil {
    return err
  }
  
  
  err = h(tx) // there are two db queries will be called in this function
  
  if err == nil {
    err = tx.Commit()
  }

h(tx):

_, err := cxt.Exec(fmt.Sprintf(`INSERT INTO hp_campaign (%s) VALUES (%s)`, proplist("", campaignProps), arglist(1, len(campaignProps))),
	id, v.Name, created, v.Updated,
)
if err != nil {
	return err
}

v.Id = id
v.Created = created

if (opts & StoreOptionStoreRelated) == StoreOptionStoreRelated {
	err := d.attach("company", "campaign_r_company", v.Companies, v.Id)
	if err != nil {
		return err
	}

}

attach():

func (d *Database) attach(entityName string, tableName string, ids []string, campaignID string) error {

	for _, id := range ids {

		stmt := fmt.Sprintf(`INSERT INTO %s (%s) VALUES ($1, $2)`, tableName, fmt.Sprintf("campaign_id, %s_id", entityName))
		_, err := d.db.Exec(stmt, campaignID, id)

		if err != nil {
			return err
		}
	}
	return nil

}

Error:

insert or update on table "campaign_r_company" violates foreign key constraint "campaign_r_company_campaign_id_fkey"

From the updated code, and subsequent comments, we know now that the issue was that the two queries were executed separately and not in a single transaction.

If you do not use manual transaction management, Go+PG does this for you. In this case any statement is a single transaction and constraints are get checked at the end of each. So exception is raised.