Add Constraints Safely in Migrations

PostgreSQL does not support ADD CONSTRAINT IF NOT EXISTS. Migrations using this syntax will fail.

Incorrect (causes syntax error):

-- ERROR: syntax error at or near "not" (SQLSTATE 42601)
alter table public.profiles
add constraint if not exists profiles_birthchart_id_unique unique (birthchart_id);

Correct (idempotent constraint creation):

-- Use DO block to check before adding
do $$
begin
  if not exists (
    select 1 from pg_constraint
    where conname = 'profiles_birthchart_id_unique'
    and conrelid = 'public.profiles'::regclass
  ) then
    alter table public.profiles
    add constraint profiles_birthchart_id_unique unique (birthchart_id);
  end if;
end $$;

For all constraint types:

-- Check constraints
do $$
begin
  if not exists (
    select 1 from pg_constraint
    where conname = 'check_age_positive'
  ) then
    alter table users add constraint check_age_positive check (age > 0);
  end if;
end $$;

-- Foreign keys
do $$
begin
  if not exists (
    select 1 from pg_constraint
    where conname = 'profiles_birthchart_id_fkey'
  ) then
    alter table profiles
    add constraint profiles_birthchart_id_fkey
    foreign key (birthchart_id) references birthcharts(id);
  end if;
end $$;

Check if constraint exists:

-- Query to check constraint existence
select conname, contype, pg_get_constraintdef(oid)
from pg_constraint
where conrelid = 'public.profiles'::regclass;

-- contype values:
-- 'p' = PRIMARY KEY
-- 'f' = FOREIGN KEY
-- 'u' = UNIQUE
-- 'c' = CHECK

Reference: Constraints