Table-based custom type does not verify check constraints

Question

Is there any way for a table-based custom type to obey the table check constraints, other than creating my own type based on the table?

Assume the following table that serves as a source for the custom type:

CREATE TABLE source
(
  id character varying(20) NOT NULL,
  weight smallint NOT NULL,
  CONSTRAINT source_pkey PRIMARY KEY (id),
  CONSTRAINT source_weight_check CHECK (weight > 0 AND weight < 6)
)

Now, I intend to create the following data table:

CREATE TABLE final
(
  name character varying(20) NOT NULL,
  data source NOT NULL,
  CONSTRAINT final_pkey PRIMARY KEY (name),
)

When inserting records into the final table, the check constraint will not be observed.

Requirements:

1. I'd like to avoid creating my own type based of source, because when source changes I'll be forced to remember to also update the custom type.
2. I realize that table-based custom types obey domain check constraints. But, on my particular case, the weight field in the source database is ill-suited for a domain. Its check constraints varies from table to table. FYI, it's a value used in calculating weighted averages.

Answer 1

A record will only be null if all of its fields are null or if the record itself is null. If you want to check if any of the fields is null then check one by one:

create table final
(
    name character varying(20) not null,
    data source check (
        (data).id is not null and (data).weight is not null
    ),
    constraint final_pkey primary key (name)
);

insert into final (name, data) values ('a', null);
ERROR:  new row for relation "final" violates check constraint "final_data_check"
DETAIL:  Failing row contains (a, null).

insert into final (name, data) values ('a', (null, null));
ERROR:  new row for relation "final" violates check constraint "final_data_check"
DETAIL:  Failing row contains (a, (,)).

insert into final (name, data) values ('a', ('1', null));
ERROR:  new row for relation "final" violates check constraint "final_data_check"
DETAIL:  Failing row contains (a, (1,)).

insert into final (name, data) values ('a', ('1', 1));
INSERT 0 1