til/postgres/show-reconstructed-constraints-for-a-table.md

Show Reconstructed Constraints For A Table

The pg_get_constraintdef function can be used to reconstruct the command for creating a given constraint. This isn't necessarily the command (or commands) that originally created the constraint, but rather a reconstruction.

We have to pass it an oid that corresponds to the constraint which we can get from the pg_constraint table. These results can be further narrowed down by the conname (constraint name) and conrelid (table name).

Here is an example of listing the constraints on a reading_statuses table.

> select
    conname,
    pg_get_constraintdef(oid)
  from pg_constraint
  where conrelid = 'reading_statuses'::regclass;

               conname               |                                                                               pg_get_constraintdef
-------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 reading_statuses_pkey               | PRIMARY KEY (id)
 fk_rails_17ee7cb2c4                 | FOREIGN KEY (user_id) REFERENCES users(id)
 fk_rails_0d3729339f                 | FOREIGN KEY (book_id) REFERENCES books(id)
 reading_statuses_valid_status_check | CHECK (((status)::text = ANY ((ARRAY['started'::character varying, 'completed'::character varying, 'abandoned'::character varying, 'already_read'::character varying])::text[])))
(4 rows)

I came across this while experimenting with an idea for a fail-fast Rails initializer check that verifies the values of the reading_statuses_valid_status_check stay in sync with the Rails version of those values that live in a constant.